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The behaviour of Przewalski's horses and its importance to their management
Applied Animal Behaviour Science, 29 ( 1991 ) 301-318
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Elsevier Science Publishers B.V., Amsterdam
The behavior of Przewalski's horses and its importance to their management Lee E. Boyd Department of Biology, Washburn University, Topeka, KS 6662• (U.S.A.)
ABSTRACT Boyd, L.E., 1991. The behavior of Przewalski's horses and its importance to their management. App[. Anita. Behav. Sci., 29: 301-318. Przewalski's horses (Equus przewalskii) are believed to be extinct in the wild; the current known population of 797 animals exists wholly in zoos. The Species Survival Commission of the International Union for Conservation of Nature and Natural Resources is proposing to reintroduce this endangered species into its former Mongolian habitat within the next decade. Knowledge of the behavior of harem-forming equids in general and of Przewalski's horses in particular, is of great importance to the captive propagation and eventual reintroduction of this species. Horses are rarely solitary by nature. Solitary captive animals are prone to pacing. Juvenile male feral horses (Equus caballus) form bachelor herds upon dispersal from their natal band. Zoos can set up bachelor herds as a way of managing surplus males. The older, more dominant feral horse bachelors are the first to acquire mares. Bachelors do not generally obtain females until they are 4 or 5 years of age. The first females acquired are usually 1- and 2-year-old fillies dispersing from their natal band. Because of the age differential, the stallions are generally dominant to their mares. Behavioral impotence may result if captive stallions are given a harem at too young an age, especially if the harem contains older, more dominant, females. Typical harem sizes in the wild are 3-5 mares. Captive stallions with too large a harem may become either apathetic or aggressive toward their mares. Wild horses spend 60-70% of their time foraging. Captive animals may quickly consume their limited amounts of food and develop vices out of boredom. Provision of hay ad libitum reduces the amount of pacing seen in captive animals, and virtually eliminates coprophagy.
INTRODUCTION
A knowledge of the behavior of a species is essential to its management. The Przewalski's horse, Equus przewalskii, is an endangered species presently believed to be extinct in the wild and now found only in zoos. The Food and Agriculture Organization of the United Nations ( 1985 ) is drawing up plans to reintroduce Przewalski's horses into their former Mongolian habitat, and this project will be documented as a model for the reintroduction of other species. Przewalski's horses are the closest living relative of domestic and feral horses, Equus caballus (Benirschke et al., 1965; Ryder et al., 1978). Much 0168-1591/91/$03.50
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information exists about the behavior of domestic and feral horses (reviewed by Waring, 1983 ). However, there is a dearth of behavioral information about Przewalski's horses, and this deficiency has long been recognized (Species Survival Commission of the International Union for Conservation of Nature and Natural Resources, 1981; Ryder and Wedemeyer, 1982; Ryder et al., 1984). The purpose of this study was to document the behavior of captive Przewalski's horses. Much of the existing behavioral information is anecdotal. Studies in which data were quantified have been of short duration (usually < 1 m o n t h ) and have taken place at one, or at most two, locations. The majority of this work is unpublished, or has been presented in publications which are difficult to obtain. Unfortunately we will never know what natural Przewalski's horse behavior was like because little behavioral information was recorded by those who observed the horses in the wild before their extinction. The current population of Przewalski's horses has been in captivity for 2-12 generations (Volf, 1988 ). Thus, we cannot be sure whether any behavioral similarities between Przewalski's and domestic horses exist because of their close evolutionary relationship, or whether similarities exist because of captivity which both species have experienced. Nevertheless, the study of Przewalski's horse behavior is important if this species is to be reintroduced into the wild. If captive Przewalski's horse behavior is similar to captive domestic horse behavior, then one would predict that they would behave like feral horses when released. Much is known about feral horse behavior (Pelligrini, 1971; Welsh, 1975; Feist and McCullough, 1976; Salter, 1978; Boyd, 1980; Miller, 1980; Kaseda, 1981; Rubenstein, 1981; Keiper, 1985; Berger, 1986 ) and this knowledge could then be used as a basis for managing Przewalski's horses after their reintroduction. Other than their restricted roaming abilities, the conditions of captive Przewalski's horses differ from those of free-ranging horses in four main ways: ( 1 ) some zoos do not provide food ad libitum, and water and food may be concentrated in one area, eliminating search time in foraging; (2) the horses come into more contact with humans than most free-ranging animals; (3) there is no male competition for females; (4) because humans decide what individuals will be grouped together, there is diminished mate choice, although some still occurs when stallions reject certain mares, or vice versa. It should be noted that points (2) to (4) above are also true to a lesser degree for some free-ranging equid populations where human intervention is frequent or where there are heavily female-biased sex ratios (Tyler, 1972; Duncan, 1980; Pollock, 1980; Keiper, 1985). ANIMALS, MATERIALS A N D M E T H O D S
This study took place from 1982 to 1987. I visited every zoo in the United States that possessed Przewalski's horses during these years, with the excep-
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tion of the Minnesota Zoo whose horses had been studied by Skiff (1982). To collect data, I modified a protocol developed by Crowell-Davis (1983). Focal animal observations of 15 min duration were conducted on each herd member in order to develop time budgets (Altmann, 1974). The next horse to be observed was chosen randomly without replacement until all herd members had been observed once. The process was then repeated. During the focal animal sample, the time of onset and end of 10 mutually exclusive behavior states were recorded to the nearest 3 s. These behavioral states were feeding (time spent eating feces was noted as a subset), nursing, drinking, standing awake, standing resting, locomotion (time spent pacing the fence was noted as a subset ), play, self-grooming, mutual grooming, recumbency. If ever these behavioral states were of < 3 s duration, they were not recorded. The mean number of behavioral states exhibited by the individual per hour was calculated. Behaviors which were typically of short duration were treated as events. These included defecation, urination, marking, flehmen, masturbation, aggression, snapping, and vocalization. For interactive behaviors such as aggression, mutual grooming, mutual play, and nursing, the participants and the contexts of the behavior were noted. At 5 min intervals during the observation, the location and nearest neighbor of the focal animal was recorded. Dominance hierarchies were determined for 10 herds using Schein and Fohrman's ( 1955 ) method, in which animals are ranked so that each individual receives threats from the smallest number of individuals below it. Grain sharing was studied in the Topeka herd. Horses were fed in rubber pans from which 2-3 horses could share. As there were five pans and 5-10 horses, sharing frequently occurred. Opportunistic notes were made of which horse was sharing with w h o m from 1982 to 1985 and were used only to determine the pattern of grain sharing by the stallion "Rolmar". In 1986 "Rolmar" was replaced by his paternal half brother, "Roccol", and in 1987 data on sharing were collected by a series of point samples at 1 min intervals whenever grain was provided. A total of 1332 h was spent watching horses, and 1020.25 h of focal animal data were gathered from a total of 92 individuals in 29 enclosures at nine zoos. These individuals represent approximately 85% of the United States population and 15% of the world population during this period (Volf, 1988 ). At each zoo, background information about enclosure size and diet was collected and supplemented by information from a questionnaire which eight zoos filled out and returned. Detailed data have been presented elsewhere (Boyd, 1986, 1988a,b; Boyd and Kasman, 1986; Boyd et al., 1988), so the following will be an overview. Management implications will be discussed in the light of these findings.
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RESULTS A N D D I S C U S S I O N
Time budgets Three or more hours of focal time budget data were collected from 15 males and 27 females in 13 enclosures at seven zoos. The stallions averaged 66 _+4 behavioral states h-~ while the females averaged only 55 __+3 (Boyd, 1988b). The stallions also vocalized 2.5 -+ 0.3 times h - ~which was significantly higher than the mares' rate of 1.4 + 0.1 times h-1 (Mann-Whitney U test ( M W U ) , P < 0.01 ). Stallions spent 14.2 + 2.0% of their time in motion which was significantly higher than the 8.5 _+0.5% of time that the mares spent in locomotion (MWU, P < 0.01 ). The difference in the amount of time spent in motion was also true for the special case of pacing the fence. Only 2 of the mares were observed pacing, as opposed to two-thirds of the stallions. Both solitary stallions paced, 5 of 8 bachelors paced, and 3 of 8 harem stallions paced. One of the harem males paced for ~ 1 h day-z in temperature of 30°C, causing profuse sweating (Boyd, 1986). The tendency of males to be more active than females, showing frequent changes of behavioral state, may be due to the fact that stallions are free of the nutritional demands of pregnancy and lactation whereas mares spend much time grazing. It may also be a legacy of the role of free-ranging stallions. In the wild, stallions must be active and vigilant in obtaining mares and in defending their harems against competitors. This tendency to be more active makes stallions more prone to pacing in captivity, a habit which might be life-threatening in hot weather. Solitary males spent more time pacing (7.1_+2.8% of their time budget) than bachelors (6.1 _+3.3%) or harem males (0.9 _+0.7%). Pasturing the horses or providing hay ad libitum virtually eliminated pacing for bachelors and for harem males and reduced the amount of time solitary males spent pacing to 4.9_+ 0.7% (Boyd, 1988b). Providing companions may reduce the amount of pacing shown by an individual. One harem stallion did not pace when hay was present, but, as a solitary male, he spent 6% of his time pacing even when hay was present. When some form of food was available, adult horses rarely showed coprophagy.
Enclosure effects Skiff (1982) studied the effects of enclosure size on the behavior of 2 herds of Przewalski's horses. The number of activities was higher in the 0.05 ha enclosure than in the 3.5 ha enclosure. When in the small enclosure, horses did more milling and fence-pacing, had higher rates of aggression, and spent more time mutually grooming than when on pasture. Coprophagy and wood chewing was seen in the small enclosure but never on pasture. Przewalski's horses isolated in small enclosures may also begin cribbing, a behavior difficult to extinguish even when the horse is moved to a pasture (Bouman-Heins-
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dijk, 1982; Boyd, 1986). In the present study, there was no significant effect of enclosure size on aggressiveness or on the amount of time spent in mutual grooming or in locomotion (Kruskal-Wallis ANOVA, P > 0 . 0 5 ) . The main effects of enclosure size were that horses in enclosures < 0.4 ha in size tended to exhibit more behavioral states per hour and spent significantly less time standing resting than animals on pasture (Boyd, 1988b ). Horses in small enclosures were in metropolitan areas, whereas those in large enclosures were not. It is possible that the noises in zoos or of the cities in which they are located, disrupt the diurnal resting behavior of the horses. A summertime 24-h time budget was compiled for 8 horses at the National Zoo's Conservation and Research Center. These horses were in a grassy 12 ha pasture. The center's staff had expressed concern that despite the spacious enclosure, the horses were only observed to use the area immediately adjacent to the barn. Over 200 h of around-the-clock focal observations indicated that the horses were making use of the entire area. They met the keepers at the barn in the late morning, when pelleted feed was provided. The horses lingered in this area during the hot hours of the day but moved out to graze by 20:00 h when the staff had gone home. The horses then spent the evening grazing and recumbent away from the barn. Grazing peaked between 20:00 and 24:00 h when the horses spent 68.2% of their time in this activity. Recumbency peaked at 21.4% of the time budget between the hours of 00:00 and 04:00 (Boyd et al., 1988). Thus, the horses were making use of their entire enclosure and were taking advantage of the forage present but were waiting until evening to do so. Grazing during s u m m e r evenings may have minimized thermal stress, and at night there was no reason to linger near the barn as no visits by staff were expected.
Dominance hierarchies The dominance hierarchies of Przewalski's horses appear to be remarkably stable over time. In the 4 years that the Topeka herd was observed, only one change in rank occurred. The exhibit herd at the San Diego Wild Animal Park was studied by Mackler and Dolan (1980) in 1975 and 1976. The relative ranks of the 4 mares still present in 1984 were the same except that 1 mare reintroduced to the herd during Mackler and Dolan's study had moved up one position from the b o t t o m of the hierarchy. Przewalski's horses have been reported to direct most of their aggression toward horses just below them in rank (Hutson, 1975; Mackler and Dolan, 1980 ). Subordinate mares are the most aggressive toward new herd members (Mackler and Dolan, 1980). In this study, 55% of the mares directed the majority of their aggression toward the horse just below them in rank (or 63%, if only the mares who were not at the bottom of their respective hierarchies are considered). For the 2 cases in which new mares were introduced to
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the harem during the course of the study, the lowest ranking adult female showed the most aggression toward the newcomers. Foals were always the lowest ranking animals in the herd but were protected when near their mother and so initiated and received little aggression. Age is important in determining position in the dominance hierarchy (Lewtas, 1973; Kolter, 1985; Feh, 1988; KJimov, 1988; Kolter and Zimmermann, 1988). Age was positively correlated with dominance order for the adults in the 10 herds studied and significantly correlated with rank in 4 of the 6 herds large enough to permit statistical testing. Following age in importance was body size. Aggressiveness was positively correlated with dominance in all but 1 herd. Horses ranking high in the hierarchy initiated more aggressive acts per hour. Factors of lesser importance were length of residence in the herd and maternal rank. Size and aggressiveness are probably initially important factors when a new herd is formed. In herds where core membership is stable over time, as is the case for most Przewalski's herds in captivity, age becomes the most important factor. Juveniles are subordinate to adults, and, if they remain in their natal band, they will have to fight their way up the ranks against individuals who have been dominant to them for at least 2 years. Maternal rank may be important if the foals inherit traits such as large size that are advantageous in competition. Foals of dominant mothers may learn confidence in their dealings with other herd members, but, if they stay in their natal band, they must still fight their way past all other mares including their mother to achieve top rank. Thus, maternal rank is not as good a predictor of offspring rank as is age relative to other herd members. Horses with longer tenure in the herd are likely to be older and have the confidence of their surroundings and their herd alliances. Nevertheless, this may not always permit them to hold their rank upon the arrival of larger, more aggressive horses. To examine what benefits might accrue to dominant animals, percent time spent feeding by 36 mares was correlated with dominance rank. The correlation was negative and significant, indicating that mares ranked Number 1 or 2 in the herd spent more time feeding (rs = - 0 . 2 9 , P < 0.05 ). The correlation between dominance rank and the amount of time spent feeding was apparently not a result of dominant mares being larger and, therefore, requiring more food. The correlation between size and amount of time spent feeding was not significant ( r s = - 0 . 0 9 , N = 2 5 , P > 0 . 0 5 ) . Additionally, mares have been observed to interfere with the stallion when he attempts to breed mares that are subordinate to them (Dolan, 1977; Mackler and Dolan, 1980; Zimmermann, 1985; Boyd, 1986). In two cases, the interfering mare had to be removed from the herd until the subordinate mares conceived. Adult Przewalski's stallions are generally dominant to mares (Dobroruka, 1961 ; Lewtas, 1973; Hutson, 1975; Klimov, 1988 ), however, young stallions may be subordinate to older mares. In 2 of the herds studied, the stallions,
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aged 3 and 7, were subordinate to some mares, and in a third herd the stallion, aged 7, was never observed to interact aggressively with any herd member so his rank could not be ascertained. All 3 of these stallions were younger than some of the mares in their herds. One of the subordinate stallions sired 3 foals in 3 years. The other, and the one whose rank could not be determined, have never sired any foals. The stallion who sired 3 foals was placed with a harem of 6 mares as a 2-year-old and was subordinate to those mares who were older than he. As a 3-year-old, this stallion impregnated a younger mare to whom he was dominant. The following year he sired 2 foals by 2 other younger mares to whom he was dominant. The older females remained dominant to him and were not impregnated. The stallions in the other 5 harems studied were the dominant animal in their herd. Four of these 5 dominant stallions, ranging in age from 4 to 21 years, had sired more than 1 foal in the past 3 years. Three of the dominant stallions who successfully reproduced were also the oldest members of their harem. The dominant stallion who did not reproduce had previously been subordinate to both of his mares. They kicked him whenever he attempted to mount and he failed to intromit. The mares were sent to another zoo to be bred, and, when they returned 1 year later, the 8-year-old stallion was now dominant to them. He became overly aggressive toward the mares, biting them severely enough to leave scars. The stallion attained an erection and mounted the mares but still failed to intromit. He was then sent to another zoo where he was solitary for 2 years. In an attempt to rehabilitate him, he was then given 2 mares. One was the same age as himself ( 11 years) and one was 2 years old. He was dominant to both mares. This small harem was then moved to a pasture adjacent to a larger harem so that the impotent stallion would have sensory, but not physical, contact with the successfully breeding stallion in the nearby enclosure. After several months the successful stallion was retired. Then the impotent stallion was allowed to merge his small harem with the 3 adult mares and their offspring in the adjoining pasture, all of whom were younger than he was. The impotent stallion remained on pasture with these mares for 2 years until his death at age 13. He was not overly aggressive toward the adult mares, although he was aggressive toward the 2- and 3-yearold pubertal fillies. He attained an erection and mounted the mares, who tolerated his attempts, but he was never observed to intromit and sired no foals. Feral horse bachelors do not have an opportunity to breed until the age of 4 or 5 years, and their first mares are usually 1- and 2-year-old fillies dispersing from their natal bands (Boyd, 1980; Keiper, 1985; Berger, 1986). Bachelors are usually 6-8 years of age before they are successful at obtaining mares through combat (Berger, 1986). The most dominant bachelors are the first to obtain mares (Feist and McCullough, 1976; Rubenstein, 1982; Keiper, 1985 ). Therefore, in the wild, harem stallions would usually be older and more dominant than their mares. As horses are harem polygynous and not all
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males are able to acquire mares, it would seem that only the most dominant males would breed and that a female would prefer to breed with a dominant male who might pass genetic factors contributing to his status to her sons. Accordingly, adult Przewalski's mares have previously been reported to reject immature stallions (Bouman-Heindsdijk, 1982; Kolter, 1985; Klimov, 1988 ). In the herds of Przewalski's horses studied, subordinate harem stallions were always younger than some of their mares and had lower reproductive success than alpha-ranking stallions. In addition to these herds, three others had harem stallions who were not the oldest members of their herd and who sired at least 1 foal. However, in another four cases such stallions sired no foals. In contrast, no cases were found in which a male initially placed with younger females failed to sire foals. As age and dominance are correlated, and as dominance affects reproductive success, zoos need to place inexperienced stallions with younger mares, at least until the stallion gains age and experience.
Relationships among females Mares in a harem form close, stable associations that persist even in the absence of stallions (Kaseda, 1981; Keiper, 1985; L.E. Boyd, personal observations of 3 herds of Przewalski's mares without a stallion present, 1986, 1988 ). It is not unusual for mares in the same harem to be relatives (mother and daughters, full or half sisters), which probably contributes to the close ties between harem females. Closely related mares preferred one another as mutual grooming partners. As mares form these long-term bonds, it seems logical that the present practice of rotating stallions from harem to harem to reduce inbreeding more closely approximates the natural social processes than would a program that traded particular mares.
Bachelor herds and harem composition As of 1980, 75 zoos worldwide maintained Przewalski's horses in a total of 97 enclosures (Bouman, 1982 ). Thirty-two enclosures housed males only. The males in 26 of these enclosures were solitary, and therefore, likely to pace. Horses are gregarious; in the wild, stallions which do not possess harems are rarely solitary but form bachelor groups instead. In captivity, this process can be mimicked. Males not currently being used for breeding can be grouped to form bachelor herds. This reduces the number of enclosures needed and makes the stallions less likely to pace. Serious aggression is not a problem with juvenile males; forming herds containing older experienced males requires more care, however. By setting up herds in areas large enough (0.4-0.8 ha) to allow subordinates to flee and take cover, Tilson et al. (1988) were able to form bachelor groups containing males ranging in age from 1 to 9 years. Including several juveniles and allowing them some time together before the introduc-
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tion of the older males permits the juveniles to form coalitions and lessens the chance that any one of them will receive the full brunt of aggression. Wild stallions seldom have harems of more than 5 or 6 females. When the harem of the Przewalski's stallion "Basil" grew to 18 members, he became apathetic about breeding (Knowles, 1980). Isolating him and reducing the harem size prior to his return restored his libido. The stallion "Bars" became aggressive toward his mares when the harem size reached 13 animals (Keverling Buisman and van Weeren, 1982 ). The plans for reintroduction of Przewalski's horses into Mongolia specify that 1- and 2-year-old fillies are to be kept together in a holding area until 3 years of age to promote social bonding (Species Survival Commission of the International Union for Conservation of Nature and Natural Resources, 1981 ), which seems reasonable. A 3-year-old stallion will be added to each harem before it is released into the wild, which, in light of the age at which feral horse stallions acquire harems, seems a little too young. Current plans call for the stallions to be approximately the same age, so that one does not monopolize all the mares. In my opinion, the critical factor affecting harem size will be the male:female sex ratio rather than male age. Even if all the stallions are the same age, a few will be dominant and monopolize all the mares unless they have competition. And no I 0-year-old stallion in the prime of life can hold more than 4 or 5 mares if he must constantly fight off 5 or more bachelors. I would r e c o m m e n d a 1 : 1 sex ratio for release, especially as males are the more expendable sex.
Masturbation All stallions, whether solitary, in bachelor groups, or in harems, were observed to masturbate by flipping the penis against the belly. As stallions masturbated throughout the breeding season and still sired foals, this behavior does not appear to reduce libido, as has been suggested for domestic horses (Waring, 1983 ). Therefore, masturbation should not be considered a problem.
Lactation The gestation length of Przewalski's horses is 11 months and foals are usually born in the late spring or early s u m m e r (Dobroruka, 1961; Mohr, 1971 ). Mares come into estrus within 1 week of foaling. The mares in this study who conceived promptly weaned their foals just prior to the birth of their next offspring. Mares failing to conceive during that breeding season, permitted their current foals to nurse much longer. Three mares whose 2-year-old daughters remained in their natal band failed to conceive during the 2 years following the birth of their daughters. Each of these mares nursed their daughters through 26 months of age. Mares continue investment in lactation for
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their older offspring until they have another offspring in utero which begins to make significant demands on their resources. The timing of weaning of older foals can, thus, be used as a non-invasive diagnosis for pregnancy, with the exception of mares on a high plane of nutrition. Mares at two zoos received 17% more digestible energy than horses at the zoo feeding the next highest energy level, and nursed both their yearlings and their latest foals. Klimov (1988) has also reported that mares may allow older offspring to continue to nurse along with their newborn foals. The high level of nutrition apparently permitted the mares to invest in more than 1 offspring simultaneously.
Grain sharing The horses at the Topeka Zoo's Forbes Conservation/Propagation Center were expected to spend 10% of their time sharing grain with each other if they shared grain randomly, and had as much chance of eating alone as with any other horse. However, random sharing was not observed. The mares' most frequent partners at a pan of grain were their offspring. Mares spent 16 + 14% of their time feeding alone, 7 + 1% sharing with one of the 4 other adults, and 77 _+ 13% of the time sharing with foals; 96.5 + 0.5% of the time these foals were their own offspring. The sire of the foals spent twice as much time sharing grain with his offspring than did the stallion who replaced him the following year with the unrelated foals born after his arrival (Fig. 1 ). The sire of the foals spent 35.8% of the time feeding by himself (expected= 16.7%), 18.0% of the time sharing grain with adult mares (expected = 50.1%), and 46.2% of the time sharing with his offspring (expected= 33.2%). The unrelated stallion spent only 24.4% of his feeding time sharing with foals (expected= 50.0%).
Paternal behavior In the wild, sires probably played an important role in protecting their offspring against predators and in retrieving foals when they strayed from the herd, as has been observed in feral horses (Feist and McCullough, 1975; Boyd, 1980; Berger and Rudman, 1985; Berger, 1986 ). In captivity, sires rarely have occasion to provide this form of parental investment. When the number of feeding stations are limited, sires do appear to aid their offspring by sharing grain with them. Mackler and Dolan (1980) also observed a Przewalski's stallion sharing food with his son but chasing adult mares away. Stallions also tolerate foals grazing in their vicinity (Dobroruka, 1961 ). Sires may take on part of the role of the missing dam for orphaned offspring. One foal's mother died when the foal was 2.5 months of age. The sire permitted the orphan to suckle his sheath over a period of several weeks until
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Fig. 1. Grain sharing. (A) A stallion (left) and mare (right) contesting a pan of grain at the Topeka Zoo's Forbes Conservation/Propagation Center. (B) The stallion was more likely to share grain with his offspring than with adult mares.
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this nursing behavior ended. The stallion was more likely to be this foal's nearest neighbor than expected under random association, and the orphan was twice as likely to be within 1 m of the stallion than mothered foals in 67% of the months after his dam died (Boyd, 1988a). When the orphaned foal was between 3 and 6 months of age, his sire was observed to lean over him 4 times while feeding from a hayrack to bite a yearling daughter. Twice the sire reached over the orphan to bit adult mares. In one case the orphan pushed in between his sire and an adult mare at the hayrack, bumping his sire. His sire reached over him and bit the mare. In the other case, the orphan walked under his sire's neck to get to the rack, interrupting his sire's feeding. The sire leaned over him and bit a mare. Three times the stallion threatened a mare away from the hay but permitted the orphan to stay next to him. Once he approached the orphan when it was sharing grain with a yearling paternal half sibling. The sire supplanted his yearling but allowed the orphan to stay and share grain with him. The stallion also threatened 1 of the adult mares twice when she approached his pile of grain but let the orphan share. The orphaned foal bumped into him, but, instead of attacking the foal, the sire threatened to kick the mare, who was still near. The stallion was dominant to all members of the herd, so he could have threatened the foals. Instead he seemed to avoid aggression against them and redirected his aggression toward older herd members. In this study, colts spent as much time mutual grooming with stallions as expected, while fillies mutual groomed with sires much less than expected. Foals depended primarily on other foals as play partners, but 2 colts played frequently with their sire. "Basil", the father of I of the colts, had been reported by Lewtas (1973) and Hutson (1975) to be very tolerant of play directed at him by some of his other sons. Play with sons may be important in helping them develop fighting skills essential in competition to obtain their own harems (Berger, 1986).
Breeding interference by juveniles Yearlings and 2-year-olds sometimes interfered when the stallion attempted to breed their mother. This interference consisted of the juvenile approaching the courting pair and putting itself between them, often kicking the stallion. This behavior was not observed to create a problem, as the stallion tolerated the juveniles' interference. While the behavior was proximately effective in that it always distracted the stallion from continuing courtship, ultimately it had no effect on conception rate. In every case, the mares became pregnant during that breeding season.
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Stallion-like behavior by mares Two mares exhibited stallion-like behavior such as mounting other herd members (Boyd, 1986 ). In both cases the mares were 3 months pregnant with male foals. Perhaps the gonads of the foals were producing large amounts of testosterone during differentiation and this produced temporary masculinization of their mothers' behavior, as has been reported to occur in domestic horses (Rossdale and Ricketts, 1974; Beaver and Amoss, 1982). Such behavior is not a problem as long as it is tolerated by other herd members and is determined to be due to pregnancy and not a tumor-induced hormonal imbalance.
Infanticide Several cases of infanticide by Przewalski's stallions have been reported. "Ulan" killed 3 colts aged 5-6 months sired by his father "Vampir" when he replaced "Vampir" as harem stallion (Keverling Buisman and van Weeren, 1982 ). "Ulan" was also aggressive toward the 2 remaining foals, a filly and a colt, who were quickly removed from the herd. One mare was 6 months pregnant when "Ulan" was introduced. Her colt was found dead shortly after it was born. It was covered with contusions, and "Ulan" was presumed to be the culprit because of his aggression toward the previous foals. "Ulan" has since been aggressive towards his own offspring when they reach puberty but has not killed any of his own foals (K.A. Houpt, personal communication, 1987 ). The stallion "Basil" sired more than 50 foals (Gruenerwald, 1982 ) without incident at several European zoos before coming to the United States. Two mares at the San Diego Wild Animal Park were carrying colts sired by other stallions when "Basil" was introduced into the herd. One of these mares foaled 3 months after "Basil" arrived. Her colt was found dead on the day of its birth. The cause of death was trauma to the neck region (Ryder and Massena, 1988). Five months after "Basil" arrived, the second mare foaled. "Basil" was observed to attack the colt by grabbing its neck, shaking it, and flinging it into the air (Ryder and Massena, 1988). The colt subsequently died of its injuries. The d a m immediately conceived by "Basil", and the next year she and another mare gave birth to colts sired by "Basil". He attacked neither of these colts. Infanticide may not be a pathological behavior resulting from captivity, as it is also seen in free-ranging domestic horses and appears to be adaptive as it is directed toward unrelated foals rather than the sire's own offspring. Although " U l a n " killed paternal half siblings, he had been separated from his natal herd for several years before being reintroduced. Unless horses use phenotype matching as a mechanism of kin recognition,"Ulan" probably had no way of knowing that he was related to the foals born after his departure (Berger, 1986; Berger and Cunningham, 1987 ). By killing unrelated foals, a stallion eliminates future competitors who do not share his genes. In domestic
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horses, unrelated male foals are more commonly victimized than unrelated female foals, perhaps because male foals grow up to be competitors whereas female foals represent future mates (Duncan, 1982 ). By killing young unrelated foals, a stallion might also benefit by freeing the mares from the physiological stress of lactation, which improves their chances of producing a foal by him the following season (Boyd, 1986). Domestic mares in the weight range of Przewalski's horses (200-300 kg) have peak lactation requirements for protein and energy that are approximately twice maintenance levels (National Research Council, 1978). The nutritional stress imposed by the demands of lactation may impair a mare's body condition and subsequent reproductive performance (Pollock, 1980; Berger, 1986). New Forest pony mares nursing foals received a body condition score that was twice as poor as that of barren mares (Pollock, 1980). Removal of the foals brought about an immediate improvement in body condition. Mares who nursed their foals for 2 years were in poor condition for at least 18 of those months; although they bred in the second year, they failed to foal. Domestic mares in poor body condition during gestation and lactation have lower conception rates and higher embryonic mortality than mares on a higher nutritional plane (Henneke et al., 1984). Keiper (1985) found that a population of feral mares in which the duration of lactation was shortened by removal of the foals at 6-12 weeks of age had an average yearly foaling rate of 74%, while mares in a population where foals were not removed had an average foaling rate of only 57%. Thus, killing foals before peak lactation occurs would improve the stallion's chances that the mare would conceive by him and carry his foal to term. Infanticide does not always occur when 1 stallion replaces another. Several stallions did not attack unrelated foals born into their harem after they replaced the former stallion. As zoos cannot predict whether a stallion will be infanticidal, precautionary measures may be warranted. Introducing stallions only to harems containing non-pregnant mares and older foals, who are less likely to be attacked, should help. Alternatively, mares pregnant by other stallions could be isolated prior to foaling, although this disrupts the normal social structure of the herd. Zoos must strike a balance between the need to rotate stallions frequently in order to promote gene flow in inbred populations and the increased risk of infanticide that this practice brings.
Dispersal As the juveniles near the end of their second year, Przewalski's stallions seem to be more tolerant of their sons than daughters. Hutson (1975) reported that the 22-month-old Przewalski's filly "Hayley" was the recipient of much aggression from her father "Basil" who chased and bit her so much that she had raw patches on her rump. Fillies are ready to breed at 2 years of age. By becoming more aggressive toward pubertal daughters, parents probably encourage their dispersal from the herd, avoiding consanguineous matings.
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In support of this hypothesis, Kaseda et al. (1984) found that feral Misaki horse fillies whose mothers belonged to harems dispersed earlier than fillies whose mothers were unaffiliated with a harem. As familiar fathers and daughters rarely mate (Berger and Cunningham, 1987 ), the fillies would be wasting breeding opportunities by remaining in natal harems. Feral fillies on Shackleford Island were driven out by the adult mares at 2 and 3 years of age, and Rubenstein ( 1982 ) hypothesized that they competed with their dams for limited forage. Sons do not begin to breed before the age of 4 years (Rubenstein, 1982; Kaseda et al., 1984; Berger, 1986) so they are not missing breeding opportunities by staying in the natal band, nor are they likely to take mating opportunities away from their father. By tolerating sons, parents permit them to postpone having to fend for themselves. Rubenstein ( 1982 ) found that at ~ 4 years of age, juvenile males on Shackleford Island left their natal bands of their own accord rather than being driven out. Thus, juvenile Przewalski's horses of both sexes can be left in their natal bands until at least 18 months of age. The juveniles probably gain social skills through continued association with adults and do not need to be removed until parental aggression becomes evident. Fillies are likely to become the first targets of this aggression. SUMMARY
In summary, what is known about the behavior of Przewalski's and domestic horses suggests that duplicating as closely as possible the processes which occur in the wild would eliminate most of the aberrant behavior seen in captive horses and would improve reproductive success. Understanding the causes of natural but undesirable behavior, such as infanticide, is helpful in planning for its prevention. ACKNOWLEDGMENTS
The author would like to thank the Catskill Game Farm, the Denver Zoological Gardens, the Los Angeles Zoo, the Memphis Zoological Gardens, the National Zoological Park's Conservation and Research Center, the New York Zoological Park, the San Diego Zoo, the San Diego Wild Animal Park, and the Topeka Zoological Park for facilitating the observation of their Przewalski's horses. C.E. Houpt developed the time budget and social interaction computer programs. K.A. Houpt and J.R. Sauer provided much advice. Various aspects of this research were supported by the Nixon-Griffis Fund for Zoological Research, a Smithsonian Graduate Student Fellowship, and Sigma Xi Grants in Aid of Research.
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REFERENCES Altmann, J., 1974. Observational study of behavior: sampling methods. Behaviour, 49: 227267. Beaver, B.V. and Amoss, Jr., M.S., 1982. Aggressive behavior associated with naturally elevated serum testosterone in mares. Appl. Anim. Ethol., 8: 425-428. Benirschke, K,, Malouf, N., Low, R,J. and Heck, H., 1965. Chromosome complement: differences between Equus caballus and Equusprzewalskii, Poliakoff. Science, 148: 382-383. Berger, J., 1986. Wild Horses of the Great Basin. University of Chicago Press, Chicago, IL, 326 Pp. Berger, J. and Cunningham, C., 1987. Influence of familiarity on frequency of inbreeding in wild horses. Evolution, 41 ( 1 ): 229-231. Berger, J. and Rudman, R., 1985. Predation and interactions between coyotes and feral horse foals. J. Mammal., 66(2): 401-402. Bouman, J., 1982. Some facets of the enclosures in which Przewalski horses arc kept. In: J. Bouman, I. Bouman and A. Groeneveld (Editors), Breeding Przewalski Horses in Captivity for Release into the Wild. Foundation for the Preservation and Protection of the Przewalski Horse, Rotterdam, pp. 161-175. Bouman-Heinsdijk, I., 1982. Is re-introduction of Przewalski horses into the wild a realistic idea? In: J. Bouman, I. Bouman and A. Groeneveld (Editors), Breeding Przewalski Horses in Captivity for Release into the Wild. Foundation for the Preservation and Protection of the Przewalski Horse, Rotterdam, pp. 177-220. Boyd, L.E., 1980, The natality, foal survivorship, and mare-foal behavior of feral horses in Wyoming's Red Desert. M.S. Thesis, University of Wyoming, Laramie, WY, 137 pp. Boyd, L., 1986. Behavior problems of equids in zoos. In: S.L. Crowell-Davis and K.A. Houpt (Editors), Behavior. The Veterinary Clinics of North America: Equine Practice 2 (3). W.B. Saunders, Philadelphia, PA, pp. 653-664. Boyd, L.E., 1988a. Ontogeny of behavior in Przewalski horses. Appl. Anim. Behav. Sci., 21:4169. Boyd, L.E., 1988b. Time budgets of adult Przewalski horses: effects of sex, reproductive status, and enclosure. Appl. Anim. Behav, Sci., 21 : 19-39. Boyd, L. and Kasman, L., 1986. The marking behavior of male Przewalski's horses. In: D. Duvail, D. Muller-Schwarze and R.M. Silverstein (Editors), Chemical Signals in Vertebrates IV. Plenum, New York, pp. 623-626. Boyd, L.E., Carbonaro, D.A, and Houpt, K.A., 1988. The twenty-four hour time budget of Przewalski horses. Appl. Anita. Behav. Sci., 21: 5-17. Crowell-Davis, S.L., 1983. The behavior of Welsh pony foals and mares. Ph.D. Thesis, Cornell University, Ithaca, NY, 287 pp. Dobroruka, L.J., 1961. Eine Verhaltensstudie des Przewalski-Urwildpferdes (Equusprzewalskii Poliakov 1881 ) in dem Zoologischen Garten Prag. Equus, 1 ( 1 ): 89-104. Dolan, J.M., 1977. The saiga, Satga tatarica, a review as a model for the management of endangered species. Int. Zoo. Yearb., 17: 25-32. Duncan, P., 1980. Time-budgets of Camargue horses. II. Time budgets of adult horses and weaned sub-adults. Behaviour, 72: 26-49. Duncan, P., 1982. Foal killing by stallions. Appl. Anita. Ethol., 8: 567-570. Feh, C., 1988. Social behaviour and relationships of Przewalski horses in Dutch semi-reserves. Appl. Anita. Behav. Sci., 21: 71-87. Feist, J.D. and McCullough, D.R., 1975. Reproduction in feral horses. J. Reprod. Fertil., Suppl., 23: 13-18. Feist, J.D. and McCullough, D.R., 1976. Behavior patterns and communication in feral horses. Z. Tierpsychol., 41 : 337-371.
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Food and Agriculture Organization, 1985. Recommendations by FAO/UNEP Expert Consultation on Restoration of Przewalski Horse to Mongolia, 29-31 May 1985, Moscow. FAO of the United Nations, Rome, 17 pp. Gruenerwald, W., 1982. A Computer Analysis of Studbook Data for the Przewalski Horse (Equus przewalskii). Canyon Colorado Equid Sanctuary, Wagon Mound, NM, 432 pp. Henneke, D.R., Potter, G.D. and Kreider, J.L., 1984. Body condition during pregnancy and lactation and reproductive efficiency of mares. Theriogenology, 21: 897-909. Hutson, A.D., 1975. Aspects of the Social Behaviour of Przewalski's Horse. Department of Biological Science, Portsmouth Polytechnic, Portsmouth, 57 pp. (unpubl.). Kaseda, Y., 1981. The structure of the groups of Misaki horses in Toi Cape. Jpn. J. Zootech. Sci., 52: 227-235. Kaseda, Y., Nozawa, K. and Mogi, K., 1984. Separation and independence of offsprings from the harem groups in Misaki horses. Jpn. J. Zootech. Sci., 55 ( 11 ): 852-857. Keiper, R.K., 1985. The Assateague Ponies. Tidewater, Centreville, MD, 101 pp. Keverling Buisman, A.K. and van Weeren, R., 1982. Breeding and management of Przewalski's horses in captivity. In: J. Bouman, I. Bouman and A. Groeneveld (Editors), Breeding Przewalski Horses in Captivity for Release into the Wild. Foundation for the Preservation and Protection of the Przewalski Horse, Rotterdam, pp. 77-160. Klimov, V.V., 1988. Spatial-ethological organization of the herd of Przewalski horses (Equus przewalskii) in Askania-Nova. Appl. Anim. Behav. Sci., 21: 99-115. Knowles, J.H., 1980. History of the Marwell Zoological Park's herd of Przewalski horses. Equus, 2: 73-78. Kolter, L., 1985. Soziale Beziehungen zwischen den Przewalskipferden (Equus p. przewalskii) im Kflner Zoo. Z. K61n. Zoo, 4(28): 193-201. Kolter, L. and Zimmermann, W., 1988. Social behaviour of the Przewalski horses (Equus p. przewalskii) in the Cologne Zoo and its consequences for management and housing. Appl. Anim. Behav. Sci., 21:117-145. Lewtas, P.N., 1973. Aspects of Social Behaviour of Mongolian Wild Horses. Department of Biological Sciences, Portsmouth Polytechnic, Portsmouth, 42 pp. (unpubl.). Mackler, S.F. and Dolan, J.M., 1980. Social structure and herd behavior ofEquus przewalskii Poliakov, 1881 at the San Diego Wild Animal Park. Equus, 2( 1): 55-69. Miller, R., 1980. The ecology of feral horses in Wyoming's Red Desert. Ph.D. Thesis, University of Wyoming, Laramie, WY, 109 pp. Mohr, E., 1971. The Asiatic Wild Horse. J.A. Allen, London, 124 pp. National Research Council, 1978. Nutrient Requirements of Horses. National Academy of Sciences, Washington, DC, 33 pp. Pelligrini, S.W., 1971. Home range, territoriality, and movement patterns of wild horses in the Wassuk Range of Western Nevada. M.S. Thesis, University of Nevada, Reno, NV, 39 pp. Pollock, J.L., 1980. Behavioural Ecology and Body Condition Changes in New Forest Ponies. RSPCA, Horsham, 119 pp. Rossdale, P.D. and Ricketts, S.W., 1974. The Practice of Equine Stud Medicine. Bailli~re Tindall, London, 421 pp. Rubenstein, D.I., 1981. Behavioural ecology of island feral horses. Equine Vet. J., 13: 27-34. Rubenstein, D.I., 1982. Reproductive value and behavioral strategies: coming of age in monkeys and horses. In: P.P.G. Bateson and P.H. Klopfer (Editors), Perspectives in Ethology, Vol. 5. Plenum, New York, pp. 469-487. Ryder, O.A. and Massena, R., 1988. A case of male infanticide in Equus przewalskii. Appl. Anim. Behav. Sci., 21: 187-190. Ryder, O.A. and Wedemeyer, E.A., 1982. A cooperative breeding program for the Mongolian wild horse Equus przewalskii in the United States. Biol. Conserv., 22:259-271.
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Ryder, O.A., Epel, N.C. and Benirschke, K., 1978. Chromosome banding studies of the Equidae. Cytogenet. Cell Genet., 20: 323-350. Ryder, O.A., Bowling, A.T., Brisbin, P.C., Carroll, P.M., Gadi, I.K., Hansen, S.K. and Wedemeyer, E.A., 1984. Genetics of Equus przewalskii Poliakov, 1881: analysis of genetic variability in breeding lines, comparison of equid DNAs, and a brief description of a cooperative breeding program in North America. Equus, 2 (2): 207-227. Salter, R.E., 1978. Ecology of feral horses in Western Alberta. M.S. Thesis, University of Alberta, Edmonton, Alta., 233 pp. Schein, M.W. and Fohrman, M.H., 1955. Social dominance relationships in a herd of dairy cattle. Br. J. Anita. Behav., 3: 45-55. Skiff, E.M., 1982. The effect of enclosure design on social interactions and daily activity patterns of the captive Asiatic wild horse (Equusprzewalskii). M.S. Thesis, University of Minnesota, St. Paul, MN, 31 pp. Species Survival Commission of the International Union for Conservation of Nature and Natural Resources, 1981, Guidelines for the development of a captive management and reintroduction plan for Equus przewalskii. Draft of a report by the Przewalski horse committee of the IUCN Survival Service Commission Captive Breeding Specialist Group, 52 pp. ( unpubl. ). Tilson, R.L., Sweeney, K.A., Binczik, G.A. and Reindl, N.J., 1988. Buddies and bullies: social structure of a bachelor group of Przewalski horses. Appl. Anita. Behav. Sci., 21:169-185. Tyler, S.J., 1972. The behaviour and social organization of the New Forest ponies. Anim. Behav. Monogr., 5(2): 85-196. Volf, J., 1988. Pedigree Book of the Przewalski Horse. Zoological Garden, Prague, pp. 12071312. Waring, G.H., 1983. Horse Behavior: the Behavioral Traits and Adaptations of Domestic and Wild Horses, Including Ponies. Noyes, Park Ridge, N J, 292 pp. Welsh, D.A., 1975. Population, behavioral and grazing ecology of the horses of Sable Island. Ph.D. Thesis, Dalhousie University, Halifax, N.S., 403 pp. Zimmermann, W., 1985.20 Jahre Przewalskipferde (Equus p. przewalskii) im K61ner Zoo. Z. K61n. Zoo, 4: 171-187.
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The behavior of Przewalski's horses and its importance to their management Lee E. Boyd Department of Biology, Washburn University, Topeka, KS 6662• (U.S.A.)
ABSTRACT Boyd, L.E., 1991. The behavior of Przewalski's horses and its importance to their management. App[. Anita. Behav. Sci., 29: 301-318. Przewalski's horses (Equus przewalskii) are believed to be extinct in the wild; the current known population of 797 animals exists wholly in zoos. The Species Survival Commission of the International Union for Conservation of Nature and Natural Resources is proposing to reintroduce this endangered species into its former Mongolian habitat within the next decade. Knowledge of the behavior of harem-forming equids in general and of Przewalski's horses in particular, is of great importance to the captive propagation and eventual reintroduction of this species. Horses are rarely solitary by nature. Solitary captive animals are prone to pacing. Juvenile male feral horses (Equus caballus) form bachelor herds upon dispersal from their natal band. Zoos can set up bachelor herds as a way of managing surplus males. The older, more dominant feral horse bachelors are the first to acquire mares. Bachelors do not generally obtain females until they are 4 or 5 years of age. The first females acquired are usually 1- and 2-year-old fillies dispersing from their natal band. Because of the age differential, the stallions are generally dominant to their mares. Behavioral impotence may result if captive stallions are given a harem at too young an age, especially if the harem contains older, more dominant, females. Typical harem sizes in the wild are 3-5 mares. Captive stallions with too large a harem may become either apathetic or aggressive toward their mares. Wild horses spend 60-70% of their time foraging. Captive animals may quickly consume their limited amounts of food and develop vices out of boredom. Provision of hay ad libitum reduces the amount of pacing seen in captive animals, and virtually eliminates coprophagy.
INTRODUCTION
A knowledge of the behavior of a species is essential to its management. The Przewalski's horse, Equus przewalskii, is an endangered species presently believed to be extinct in the wild and now found only in zoos. The Food and Agriculture Organization of the United Nations ( 1985 ) is drawing up plans to reintroduce Przewalski's horses into their former Mongolian habitat, and this project will be documented as a model for the reintroduction of other species. Przewalski's horses are the closest living relative of domestic and feral horses, Equus caballus (Benirschke et al., 1965; Ryder et al., 1978). Much 0168-1591/91/$03.50
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information exists about the behavior of domestic and feral horses (reviewed by Waring, 1983 ). However, there is a dearth of behavioral information about Przewalski's horses, and this deficiency has long been recognized (Species Survival Commission of the International Union for Conservation of Nature and Natural Resources, 1981; Ryder and Wedemeyer, 1982; Ryder et al., 1984). The purpose of this study was to document the behavior of captive Przewalski's horses. Much of the existing behavioral information is anecdotal. Studies in which data were quantified have been of short duration (usually < 1 m o n t h ) and have taken place at one, or at most two, locations. The majority of this work is unpublished, or has been presented in publications which are difficult to obtain. Unfortunately we will never know what natural Przewalski's horse behavior was like because little behavioral information was recorded by those who observed the horses in the wild before their extinction. The current population of Przewalski's horses has been in captivity for 2-12 generations (Volf, 1988 ). Thus, we cannot be sure whether any behavioral similarities between Przewalski's and domestic horses exist because of their close evolutionary relationship, or whether similarities exist because of captivity which both species have experienced. Nevertheless, the study of Przewalski's horse behavior is important if this species is to be reintroduced into the wild. If captive Przewalski's horse behavior is similar to captive domestic horse behavior, then one would predict that they would behave like feral horses when released. Much is known about feral horse behavior (Pelligrini, 1971; Welsh, 1975; Feist and McCullough, 1976; Salter, 1978; Boyd, 1980; Miller, 1980; Kaseda, 1981; Rubenstein, 1981; Keiper, 1985; Berger, 1986 ) and this knowledge could then be used as a basis for managing Przewalski's horses after their reintroduction. Other than their restricted roaming abilities, the conditions of captive Przewalski's horses differ from those of free-ranging horses in four main ways: ( 1 ) some zoos do not provide food ad libitum, and water and food may be concentrated in one area, eliminating search time in foraging; (2) the horses come into more contact with humans than most free-ranging animals; (3) there is no male competition for females; (4) because humans decide what individuals will be grouped together, there is diminished mate choice, although some still occurs when stallions reject certain mares, or vice versa. It should be noted that points (2) to (4) above are also true to a lesser degree for some free-ranging equid populations where human intervention is frequent or where there are heavily female-biased sex ratios (Tyler, 1972; Duncan, 1980; Pollock, 1980; Keiper, 1985). ANIMALS, MATERIALS A N D M E T H O D S
This study took place from 1982 to 1987. I visited every zoo in the United States that possessed Przewalski's horses during these years, with the excep-
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tion of the Minnesota Zoo whose horses had been studied by Skiff (1982). To collect data, I modified a protocol developed by Crowell-Davis (1983). Focal animal observations of 15 min duration were conducted on each herd member in order to develop time budgets (Altmann, 1974). The next horse to be observed was chosen randomly without replacement until all herd members had been observed once. The process was then repeated. During the focal animal sample, the time of onset and end of 10 mutually exclusive behavior states were recorded to the nearest 3 s. These behavioral states were feeding (time spent eating feces was noted as a subset), nursing, drinking, standing awake, standing resting, locomotion (time spent pacing the fence was noted as a subset ), play, self-grooming, mutual grooming, recumbency. If ever these behavioral states were of < 3 s duration, they were not recorded. The mean number of behavioral states exhibited by the individual per hour was calculated. Behaviors which were typically of short duration were treated as events. These included defecation, urination, marking, flehmen, masturbation, aggression, snapping, and vocalization. For interactive behaviors such as aggression, mutual grooming, mutual play, and nursing, the participants and the contexts of the behavior were noted. At 5 min intervals during the observation, the location and nearest neighbor of the focal animal was recorded. Dominance hierarchies were determined for 10 herds using Schein and Fohrman's ( 1955 ) method, in which animals are ranked so that each individual receives threats from the smallest number of individuals below it. Grain sharing was studied in the Topeka herd. Horses were fed in rubber pans from which 2-3 horses could share. As there were five pans and 5-10 horses, sharing frequently occurred. Opportunistic notes were made of which horse was sharing with w h o m from 1982 to 1985 and were used only to determine the pattern of grain sharing by the stallion "Rolmar". In 1986 "Rolmar" was replaced by his paternal half brother, "Roccol", and in 1987 data on sharing were collected by a series of point samples at 1 min intervals whenever grain was provided. A total of 1332 h was spent watching horses, and 1020.25 h of focal animal data were gathered from a total of 92 individuals in 29 enclosures at nine zoos. These individuals represent approximately 85% of the United States population and 15% of the world population during this period (Volf, 1988 ). At each zoo, background information about enclosure size and diet was collected and supplemented by information from a questionnaire which eight zoos filled out and returned. Detailed data have been presented elsewhere (Boyd, 1986, 1988a,b; Boyd and Kasman, 1986; Boyd et al., 1988), so the following will be an overview. Management implications will be discussed in the light of these findings.
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RESULTS A N D D I S C U S S I O N
Time budgets Three or more hours of focal time budget data were collected from 15 males and 27 females in 13 enclosures at seven zoos. The stallions averaged 66 _+4 behavioral states h-~ while the females averaged only 55 __+3 (Boyd, 1988b). The stallions also vocalized 2.5 -+ 0.3 times h - ~which was significantly higher than the mares' rate of 1.4 + 0.1 times h-1 (Mann-Whitney U test ( M W U ) , P < 0.01 ). Stallions spent 14.2 + 2.0% of their time in motion which was significantly higher than the 8.5 _+0.5% of time that the mares spent in locomotion (MWU, P < 0.01 ). The difference in the amount of time spent in motion was also true for the special case of pacing the fence. Only 2 of the mares were observed pacing, as opposed to two-thirds of the stallions. Both solitary stallions paced, 5 of 8 bachelors paced, and 3 of 8 harem stallions paced. One of the harem males paced for ~ 1 h day-z in temperature of 30°C, causing profuse sweating (Boyd, 1986). The tendency of males to be more active than females, showing frequent changes of behavioral state, may be due to the fact that stallions are free of the nutritional demands of pregnancy and lactation whereas mares spend much time grazing. It may also be a legacy of the role of free-ranging stallions. In the wild, stallions must be active and vigilant in obtaining mares and in defending their harems against competitors. This tendency to be more active makes stallions more prone to pacing in captivity, a habit which might be life-threatening in hot weather. Solitary males spent more time pacing (7.1_+2.8% of their time budget) than bachelors (6.1 _+3.3%) or harem males (0.9 _+0.7%). Pasturing the horses or providing hay ad libitum virtually eliminated pacing for bachelors and for harem males and reduced the amount of time solitary males spent pacing to 4.9_+ 0.7% (Boyd, 1988b). Providing companions may reduce the amount of pacing shown by an individual. One harem stallion did not pace when hay was present, but, as a solitary male, he spent 6% of his time pacing even when hay was present. When some form of food was available, adult horses rarely showed coprophagy.
Enclosure effects Skiff (1982) studied the effects of enclosure size on the behavior of 2 herds of Przewalski's horses. The number of activities was higher in the 0.05 ha enclosure than in the 3.5 ha enclosure. When in the small enclosure, horses did more milling and fence-pacing, had higher rates of aggression, and spent more time mutually grooming than when on pasture. Coprophagy and wood chewing was seen in the small enclosure but never on pasture. Przewalski's horses isolated in small enclosures may also begin cribbing, a behavior difficult to extinguish even when the horse is moved to a pasture (Bouman-Heins-
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dijk, 1982; Boyd, 1986). In the present study, there was no significant effect of enclosure size on aggressiveness or on the amount of time spent in mutual grooming or in locomotion (Kruskal-Wallis ANOVA, P > 0 . 0 5 ) . The main effects of enclosure size were that horses in enclosures < 0.4 ha in size tended to exhibit more behavioral states per hour and spent significantly less time standing resting than animals on pasture (Boyd, 1988b ). Horses in small enclosures were in metropolitan areas, whereas those in large enclosures were not. It is possible that the noises in zoos or of the cities in which they are located, disrupt the diurnal resting behavior of the horses. A summertime 24-h time budget was compiled for 8 horses at the National Zoo's Conservation and Research Center. These horses were in a grassy 12 ha pasture. The center's staff had expressed concern that despite the spacious enclosure, the horses were only observed to use the area immediately adjacent to the barn. Over 200 h of around-the-clock focal observations indicated that the horses were making use of the entire area. They met the keepers at the barn in the late morning, when pelleted feed was provided. The horses lingered in this area during the hot hours of the day but moved out to graze by 20:00 h when the staff had gone home. The horses then spent the evening grazing and recumbent away from the barn. Grazing peaked between 20:00 and 24:00 h when the horses spent 68.2% of their time in this activity. Recumbency peaked at 21.4% of the time budget between the hours of 00:00 and 04:00 (Boyd et al., 1988). Thus, the horses were making use of their entire enclosure and were taking advantage of the forage present but were waiting until evening to do so. Grazing during s u m m e r evenings may have minimized thermal stress, and at night there was no reason to linger near the barn as no visits by staff were expected.
Dominance hierarchies The dominance hierarchies of Przewalski's horses appear to be remarkably stable over time. In the 4 years that the Topeka herd was observed, only one change in rank occurred. The exhibit herd at the San Diego Wild Animal Park was studied by Mackler and Dolan (1980) in 1975 and 1976. The relative ranks of the 4 mares still present in 1984 were the same except that 1 mare reintroduced to the herd during Mackler and Dolan's study had moved up one position from the b o t t o m of the hierarchy. Przewalski's horses have been reported to direct most of their aggression toward horses just below them in rank (Hutson, 1975; Mackler and Dolan, 1980 ). Subordinate mares are the most aggressive toward new herd members (Mackler and Dolan, 1980). In this study, 55% of the mares directed the majority of their aggression toward the horse just below them in rank (or 63%, if only the mares who were not at the bottom of their respective hierarchies are considered). For the 2 cases in which new mares were introduced to
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the harem during the course of the study, the lowest ranking adult female showed the most aggression toward the newcomers. Foals were always the lowest ranking animals in the herd but were protected when near their mother and so initiated and received little aggression. Age is important in determining position in the dominance hierarchy (Lewtas, 1973; Kolter, 1985; Feh, 1988; KJimov, 1988; Kolter and Zimmermann, 1988). Age was positively correlated with dominance order for the adults in the 10 herds studied and significantly correlated with rank in 4 of the 6 herds large enough to permit statistical testing. Following age in importance was body size. Aggressiveness was positively correlated with dominance in all but 1 herd. Horses ranking high in the hierarchy initiated more aggressive acts per hour. Factors of lesser importance were length of residence in the herd and maternal rank. Size and aggressiveness are probably initially important factors when a new herd is formed. In herds where core membership is stable over time, as is the case for most Przewalski's herds in captivity, age becomes the most important factor. Juveniles are subordinate to adults, and, if they remain in their natal band, they will have to fight their way up the ranks against individuals who have been dominant to them for at least 2 years. Maternal rank may be important if the foals inherit traits such as large size that are advantageous in competition. Foals of dominant mothers may learn confidence in their dealings with other herd members, but, if they stay in their natal band, they must still fight their way past all other mares including their mother to achieve top rank. Thus, maternal rank is not as good a predictor of offspring rank as is age relative to other herd members. Horses with longer tenure in the herd are likely to be older and have the confidence of their surroundings and their herd alliances. Nevertheless, this may not always permit them to hold their rank upon the arrival of larger, more aggressive horses. To examine what benefits might accrue to dominant animals, percent time spent feeding by 36 mares was correlated with dominance rank. The correlation was negative and significant, indicating that mares ranked Number 1 or 2 in the herd spent more time feeding (rs = - 0 . 2 9 , P < 0.05 ). The correlation between dominance rank and the amount of time spent feeding was apparently not a result of dominant mares being larger and, therefore, requiring more food. The correlation between size and amount of time spent feeding was not significant ( r s = - 0 . 0 9 , N = 2 5 , P > 0 . 0 5 ) . Additionally, mares have been observed to interfere with the stallion when he attempts to breed mares that are subordinate to them (Dolan, 1977; Mackler and Dolan, 1980; Zimmermann, 1985; Boyd, 1986). In two cases, the interfering mare had to be removed from the herd until the subordinate mares conceived. Adult Przewalski's stallions are generally dominant to mares (Dobroruka, 1961 ; Lewtas, 1973; Hutson, 1975; Klimov, 1988 ), however, young stallions may be subordinate to older mares. In 2 of the herds studied, the stallions,
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aged 3 and 7, were subordinate to some mares, and in a third herd the stallion, aged 7, was never observed to interact aggressively with any herd member so his rank could not be ascertained. All 3 of these stallions were younger than some of the mares in their herds. One of the subordinate stallions sired 3 foals in 3 years. The other, and the one whose rank could not be determined, have never sired any foals. The stallion who sired 3 foals was placed with a harem of 6 mares as a 2-year-old and was subordinate to those mares who were older than he. As a 3-year-old, this stallion impregnated a younger mare to whom he was dominant. The following year he sired 2 foals by 2 other younger mares to whom he was dominant. The older females remained dominant to him and were not impregnated. The stallions in the other 5 harems studied were the dominant animal in their herd. Four of these 5 dominant stallions, ranging in age from 4 to 21 years, had sired more than 1 foal in the past 3 years. Three of the dominant stallions who successfully reproduced were also the oldest members of their harem. The dominant stallion who did not reproduce had previously been subordinate to both of his mares. They kicked him whenever he attempted to mount and he failed to intromit. The mares were sent to another zoo to be bred, and, when they returned 1 year later, the 8-year-old stallion was now dominant to them. He became overly aggressive toward the mares, biting them severely enough to leave scars. The stallion attained an erection and mounted the mares but still failed to intromit. He was then sent to another zoo where he was solitary for 2 years. In an attempt to rehabilitate him, he was then given 2 mares. One was the same age as himself ( 11 years) and one was 2 years old. He was dominant to both mares. This small harem was then moved to a pasture adjacent to a larger harem so that the impotent stallion would have sensory, but not physical, contact with the successfully breeding stallion in the nearby enclosure. After several months the successful stallion was retired. Then the impotent stallion was allowed to merge his small harem with the 3 adult mares and their offspring in the adjoining pasture, all of whom were younger than he was. The impotent stallion remained on pasture with these mares for 2 years until his death at age 13. He was not overly aggressive toward the adult mares, although he was aggressive toward the 2- and 3-yearold pubertal fillies. He attained an erection and mounted the mares, who tolerated his attempts, but he was never observed to intromit and sired no foals. Feral horse bachelors do not have an opportunity to breed until the age of 4 or 5 years, and their first mares are usually 1- and 2-year-old fillies dispersing from their natal bands (Boyd, 1980; Keiper, 1985; Berger, 1986). Bachelors are usually 6-8 years of age before they are successful at obtaining mares through combat (Berger, 1986). The most dominant bachelors are the first to obtain mares (Feist and McCullough, 1976; Rubenstein, 1982; Keiper, 1985 ). Therefore, in the wild, harem stallions would usually be older and more dominant than their mares. As horses are harem polygynous and not all
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males are able to acquire mares, it would seem that only the most dominant males would breed and that a female would prefer to breed with a dominant male who might pass genetic factors contributing to his status to her sons. Accordingly, adult Przewalski's mares have previously been reported to reject immature stallions (Bouman-Heindsdijk, 1982; Kolter, 1985; Klimov, 1988 ). In the herds of Przewalski's horses studied, subordinate harem stallions were always younger than some of their mares and had lower reproductive success than alpha-ranking stallions. In addition to these herds, three others had harem stallions who were not the oldest members of their herd and who sired at least 1 foal. However, in another four cases such stallions sired no foals. In contrast, no cases were found in which a male initially placed with younger females failed to sire foals. As age and dominance are correlated, and as dominance affects reproductive success, zoos need to place inexperienced stallions with younger mares, at least until the stallion gains age and experience.
Relationships among females Mares in a harem form close, stable associations that persist even in the absence of stallions (Kaseda, 1981; Keiper, 1985; L.E. Boyd, personal observations of 3 herds of Przewalski's mares without a stallion present, 1986, 1988 ). It is not unusual for mares in the same harem to be relatives (mother and daughters, full or half sisters), which probably contributes to the close ties between harem females. Closely related mares preferred one another as mutual grooming partners. As mares form these long-term bonds, it seems logical that the present practice of rotating stallions from harem to harem to reduce inbreeding more closely approximates the natural social processes than would a program that traded particular mares.
Bachelor herds and harem composition As of 1980, 75 zoos worldwide maintained Przewalski's horses in a total of 97 enclosures (Bouman, 1982 ). Thirty-two enclosures housed males only. The males in 26 of these enclosures were solitary, and therefore, likely to pace. Horses are gregarious; in the wild, stallions which do not possess harems are rarely solitary but form bachelor groups instead. In captivity, this process can be mimicked. Males not currently being used for breeding can be grouped to form bachelor herds. This reduces the number of enclosures needed and makes the stallions less likely to pace. Serious aggression is not a problem with juvenile males; forming herds containing older experienced males requires more care, however. By setting up herds in areas large enough (0.4-0.8 ha) to allow subordinates to flee and take cover, Tilson et al. (1988) were able to form bachelor groups containing males ranging in age from 1 to 9 years. Including several juveniles and allowing them some time together before the introduc-
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tion of the older males permits the juveniles to form coalitions and lessens the chance that any one of them will receive the full brunt of aggression. Wild stallions seldom have harems of more than 5 or 6 females. When the harem of the Przewalski's stallion "Basil" grew to 18 members, he became apathetic about breeding (Knowles, 1980). Isolating him and reducing the harem size prior to his return restored his libido. The stallion "Bars" became aggressive toward his mares when the harem size reached 13 animals (Keverling Buisman and van Weeren, 1982 ). The plans for reintroduction of Przewalski's horses into Mongolia specify that 1- and 2-year-old fillies are to be kept together in a holding area until 3 years of age to promote social bonding (Species Survival Commission of the International Union for Conservation of Nature and Natural Resources, 1981 ), which seems reasonable. A 3-year-old stallion will be added to each harem before it is released into the wild, which, in light of the age at which feral horse stallions acquire harems, seems a little too young. Current plans call for the stallions to be approximately the same age, so that one does not monopolize all the mares. In my opinion, the critical factor affecting harem size will be the male:female sex ratio rather than male age. Even if all the stallions are the same age, a few will be dominant and monopolize all the mares unless they have competition. And no I 0-year-old stallion in the prime of life can hold more than 4 or 5 mares if he must constantly fight off 5 or more bachelors. I would r e c o m m e n d a 1 : 1 sex ratio for release, especially as males are the more expendable sex.
Masturbation All stallions, whether solitary, in bachelor groups, or in harems, were observed to masturbate by flipping the penis against the belly. As stallions masturbated throughout the breeding season and still sired foals, this behavior does not appear to reduce libido, as has been suggested for domestic horses (Waring, 1983 ). Therefore, masturbation should not be considered a problem.
Lactation The gestation length of Przewalski's horses is 11 months and foals are usually born in the late spring or early s u m m e r (Dobroruka, 1961; Mohr, 1971 ). Mares come into estrus within 1 week of foaling. The mares in this study who conceived promptly weaned their foals just prior to the birth of their next offspring. Mares failing to conceive during that breeding season, permitted their current foals to nurse much longer. Three mares whose 2-year-old daughters remained in their natal band failed to conceive during the 2 years following the birth of their daughters. Each of these mares nursed their daughters through 26 months of age. Mares continue investment in lactation for
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their older offspring until they have another offspring in utero which begins to make significant demands on their resources. The timing of weaning of older foals can, thus, be used as a non-invasive diagnosis for pregnancy, with the exception of mares on a high plane of nutrition. Mares at two zoos received 17% more digestible energy than horses at the zoo feeding the next highest energy level, and nursed both their yearlings and their latest foals. Klimov (1988) has also reported that mares may allow older offspring to continue to nurse along with their newborn foals. The high level of nutrition apparently permitted the mares to invest in more than 1 offspring simultaneously.
Grain sharing The horses at the Topeka Zoo's Forbes Conservation/Propagation Center were expected to spend 10% of their time sharing grain with each other if they shared grain randomly, and had as much chance of eating alone as with any other horse. However, random sharing was not observed. The mares' most frequent partners at a pan of grain were their offspring. Mares spent 16 + 14% of their time feeding alone, 7 + 1% sharing with one of the 4 other adults, and 77 _+ 13% of the time sharing with foals; 96.5 + 0.5% of the time these foals were their own offspring. The sire of the foals spent twice as much time sharing grain with his offspring than did the stallion who replaced him the following year with the unrelated foals born after his arrival (Fig. 1 ). The sire of the foals spent 35.8% of the time feeding by himself (expected= 16.7%), 18.0% of the time sharing grain with adult mares (expected = 50.1%), and 46.2% of the time sharing with his offspring (expected= 33.2%). The unrelated stallion spent only 24.4% of his feeding time sharing with foals (expected= 50.0%).
Paternal behavior In the wild, sires probably played an important role in protecting their offspring against predators and in retrieving foals when they strayed from the herd, as has been observed in feral horses (Feist and McCullough, 1975; Boyd, 1980; Berger and Rudman, 1985; Berger, 1986 ). In captivity, sires rarely have occasion to provide this form of parental investment. When the number of feeding stations are limited, sires do appear to aid their offspring by sharing grain with them. Mackler and Dolan (1980) also observed a Przewalski's stallion sharing food with his son but chasing adult mares away. Stallions also tolerate foals grazing in their vicinity (Dobroruka, 1961 ). Sires may take on part of the role of the missing dam for orphaned offspring. One foal's mother died when the foal was 2.5 months of age. The sire permitted the orphan to suckle his sheath over a period of several weeks until
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Fig. 1. Grain sharing. (A) A stallion (left) and mare (right) contesting a pan of grain at the Topeka Zoo's Forbes Conservation/Propagation Center. (B) The stallion was more likely to share grain with his offspring than with adult mares.
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this nursing behavior ended. The stallion was more likely to be this foal's nearest neighbor than expected under random association, and the orphan was twice as likely to be within 1 m of the stallion than mothered foals in 67% of the months after his dam died (Boyd, 1988a). When the orphaned foal was between 3 and 6 months of age, his sire was observed to lean over him 4 times while feeding from a hayrack to bite a yearling daughter. Twice the sire reached over the orphan to bit adult mares. In one case the orphan pushed in between his sire and an adult mare at the hayrack, bumping his sire. His sire reached over him and bit the mare. In the other case, the orphan walked under his sire's neck to get to the rack, interrupting his sire's feeding. The sire leaned over him and bit a mare. Three times the stallion threatened a mare away from the hay but permitted the orphan to stay next to him. Once he approached the orphan when it was sharing grain with a yearling paternal half sibling. The sire supplanted his yearling but allowed the orphan to stay and share grain with him. The stallion also threatened 1 of the adult mares twice when she approached his pile of grain but let the orphan share. The orphaned foal bumped into him, but, instead of attacking the foal, the sire threatened to kick the mare, who was still near. The stallion was dominant to all members of the herd, so he could have threatened the foals. Instead he seemed to avoid aggression against them and redirected his aggression toward older herd members. In this study, colts spent as much time mutual grooming with stallions as expected, while fillies mutual groomed with sires much less than expected. Foals depended primarily on other foals as play partners, but 2 colts played frequently with their sire. "Basil", the father of I of the colts, had been reported by Lewtas (1973) and Hutson (1975) to be very tolerant of play directed at him by some of his other sons. Play with sons may be important in helping them develop fighting skills essential in competition to obtain their own harems (Berger, 1986).
Breeding interference by juveniles Yearlings and 2-year-olds sometimes interfered when the stallion attempted to breed their mother. This interference consisted of the juvenile approaching the courting pair and putting itself between them, often kicking the stallion. This behavior was not observed to create a problem, as the stallion tolerated the juveniles' interference. While the behavior was proximately effective in that it always distracted the stallion from continuing courtship, ultimately it had no effect on conception rate. In every case, the mares became pregnant during that breeding season.
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Stallion-like behavior by mares Two mares exhibited stallion-like behavior such as mounting other herd members (Boyd, 1986 ). In both cases the mares were 3 months pregnant with male foals. Perhaps the gonads of the foals were producing large amounts of testosterone during differentiation and this produced temporary masculinization of their mothers' behavior, as has been reported to occur in domestic horses (Rossdale and Ricketts, 1974; Beaver and Amoss, 1982). Such behavior is not a problem as long as it is tolerated by other herd members and is determined to be due to pregnancy and not a tumor-induced hormonal imbalance.
Infanticide Several cases of infanticide by Przewalski's stallions have been reported. "Ulan" killed 3 colts aged 5-6 months sired by his father "Vampir" when he replaced "Vampir" as harem stallion (Keverling Buisman and van Weeren, 1982 ). "Ulan" was also aggressive toward the 2 remaining foals, a filly and a colt, who were quickly removed from the herd. One mare was 6 months pregnant when "Ulan" was introduced. Her colt was found dead shortly after it was born. It was covered with contusions, and "Ulan" was presumed to be the culprit because of his aggression toward the previous foals. "Ulan" has since been aggressive towards his own offspring when they reach puberty but has not killed any of his own foals (K.A. Houpt, personal communication, 1987 ). The stallion "Basil" sired more than 50 foals (Gruenerwald, 1982 ) without incident at several European zoos before coming to the United States. Two mares at the San Diego Wild Animal Park were carrying colts sired by other stallions when "Basil" was introduced into the herd. One of these mares foaled 3 months after "Basil" arrived. Her colt was found dead on the day of its birth. The cause of death was trauma to the neck region (Ryder and Massena, 1988). Five months after "Basil" arrived, the second mare foaled. "Basil" was observed to attack the colt by grabbing its neck, shaking it, and flinging it into the air (Ryder and Massena, 1988). The colt subsequently died of its injuries. The d a m immediately conceived by "Basil", and the next year she and another mare gave birth to colts sired by "Basil". He attacked neither of these colts. Infanticide may not be a pathological behavior resulting from captivity, as it is also seen in free-ranging domestic horses and appears to be adaptive as it is directed toward unrelated foals rather than the sire's own offspring. Although " U l a n " killed paternal half siblings, he had been separated from his natal herd for several years before being reintroduced. Unless horses use phenotype matching as a mechanism of kin recognition,"Ulan" probably had no way of knowing that he was related to the foals born after his departure (Berger, 1986; Berger and Cunningham, 1987 ). By killing unrelated foals, a stallion eliminates future competitors who do not share his genes. In domestic
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horses, unrelated male foals are more commonly victimized than unrelated female foals, perhaps because male foals grow up to be competitors whereas female foals represent future mates (Duncan, 1982 ). By killing young unrelated foals, a stallion might also benefit by freeing the mares from the physiological stress of lactation, which improves their chances of producing a foal by him the following season (Boyd, 1986). Domestic mares in the weight range of Przewalski's horses (200-300 kg) have peak lactation requirements for protein and energy that are approximately twice maintenance levels (National Research Council, 1978). The nutritional stress imposed by the demands of lactation may impair a mare's body condition and subsequent reproductive performance (Pollock, 1980; Berger, 1986). New Forest pony mares nursing foals received a body condition score that was twice as poor as that of barren mares (Pollock, 1980). Removal of the foals brought about an immediate improvement in body condition. Mares who nursed their foals for 2 years were in poor condition for at least 18 of those months; although they bred in the second year, they failed to foal. Domestic mares in poor body condition during gestation and lactation have lower conception rates and higher embryonic mortality than mares on a higher nutritional plane (Henneke et al., 1984). Keiper (1985) found that a population of feral mares in which the duration of lactation was shortened by removal of the foals at 6-12 weeks of age had an average yearly foaling rate of 74%, while mares in a population where foals were not removed had an average foaling rate of only 57%. Thus, killing foals before peak lactation occurs would improve the stallion's chances that the mare would conceive by him and carry his foal to term. Infanticide does not always occur when 1 stallion replaces another. Several stallions did not attack unrelated foals born into their harem after they replaced the former stallion. As zoos cannot predict whether a stallion will be infanticidal, precautionary measures may be warranted. Introducing stallions only to harems containing non-pregnant mares and older foals, who are less likely to be attacked, should help. Alternatively, mares pregnant by other stallions could be isolated prior to foaling, although this disrupts the normal social structure of the herd. Zoos must strike a balance between the need to rotate stallions frequently in order to promote gene flow in inbred populations and the increased risk of infanticide that this practice brings.
Dispersal As the juveniles near the end of their second year, Przewalski's stallions seem to be more tolerant of their sons than daughters. Hutson (1975) reported that the 22-month-old Przewalski's filly "Hayley" was the recipient of much aggression from her father "Basil" who chased and bit her so much that she had raw patches on her rump. Fillies are ready to breed at 2 years of age. By becoming more aggressive toward pubertal daughters, parents probably encourage their dispersal from the herd, avoiding consanguineous matings.
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In support of this hypothesis, Kaseda et al. (1984) found that feral Misaki horse fillies whose mothers belonged to harems dispersed earlier than fillies whose mothers were unaffiliated with a harem. As familiar fathers and daughters rarely mate (Berger and Cunningham, 1987 ), the fillies would be wasting breeding opportunities by remaining in natal harems. Feral fillies on Shackleford Island were driven out by the adult mares at 2 and 3 years of age, and Rubenstein ( 1982 ) hypothesized that they competed with their dams for limited forage. Sons do not begin to breed before the age of 4 years (Rubenstein, 1982; Kaseda et al., 1984; Berger, 1986) so they are not missing breeding opportunities by staying in the natal band, nor are they likely to take mating opportunities away from their father. By tolerating sons, parents permit them to postpone having to fend for themselves. Rubenstein ( 1982 ) found that at ~ 4 years of age, juvenile males on Shackleford Island left their natal bands of their own accord rather than being driven out. Thus, juvenile Przewalski's horses of both sexes can be left in their natal bands until at least 18 months of age. The juveniles probably gain social skills through continued association with adults and do not need to be removed until parental aggression becomes evident. Fillies are likely to become the first targets of this aggression. SUMMARY
In summary, what is known about the behavior of Przewalski's and domestic horses suggests that duplicating as closely as possible the processes which occur in the wild would eliminate most of the aberrant behavior seen in captive horses and would improve reproductive success. Understanding the causes of natural but undesirable behavior, such as infanticide, is helpful in planning for its prevention. ACKNOWLEDGMENTS
The author would like to thank the Catskill Game Farm, the Denver Zoological Gardens, the Los Angeles Zoo, the Memphis Zoological Gardens, the National Zoological Park's Conservation and Research Center, the New York Zoological Park, the San Diego Zoo, the San Diego Wild Animal Park, and the Topeka Zoological Park for facilitating the observation of their Przewalski's horses. C.E. Houpt developed the time budget and social interaction computer programs. K.A. Houpt and J.R. Sauer provided much advice. Various aspects of this research were supported by the Nixon-Griffis Fund for Zoological Research, a Smithsonian Graduate Student Fellowship, and Sigma Xi Grants in Aid of Research.
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