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Harem size reproductive success of stallions in Misaki feral horses
APPLIED ANIMAL BEHAVIOUR SCIENCE
ELSEVIER
Applied Animal Behaviour Science 47 (1996) 163-173
Harem size and reproductive success of stallions in Misaki feral horses Yujiro Kaseda *, Ashraf M. Khalil Faculty of Agriculture, Miyazaki University, Miyazaki-shi 889-21, Japan Accepted 25 October 1995
Abstract Over a 16-year period (1979-1994), long-term investigations were carried out on 14 Misaki feral stallions to analyze changes in harem size and the reproductive success. Harem size changed with the age of the stallions. Most stallions formed harem groups with four to five mares at the age of 4 - 6 and then the number of mares increased rapidly to the maximum at the age of 6 - 9 years. Thereafter, harem size decreased gradually to a minimum with advancing age. The harem size of 60 stable harem groups ranged from 1 to 9, and the average varied from a minimum mean of 1.8 in 1988 to a maximum mean of 5.3 in 1982. Mean harem size increased as adult sex ratio increased and a significant and positive correlation was found between them. One hundred and ninety-eight sire-foal pairs were determined by a paternity test with blood types and consort relations between stallions and mares during the study period. Out of 99 foals which were born in the stable harem groups, the true sires of 84 foals (85%) were the harem stallions in which the foals were born but the remaining 15 foals (15%) were sired by other harem stallions. Two out of three stallions which were studied throughout their lifetime produced 24 and 25 foals in 10 and 11 years of their reproductive lifespan, respectively. Another one produced only five foals in 6 years. The number of foals sired by the harem stallions was less than two over harem size 7 and some of the foals born in the harem were sired by other harem stallions. These results suggest that if a particular stallion monopolizes too many mares, he could not sire so many offspring because he could not always prevent his rival stallions from mating with his mares in wild or feral circumstances.
Keywords: Harem size; Horse social organization; Reproduction; Blood types; Misaki feral horse
* Corresponding author. 0168-1591/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 01 68- 1 5 9 1 ( 9 5 ) 0 0 6 7 5 - 3
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1. Introduction
Equine harem groups usually consist of one adult male, one to several adult mares and their offspring, and are stable in adult membership during periods from months to years (Berger, 1977; McCort, 1984; Penzhorn, 1984). The harem group is therefore considered to be the basic reproductive unit in feral horse populations (Klingel, 1979; Salter and Hudson, 1982). Foal production in stable harem groups exceeds that in unstable ones (Berger, 1983a, Berger, 1986; Kaseda et al., 1995), and the reproductive success of a stallion positively relates to the number of adult mares in his harem group (Klingel, 1982; Stevens, 1990). In other words, relationships between harem stallions and mares in the harem (hereafter defined as consort relations) and the number of mares in the harem are important factors for the reproductive success of stallions. Reports in the literature on the definition of harem size are not consistent (Waring, 1983; McCort, 1984). Although harem size should be considered in a discussion of the reproductive success of harem stallions (Klingel, 1975; Berger, 1983b, Berger, 1986), few authors have referred to the number of adult mares in the harem groups (hereafter defined as harem size) and to the factors affecting harem size in feral or wild equids. Ideally, lifetime investigation is necessary to measure the exact reproductive performance of stallions (Berger, 1986), but no investigations have been carried out on the reproductive success of harem stallions over their reproductive lifespan for wild or feral equids. Some researchers report that almost all matings take place only between a harem stallion and his mares (Klingel, 1969, Klingel, 1975; Feist and McCullough, 1975, Feist and McCullough, 1976), but others report that not all matings in the harem groups are performed by the harem stallions (Miller, 1981; Kaseda et al., 1982; McCort, 1984). Therefore, a paternity test with blood types for harem stallions and foals born in the harem groups is necessary to measure the exact reproductive performance of stallions (Duncan et al., 1984; Berger, 1986). In the present study, long-term investigations over a 16-year period (1979-1994) were carried out on 14 stallions to analyze changes in harem size and reproductive success in Misaki feral horses.
2. Materials and methods
2.1. Study area The Misaki horse range covers an area of about 500 ha in Cape Toi (located on the southeast end of Kyushu Island). Two small mountains (Komatsu hill and Oogi hill, about 290 m above sea-level) are located in the center of the Misaki range. The range is interrupted by two large valleys which run down from the two mountains into the sea dividing the range into two areas: Komatsu area and Oogi area. 2.2. Study animals and population characteristics Misaki horses have been living freely in the range since 1697. Management practices are minimal and a small amount of supplementary grass is fed during severe winters
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165
Table 1 Changes in the number of stallions, mares and geldings in the Komatsu from 1979 Io 1994 Sex Stallion a Mare b Gelding c
Year '79 '80
'8l
'82
'83
'84
'85
'86
'87
'88
'89
'90
'91
'92
'93
'94
4 16 4
4 23 3
3 23 6
3 21 8
4 30 5
4 25 2
4 25 3
5 23 4
5 24 2
6 21 2
5 16 2
5 20 2
6 22 2
5 22 2
7 18 2
5 19 4
a 4 years and older. b 2 years and older. c Some males were castrated during the third autumn.
only. The horses are treated for intemal and external parasites once a year (usually in June). All horses are driven into a paddock settled in the Komatsu or Oogi area, and each individual horse is driven into a crush enclosed with a palisade and tied to the crush with ropes. At the same time, the blood of each horse was sampled to analyze blood types. The range has been patrolled daily by two rangers since 1972, in order to record any births, deaths or accidents among the animals. The population dynamics of the horses, and the individual reproductive success and home range of each animal have also been recorded since 1972. The number of horses ranged from 73 to 100 during the study period from 1979 to 1994 (Kaseda, 1981, Kaseda, 1983, Kaseda, 1991). Most animals have been observed to move between the Komatsu area and the Oogi area from late September to late March, but very little movement between the two areas is seen during the breeding season from early April to late August (Kaseda, 1983). The Misaki horses are separated into two herds according to their home range during the breeding season; the Komatsu herd live in the Komatsu area and the Oogi herd in the Oogi area (a herd was defined in this context as a structured social unit made up of harem groups which follow similar movement patterns within a common home range, as proposed by Miller (1979a). During the study period, stallions ranged in number from three to six and mares from 16 to 30 in the Komatsu area (Table 1). A few males were castrated during their third autumn from 1973 to 1979. All young males (1-3 years) which left the natal groups of their mothers and most geldings migrated from the Komatsu area to the Oogi area in the non-breeding season, and all stallions and adult mares formed harem groups in the Komatsu area. So the Komatsu herd is useful to investigate changes in harem size and reproductive success of feral stallions. The present study was therefore restricted to the Komatsu herd from 1979 to 1994.
2.3, Field observation of social behavior Misaki horses are identified by their coat color, sex, body size, markings and the number branded on the hip (Kaseda, 1981). Every animal has been recorded individually in the stud book since 1972. Field observations on the social behavior were made once or twice a month between 1979 and 1987, and once a week between 1988 and 1994.
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When a horse was sighted in the field, the location and branded number of the animal, and its behavior were recorded on field maps. In this way data on the members of social groups were collected throughout the study period. Detailed observations were made on social behavior such as the social relationships between stallions and mares and between group members, and the harem group formation of young stallions and mares.
2.4. Definitions of the stability of consort relations between harem stallions and mares The horses were observed at least eight times in the breeding season from early April to late August (at least once in April and August, and at least twice in May, June or July). A consort relation between a harem stallion and a mare ( > 2 years) was defined as stable if a particular mare was seen with the same stallion on any observation throughout the breeding season of 1 year. Consort relations were defined as unstable if a mare was found to change stallions or if she was seen to live solitarily or with her offspring on any observation. Some unstable mares were named wandering mares because they lived very often solitarily or with their offspring.
2.5. Paternity test by blood types Blood protein types (28 systems) of each horse were determined by electrophoresis (Kaseda et al., 1982). The true sire was determined by the paternity exclusion method in which blood types and consort relation between a harem stallion and mares in the breeding season. Firstly, paternity tests with blood types were conducted for all foals each year. If all but one potential stallion were excluded by the paternity test, that stallion was determined the true sire of the foal in question. Secondly, in the case that two or more possible sires were found in the paternity test with blood types, if all but one stallion lived in the different home range (the Oogi area) from the dam of the foal during the previous breeding season, that stallion was assumed the true sire of the foal. Thirdly, in the case that two or more possible sires were found in the paternity test with blood types and the home range in the breeding season, if all but one stallion had not formed a stable consort relation with the dam of the foal throughout the previous breeding season, that stallion was assumed the true sire. The true sires could not be determined in other cases. Paternity tests were conducted on 131 foals and the true sires of 12 foals could not be determined in the Komatsu herd.
3. Results
3.1. Changes in harem size Changes in harem size were shown in relation to age for five stallions which were observed consecutively for 4 or more years since the start of the harem groups (Fig. 1). Harem size changed with the age of the stallions. Most stallions formed harem groups with four to five mares at the age of 4 - 6 years and then the number of mares increased rapidly to the maximum at the age of 6 - 9 years. Thereafter, harem size decreased
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8 ,6
167
iiii
2
4
6
8
10
12
t4
16
Age(year) Fig. 1. Changes in harem size in relation to the age of five stallions. Three stallions (62S, 53S and 79S) were observed throughout their reproductive lifespan. Stallion 48S was killed by a traffic accident at the age of 9 years.
gradually to the minimum with advancing age. Some stallions lost almost all mares a few years before their deaths, but others retained some mares until just before their deaths. The largest number of mares was seven for stallion 62S at the age of 9 years, nine for stallion 53S at the age of 7 years, six for stallion 79S at the age of 6 years, seven for stallion 56S at the age of 6 - 7 years, six for stallion 48S at the age of 8 years, four for stallion 75S at the age of 8 years and only one for stallion 87S at the age of 9 years, respectively (Table 2). There was a significant difference in harem size among these seven stallions between the ages of 6 - 9 years (Kruskal-Wallis test; H--- 17.98, df = 6, P < 0.01). Almost all harem groups split into smaller groups which were composed of only familiar members during the late fall and early winter and moved to their winter habitats. In these cases, most stallions associated with one to two mares which had been the nuclei since their harem formation. These small groups returned to their summer habitats early in the next breeding season, and stable harem groups were formed by a stallion and one or more mares during late April and early May. During the reformation of harem groups, a harem group increased in size by addition of mares which left other harem groups in the previous winter or wandering mares, but the harem size decreased by transference of some of the original mares to other harem groups or death of the mares before the reformation. When harem groups were formed again or anew early in the breeding season, aggression, fighting or such behavior as active herding or driving mares was rarely observed among stallions. Almost all mares were seen to associate, of their own accord, transiently with some partner stallions before the formation of stable harem groups. However, they spontaneously formed a stable consort relation with a particular stallion by early May. After formation of a stable harem group, every stallion tried actively to retain his mares by herding them or driving other stallions away.
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Table 2 Observation periods, number of mares with which each stallion bad stable consort relations and the number of foals sired for 14 stallions Stallion
Observation period (year) a
No. of mares b
No. of foals sired c
18A 37 18B 14 62 53 76 79 87 56 48 75 2 74
3 (14-18) 2 (13-14) 6 (12-17) 1 (12) 11 (6-16) l0 (4-13) 1 (13) 6 (5-10) 5 (6-10) 7 (5-11) 4 (6-9) 4 (5-8) 3 (5-7) 2 (5-6)
8 2 2l 2 40 (23) 52 (30) 2 21 (14) 2(1) 30 (18) 19 (19) 10 (11) 5 10
9 (7) 1 (0) 15 (8) 2 (0) 24 (16) 25 (15) 1 (1) 5 (2) 4(1) 14(8) 9 (4) 6 (3) 3 (1) 4 (3)
a Age. Number of mares between the ages of 6 to 9 years. c Number of foals sired in his harem group.
Sixty stable harem groups ranged in size from 1 to 9, and the average varied from a minimum mean o f 1.8 in 1988 to a maximum mean of 5.3 in 1982. Harem sizes over six were observed while stallions were between the ages of 6 and 9 years, and the highest values of nine or eight were observed for stallion 53S in 1984-1986 when he was in his prime, while the lowest values of zero or one were found for other stallions during the same seasons. A harem size of 4 was most frequently observed, followed by a harem size 2 (Fig. 2).
(91
(~2)
(61
2
3
(14)
(7)
(6)
(3)
(2)
6
7
$
I0
2O
g,
F3 Harem
9
Size
Fig. 2. Frequency distribution of harem sizes in stable harem groups. Sample size for each harem size is given in parentheses.
Y. Kaseda A.M. Khalil / Applied Animal Behaviour Science 47 (1996) 163-173 ,
~
, O:Sel
,
1 7 r*tho
@: H i r a m st~e
,i
! ~s i
I
°i
~
169
6
.2
4
1919
1991 r 19~3 ' 1985
1987
L
1959
1991
I 1993
Fig. 3. Changes in mean harem size (mean + standard deviation) and adult sex ratio (mares/stallion) from 1979 to 1994. A significant and positive correlation was found between mean harem size and adult sex ratio ( r s = 0.751, d f = 16, P < 0.05).
3.2. Adult sex ratio and harem size The number of stallions in the Komatsu area was nearly constant, ranging from three to six, but the number of mares varied greatly, ranging from 16 to 30 (Table 1). In particular the number of mares increased from 19 in 1980 to 30 in 1984, and the adult sex ratio (mares/stallion) increased rapidly from 3.8 in 1980 to 7.5 in 1984. Thereafter, the high level of sex ratio continued until 1986 and decreased gradually to the minimum value of 3.2 in 1990 (Table 1). Variation in harem size changed depending on the year. The variation between 1984 and 1986 was very large because eight to nine mares formed consort relations with 53S, but only one mare formed a consort relation with the oldest stallion. Another young stallion had no consort relations during the period. On the other hand, variation between 1981 and 1983 was small because three to four stallions had nearly the same number of mares during the period (Fig. 3). A significant and positive correlation was found between mean harem size and adult sex ratio (r s = 0.751, df = 16, P < 0.05) (Fig. 3). Mean harem size in the years when the sex ratio was higher than 6.0 was significantly larger than that in the years when the sex ratio was lower than 4.0 (t = 2.65, df = 39, P < 0.01). 3.3. Reproductive success of stallions One hundred and ninety-eight sire-foal pairs for 14 stallions and their offspring were determined by a paternity test with blood types and consort relations between stallions and mares during the study period. Out of 99 foals which were born in the stable harem groups, the true sires of 84 foals (85%) were the stallions of the harem group in which the foals were born, but the remaining 15 foals (15%) were sired by other harem stallions. The total number of foals sired was 24 (16 in the harem) for stallion 62S, 25 (15 in the harem) for stallion 53S, and 5 (2 in the harem) for stallion 79S. The difference
170
Y. Kaseda, A.M. Khalil / Applied Animal Behaviour Science 47 (1996) 163-173
3
1 O : Fot]s In Harem
1
2
]
4
5
6
7
i
9
Harem Size Fig. 4. Relationships between harem size and the mean number of foals sired by the harem stallions in the harem groups (black circle), and mean number of foals born in the harem groups (white circle). Significant and positive correlations were found between harem size and (1) the mean number of foals sired by the harem stallions ( r s = 0.886, d f = 6, P <0.05), and (2) the mean number of foals born in the harem groups ( r s = 0.943, df = 6, P < 0.01)
between the total number and the number in the harem was the number of foals which were born by matings between harem stallions and other harem mares or unstable wandering mares. There was a positive and significant correlation between the total number of stable mares and the total number of foals sired by the harem stallion in the harem groups (r~ = 0.875, df = 7, P < 0.05) for seven stallions which were observed for 4 or more consecutive years since the harem formation. There was a significant difference in the number of foals sired in the harem among these seven stallions between the ages of 6 - 9 years (Kruskal-Wallis test; H = 14.63, df = 6, P < 0.05). Positive and significant correlations existed between harem size and the mean number of foals sired by the harem stallion ( r s = 0.879, df = 9, P < 0.05), and between harem size and the mean number of foals born in the harem (r~ = 0.729, df = 9, P < 0.05) (Fig. 4). However, the mean number of foals sired by the harem stallions was less than two over harem size 7, and the mean number of foals born in the harem also was only two at harem sizes 8 and 9. About half of the foals born in the harem were sired by other harem stallions at harem size 7.
4. Discussion
4.1. Change in harem size Since almost all harem mares are mated exclusively by the harem stallions (Miller, 1981; McCort, 1984), it is certain that the reproductive success of stallions depends on the number of harem mares (harem size) (Klingel, 1982; Stevens, 1990). It is possible that changes in harem size or individual difference in harem size among stallions may be
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influenced by some factors which affect the reproductive success of stallions such as age, body weight and fighting abilities (Berger, 1986), or dominance and aggressive behavior (Miller, 1981). In the present study, the results of investigations on the changes in harem size of stallions showed that harem size changes with the age of stallions and reaches the maximum at the age of 6 to 9 years and then gradually decreases with advancing age. The correlation between mean harem size and adult sex ratio (mares per stallion) was significant, although variation in harem size was very large between years. Mean harem size was significantly higher in the years when the sex ratio was higher. This indicates that harem size increases with the number of mares relative to the stallions in the herd. Berger (1986) reported that stallions in the Great Basin acquire or enlarge harems in three basic ways: appropriation of wandering mares, deposition of resident stallions through escalated aggression and fighting and cooperation and alliances. In the present study, aggression or fighting between stallions for mares, or active herding behavior toward mares were rarely observed in reformations or new formations of harem groups early in the breeding season of each year. Some mares which did not remain with any particular stallion seemed to spontaneously select partner stallions or harem groups. Some mares which left their harem groups in the previous winter were confronted with a choice to select either the same stallion as in the previous year or the other stallion early in the breeding season. Some young mares which left their native harem groups and reached sexual maturity faced the same problems. Those mares may determine their partner stallions according to a certain selective criteria in such cases. Mares should try to select potential stallions who have the ability to keep a stable consort relation with them, because the reproductive success of mares in stable harem groups is higher than that of mares in unstable ones (Berger, 1983a; Kaseda et al., 1995). It is possible that mares may select their consort partner stallions by evaluating them according to their relative abilities such as dominance (Feist and McCuliough, 1976; Berger, 1977; Miller, 1981) in some cases, aggressive behavior (Miller, 1981) or attentiveness to their mares (Rutberg, 1990). A stallion's abilities may change with advancing age or with the stallion's physical strength and experience. Change in harem size due to the stallion's age or change in sex ratio is considered the result of a relative evaluation of stallions by mares according to such selective criteria.
4.2. Relationships between harem size and reproductive success of stallions Previous authors have stated that stable consort relations between harem stallions and mares are usually kept for several months to years and almost all matings are exclusively done between them (Feist and McCullough, 1975; Klingel, 1982). Meanwhile, Miller (1981) reported that the band stallion bred in 18 of the 21 observed matings and the other three mares were bred by males from other bands. However, paternity tests with blood types were not done in these previous studies. In the present study, 15 of the 99 foals born in the stable harem groups were sired by other harem stallions. The present result supports, by paternity tests with blood types, the previous observations that some mares in the stable harem groups were mated by other harem stallions (Miller, 1981; McCort, 1984).
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Reproductive contributions over a lifetime are the best measure of a stallion's reproductive success (Berger, 1986). Changes in harem size and the number of foals sired over the lifetimes of three stallions were observed in the present study. Two stallions produced 24 and 25 foals for 10 and 11 years of reproductive lifespan, respectively. Another one produced only five foals in 6 years of reproductive lifespan. Berger (1986) reported that reproductive success was affected by age, body weight, fighting abilities, home-range locations and reproductive lifespan, and that the differences in reproductive success among six stallions between the ages of 7 and 11 years were great. In the present study, the differences in number of mares with which a stallion bad consort relations and number of foals sired were significant among seven stallions between the ages of 6 to 9 years. These differences in reproductive success are probably due to factors other than age, home-range or reproductive lifespan as these three factors were controlled in this comparison. A stallion's reproductive success is observed to relate positively to the number of mares in his harem (Klingel, 1982; Stevens, 1990). In this population too, there was a significant and positive correlation between the total number of mares with which the harem stallion had stable consort relations and the total number of foals sired throughout the observation period for seven stallions between the ages of 6 to 9 years. The results indicate that the larger the harem and the longer the tenure a stallion has, the higher his reproductive success. It is especially important for stallions to attract as many mares as possible and to keep stable consort relations with them as long as possible (Stevens, 1990). The number of foals sired increased with harem size under size 6, but not over size 6. This indicates that the reproductive success of stallions may be restricted if harem size is too large. The number of foals sired by the harem stallions and born in the harem were very few in spite of many harem mares over harem size 7, and some of the foals born in the harem were sired by other harem stallions. If a particular stallion monopolizes too many mares, it is highly possible that he could not sire so many offspring because he could not always prevent his rival stallions from mating with his mares in wild or feral circumstances. A certain optimum harem size may exist for the reproductive success of wild or feral equids.
Acknowledgements The authors wish to thank the Japan Equine Affairs Association for their support of the present study. This investigation was supported in part by a Grant-in-Aids for Research (No. 03660283) from the Ministry of Education, Science, and Culture in Japan.
References Berger, J., 1977. Organizational systems and dominance in feral horses in the Grand Canyon. Behav. Ecol. Sociobiol., 2: 131-146. Berger, J., 1983a. Induced abortion and social factor in wild horses. Nature, 303: 59-61.
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Berger, J., 1983b. Predation, sex ratios, and male competition in equids (Mammalia: Pressodactyla). J. Zool. Lond., 201: 205-216, Berger, J., 1986. Wild Horses of the Great Basin. Social Competition and Population Size. The University of Chicago Press, Chicago, pp. 89-95, 128-147, 196-231. Duncan, P., Feh, C., Gleize, J.C., Malkas, P. and Scott, A.M., 1984. Reduction of inbreeding in a natural herd of horses. Anim. Behav., 32: 520-527. Feist, J.D. and McCullough, D.R., 1975. Reproduction in feral horses. J. Reprod. Fert. Suppl., 23: 13-18. Feist, J.D. and McCuUough, D.R., 1976. Behaviour patterns and communication in feral horses. Z. Tierpsychol., 41: 337-371. Kaseda, Y., 1981. The structure of the groups of Misaki horses in Toi Cape. Jpn. J. Zootech. Sci., 52: 227-235. Kaseda, Y., 1983. Seasonal changes in the home range and the size of harem groups of Misaki horses. Jpn. J. Zootech. Sci., 54: 254-262. Kaseda, Y., 1991. Some factors affecting on the population dynamics of two herds in Misaki feral horses. Anim. Sci. Technol. (Jpn), 62:1171-1178. Kaseda, Y., Nozawa, K. and Mogi, K., 1982. Sire-foal relationships between harem stallions and foals in Misaki horses. Jpn. J. Zootech. Sci., 53: 822-830. Kaseda, Y., Khalil, A.M. and Ogawa, H., 1995. Harem stability and reproductive success of Misaki feral mares. Equine Vet, J., 27: 368-372. Klingel, H., 1969. Reproduction in the Plains zebra, Equus Burdielli Boehmi: Behaviour and ecological factors. J. Reprod. Fert. Suppl., 6: 339-345. Klingel, H., 1975. Social organization and reproduction in equids. J. Reprod. Fert. Suppl., 23: 7-11. Klingel, H., 1979. A comparison of the social organization of the equids. In: R.H. Denniston (Editor), Symposium on the Ecology and Behaviour of Wild and Feral Equids. University of Wyoming, Laramie, pp. 113-123. Klingel, H., 1982. Social organization of feral horses. J. Reprod. Fert. Suppl., 32: 89-95. McCort, W.D., 1984. Behavior of feral horses and ponies. J. Anita. Sci., 58: 493-499. Miller, R., 1979a. Interband dominance in feral horses. Z. Tierpsychol., 51: 41-47. Miller, R., 1981. Male aggression, dominance and breeding behavior in Red Desert feral horses. Z. Tierpsychol., 57:340-351. Penzhorn, B.L., 1984. A long-term study of social organization and behavior of Cape Mountain Zebras Equus zebra zebra. Z. Tierpsychol., 64: 97-146. Rutberg, A.T., 1990. Inter-group transfer in Assateague pony mares. Anim. Behav., 40: 945-952. Salter, R.E. and Hudson, R.J., 1982. Social organization of feral horses in western Canada. Appl. Anim. Ethol., 8: 207-223. Stevens, E.F., 1990. Instability of harems of feral horses in relation to season and presence of subordinate stallions. Behav., 112: 149-16l. Waring, G.H., 1983. Horse Behavior. The Behavioral Traits and Adaptations of Domestic and Wild Horses, Including Ponies. Noyes Publications, New Jersey, USA, pp. 142-147.
ELSEVIER
Applied Animal Behaviour Science 47 (1996) 163-173
Harem size and reproductive success of stallions in Misaki feral horses Yujiro Kaseda *, Ashraf M. Khalil Faculty of Agriculture, Miyazaki University, Miyazaki-shi 889-21, Japan Accepted 25 October 1995
Abstract Over a 16-year period (1979-1994), long-term investigations were carried out on 14 Misaki feral stallions to analyze changes in harem size and the reproductive success. Harem size changed with the age of the stallions. Most stallions formed harem groups with four to five mares at the age of 4 - 6 and then the number of mares increased rapidly to the maximum at the age of 6 - 9 years. Thereafter, harem size decreased gradually to a minimum with advancing age. The harem size of 60 stable harem groups ranged from 1 to 9, and the average varied from a minimum mean of 1.8 in 1988 to a maximum mean of 5.3 in 1982. Mean harem size increased as adult sex ratio increased and a significant and positive correlation was found between them. One hundred and ninety-eight sire-foal pairs were determined by a paternity test with blood types and consort relations between stallions and mares during the study period. Out of 99 foals which were born in the stable harem groups, the true sires of 84 foals (85%) were the harem stallions in which the foals were born but the remaining 15 foals (15%) were sired by other harem stallions. Two out of three stallions which were studied throughout their lifetime produced 24 and 25 foals in 10 and 11 years of their reproductive lifespan, respectively. Another one produced only five foals in 6 years. The number of foals sired by the harem stallions was less than two over harem size 7 and some of the foals born in the harem were sired by other harem stallions. These results suggest that if a particular stallion monopolizes too many mares, he could not sire so many offspring because he could not always prevent his rival stallions from mating with his mares in wild or feral circumstances.
Keywords: Harem size; Horse social organization; Reproduction; Blood types; Misaki feral horse
* Corresponding author. 0168-1591/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 01 68- 1 5 9 1 ( 9 5 ) 0 0 6 7 5 - 3
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1. Introduction
Equine harem groups usually consist of one adult male, one to several adult mares and their offspring, and are stable in adult membership during periods from months to years (Berger, 1977; McCort, 1984; Penzhorn, 1984). The harem group is therefore considered to be the basic reproductive unit in feral horse populations (Klingel, 1979; Salter and Hudson, 1982). Foal production in stable harem groups exceeds that in unstable ones (Berger, 1983a, Berger, 1986; Kaseda et al., 1995), and the reproductive success of a stallion positively relates to the number of adult mares in his harem group (Klingel, 1982; Stevens, 1990). In other words, relationships between harem stallions and mares in the harem (hereafter defined as consort relations) and the number of mares in the harem are important factors for the reproductive success of stallions. Reports in the literature on the definition of harem size are not consistent (Waring, 1983; McCort, 1984). Although harem size should be considered in a discussion of the reproductive success of harem stallions (Klingel, 1975; Berger, 1983b, Berger, 1986), few authors have referred to the number of adult mares in the harem groups (hereafter defined as harem size) and to the factors affecting harem size in feral or wild equids. Ideally, lifetime investigation is necessary to measure the exact reproductive performance of stallions (Berger, 1986), but no investigations have been carried out on the reproductive success of harem stallions over their reproductive lifespan for wild or feral equids. Some researchers report that almost all matings take place only between a harem stallion and his mares (Klingel, 1969, Klingel, 1975; Feist and McCullough, 1975, Feist and McCullough, 1976), but others report that not all matings in the harem groups are performed by the harem stallions (Miller, 1981; Kaseda et al., 1982; McCort, 1984). Therefore, a paternity test with blood types for harem stallions and foals born in the harem groups is necessary to measure the exact reproductive performance of stallions (Duncan et al., 1984; Berger, 1986). In the present study, long-term investigations over a 16-year period (1979-1994) were carried out on 14 stallions to analyze changes in harem size and reproductive success in Misaki feral horses.
2. Materials and methods
2.1. Study area The Misaki horse range covers an area of about 500 ha in Cape Toi (located on the southeast end of Kyushu Island). Two small mountains (Komatsu hill and Oogi hill, about 290 m above sea-level) are located in the center of the Misaki range. The range is interrupted by two large valleys which run down from the two mountains into the sea dividing the range into two areas: Komatsu area and Oogi area. 2.2. Study animals and population characteristics Misaki horses have been living freely in the range since 1697. Management practices are minimal and a small amount of supplementary grass is fed during severe winters
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165
Table 1 Changes in the number of stallions, mares and geldings in the Komatsu from 1979 Io 1994 Sex Stallion a Mare b Gelding c
Year '79 '80
'8l
'82
'83
'84
'85
'86
'87
'88
'89
'90
'91
'92
'93
'94
4 16 4
4 23 3
3 23 6
3 21 8
4 30 5
4 25 2
4 25 3
5 23 4
5 24 2
6 21 2
5 16 2
5 20 2
6 22 2
5 22 2
7 18 2
5 19 4
a 4 years and older. b 2 years and older. c Some males were castrated during the third autumn.
only. The horses are treated for intemal and external parasites once a year (usually in June). All horses are driven into a paddock settled in the Komatsu or Oogi area, and each individual horse is driven into a crush enclosed with a palisade and tied to the crush with ropes. At the same time, the blood of each horse was sampled to analyze blood types. The range has been patrolled daily by two rangers since 1972, in order to record any births, deaths or accidents among the animals. The population dynamics of the horses, and the individual reproductive success and home range of each animal have also been recorded since 1972. The number of horses ranged from 73 to 100 during the study period from 1979 to 1994 (Kaseda, 1981, Kaseda, 1983, Kaseda, 1991). Most animals have been observed to move between the Komatsu area and the Oogi area from late September to late March, but very little movement between the two areas is seen during the breeding season from early April to late August (Kaseda, 1983). The Misaki horses are separated into two herds according to their home range during the breeding season; the Komatsu herd live in the Komatsu area and the Oogi herd in the Oogi area (a herd was defined in this context as a structured social unit made up of harem groups which follow similar movement patterns within a common home range, as proposed by Miller (1979a). During the study period, stallions ranged in number from three to six and mares from 16 to 30 in the Komatsu area (Table 1). A few males were castrated during their third autumn from 1973 to 1979. All young males (1-3 years) which left the natal groups of their mothers and most geldings migrated from the Komatsu area to the Oogi area in the non-breeding season, and all stallions and adult mares formed harem groups in the Komatsu area. So the Komatsu herd is useful to investigate changes in harem size and reproductive success of feral stallions. The present study was therefore restricted to the Komatsu herd from 1979 to 1994.
2.3, Field observation of social behavior Misaki horses are identified by their coat color, sex, body size, markings and the number branded on the hip (Kaseda, 1981). Every animal has been recorded individually in the stud book since 1972. Field observations on the social behavior were made once or twice a month between 1979 and 1987, and once a week between 1988 and 1994.
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When a horse was sighted in the field, the location and branded number of the animal, and its behavior were recorded on field maps. In this way data on the members of social groups were collected throughout the study period. Detailed observations were made on social behavior such as the social relationships between stallions and mares and between group members, and the harem group formation of young stallions and mares.
2.4. Definitions of the stability of consort relations between harem stallions and mares The horses were observed at least eight times in the breeding season from early April to late August (at least once in April and August, and at least twice in May, June or July). A consort relation between a harem stallion and a mare ( > 2 years) was defined as stable if a particular mare was seen with the same stallion on any observation throughout the breeding season of 1 year. Consort relations were defined as unstable if a mare was found to change stallions or if she was seen to live solitarily or with her offspring on any observation. Some unstable mares were named wandering mares because they lived very often solitarily or with their offspring.
2.5. Paternity test by blood types Blood protein types (28 systems) of each horse were determined by electrophoresis (Kaseda et al., 1982). The true sire was determined by the paternity exclusion method in which blood types and consort relation between a harem stallion and mares in the breeding season. Firstly, paternity tests with blood types were conducted for all foals each year. If all but one potential stallion were excluded by the paternity test, that stallion was determined the true sire of the foal in question. Secondly, in the case that two or more possible sires were found in the paternity test with blood types, if all but one stallion lived in the different home range (the Oogi area) from the dam of the foal during the previous breeding season, that stallion was assumed the true sire of the foal. Thirdly, in the case that two or more possible sires were found in the paternity test with blood types and the home range in the breeding season, if all but one stallion had not formed a stable consort relation with the dam of the foal throughout the previous breeding season, that stallion was assumed the true sire. The true sires could not be determined in other cases. Paternity tests were conducted on 131 foals and the true sires of 12 foals could not be determined in the Komatsu herd.
3. Results
3.1. Changes in harem size Changes in harem size were shown in relation to age for five stallions which were observed consecutively for 4 or more years since the start of the harem groups (Fig. 1). Harem size changed with the age of the stallions. Most stallions formed harem groups with four to five mares at the age of 4 - 6 years and then the number of mares increased rapidly to the maximum at the age of 6 - 9 years. Thereafter, harem size decreased
Y. K aseda, A.M. Khalil / Applied Animal Behaviour Science 47 (1996) 163-173
8 ,6
167
iiii
2
4
6
8
10
12
t4
16
Age(year) Fig. 1. Changes in harem size in relation to the age of five stallions. Three stallions (62S, 53S and 79S) were observed throughout their reproductive lifespan. Stallion 48S was killed by a traffic accident at the age of 9 years.
gradually to the minimum with advancing age. Some stallions lost almost all mares a few years before their deaths, but others retained some mares until just before their deaths. The largest number of mares was seven for stallion 62S at the age of 9 years, nine for stallion 53S at the age of 7 years, six for stallion 79S at the age of 6 years, seven for stallion 56S at the age of 6 - 7 years, six for stallion 48S at the age of 8 years, four for stallion 75S at the age of 8 years and only one for stallion 87S at the age of 9 years, respectively (Table 2). There was a significant difference in harem size among these seven stallions between the ages of 6 - 9 years (Kruskal-Wallis test; H--- 17.98, df = 6, P < 0.01). Almost all harem groups split into smaller groups which were composed of only familiar members during the late fall and early winter and moved to their winter habitats. In these cases, most stallions associated with one to two mares which had been the nuclei since their harem formation. These small groups returned to their summer habitats early in the next breeding season, and stable harem groups were formed by a stallion and one or more mares during late April and early May. During the reformation of harem groups, a harem group increased in size by addition of mares which left other harem groups in the previous winter or wandering mares, but the harem size decreased by transference of some of the original mares to other harem groups or death of the mares before the reformation. When harem groups were formed again or anew early in the breeding season, aggression, fighting or such behavior as active herding or driving mares was rarely observed among stallions. Almost all mares were seen to associate, of their own accord, transiently with some partner stallions before the formation of stable harem groups. However, they spontaneously formed a stable consort relation with a particular stallion by early May. After formation of a stable harem group, every stallion tried actively to retain his mares by herding them or driving other stallions away.
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Table 2 Observation periods, number of mares with which each stallion bad stable consort relations and the number of foals sired for 14 stallions Stallion
Observation period (year) a
No. of mares b
No. of foals sired c
18A 37 18B 14 62 53 76 79 87 56 48 75 2 74
3 (14-18) 2 (13-14) 6 (12-17) 1 (12) 11 (6-16) l0 (4-13) 1 (13) 6 (5-10) 5 (6-10) 7 (5-11) 4 (6-9) 4 (5-8) 3 (5-7) 2 (5-6)
8 2 2l 2 40 (23) 52 (30) 2 21 (14) 2(1) 30 (18) 19 (19) 10 (11) 5 10
9 (7) 1 (0) 15 (8) 2 (0) 24 (16) 25 (15) 1 (1) 5 (2) 4(1) 14(8) 9 (4) 6 (3) 3 (1) 4 (3)
a Age. Number of mares between the ages of 6 to 9 years. c Number of foals sired in his harem group.
Sixty stable harem groups ranged in size from 1 to 9, and the average varied from a minimum mean o f 1.8 in 1988 to a maximum mean of 5.3 in 1982. Harem sizes over six were observed while stallions were between the ages of 6 and 9 years, and the highest values of nine or eight were observed for stallion 53S in 1984-1986 when he was in his prime, while the lowest values of zero or one were found for other stallions during the same seasons. A harem size of 4 was most frequently observed, followed by a harem size 2 (Fig. 2).
(91
(~2)
(61
2
3
(14)
(7)
(6)
(3)
(2)
6
7
$
I0
2O
g,
F3 Harem
9
Size
Fig. 2. Frequency distribution of harem sizes in stable harem groups. Sample size for each harem size is given in parentheses.
Y. Kaseda A.M. Khalil / Applied Animal Behaviour Science 47 (1996) 163-173 ,
~
, O:Sel
,
1 7 r*tho
@: H i r a m st~e
,i
! ~s i
I
°i
~
169
6
.2
4
1919
1991 r 19~3 ' 1985
1987
L
1959
1991
I 1993
Fig. 3. Changes in mean harem size (mean + standard deviation) and adult sex ratio (mares/stallion) from 1979 to 1994. A significant and positive correlation was found between mean harem size and adult sex ratio ( r s = 0.751, d f = 16, P < 0.05).
3.2. Adult sex ratio and harem size The number of stallions in the Komatsu area was nearly constant, ranging from three to six, but the number of mares varied greatly, ranging from 16 to 30 (Table 1). In particular the number of mares increased from 19 in 1980 to 30 in 1984, and the adult sex ratio (mares/stallion) increased rapidly from 3.8 in 1980 to 7.5 in 1984. Thereafter, the high level of sex ratio continued until 1986 and decreased gradually to the minimum value of 3.2 in 1990 (Table 1). Variation in harem size changed depending on the year. The variation between 1984 and 1986 was very large because eight to nine mares formed consort relations with 53S, but only one mare formed a consort relation with the oldest stallion. Another young stallion had no consort relations during the period. On the other hand, variation between 1981 and 1983 was small because three to four stallions had nearly the same number of mares during the period (Fig. 3). A significant and positive correlation was found between mean harem size and adult sex ratio (r s = 0.751, df = 16, P < 0.05) (Fig. 3). Mean harem size in the years when the sex ratio was higher than 6.0 was significantly larger than that in the years when the sex ratio was lower than 4.0 (t = 2.65, df = 39, P < 0.01). 3.3. Reproductive success of stallions One hundred and ninety-eight sire-foal pairs for 14 stallions and their offspring were determined by a paternity test with blood types and consort relations between stallions and mares during the study period. Out of 99 foals which were born in the stable harem groups, the true sires of 84 foals (85%) were the stallions of the harem group in which the foals were born, but the remaining 15 foals (15%) were sired by other harem stallions. The total number of foals sired was 24 (16 in the harem) for stallion 62S, 25 (15 in the harem) for stallion 53S, and 5 (2 in the harem) for stallion 79S. The difference
170
Y. Kaseda, A.M. Khalil / Applied Animal Behaviour Science 47 (1996) 163-173
3
1 O : Fot]s In Harem
1
2
]
4
5
6
7
i
9
Harem Size Fig. 4. Relationships between harem size and the mean number of foals sired by the harem stallions in the harem groups (black circle), and mean number of foals born in the harem groups (white circle). Significant and positive correlations were found between harem size and (1) the mean number of foals sired by the harem stallions ( r s = 0.886, d f = 6, P <0.05), and (2) the mean number of foals born in the harem groups ( r s = 0.943, df = 6, P < 0.01)
between the total number and the number in the harem was the number of foals which were born by matings between harem stallions and other harem mares or unstable wandering mares. There was a positive and significant correlation between the total number of stable mares and the total number of foals sired by the harem stallion in the harem groups (r~ = 0.875, df = 7, P < 0.05) for seven stallions which were observed for 4 or more consecutive years since the harem formation. There was a significant difference in the number of foals sired in the harem among these seven stallions between the ages of 6 - 9 years (Kruskal-Wallis test; H = 14.63, df = 6, P < 0.05). Positive and significant correlations existed between harem size and the mean number of foals sired by the harem stallion ( r s = 0.879, df = 9, P < 0.05), and between harem size and the mean number of foals born in the harem (r~ = 0.729, df = 9, P < 0.05) (Fig. 4). However, the mean number of foals sired by the harem stallions was less than two over harem size 7, and the mean number of foals born in the harem also was only two at harem sizes 8 and 9. About half of the foals born in the harem were sired by other harem stallions at harem size 7.
4. Discussion
4.1. Change in harem size Since almost all harem mares are mated exclusively by the harem stallions (Miller, 1981; McCort, 1984), it is certain that the reproductive success of stallions depends on the number of harem mares (harem size) (Klingel, 1982; Stevens, 1990). It is possible that changes in harem size or individual difference in harem size among stallions may be
Y. Kaseda, A.M. Khalil / Applied Animal Behaviour Science 47 (1996) 163-173
171
influenced by some factors which affect the reproductive success of stallions such as age, body weight and fighting abilities (Berger, 1986), or dominance and aggressive behavior (Miller, 1981). In the present study, the results of investigations on the changes in harem size of stallions showed that harem size changes with the age of stallions and reaches the maximum at the age of 6 to 9 years and then gradually decreases with advancing age. The correlation between mean harem size and adult sex ratio (mares per stallion) was significant, although variation in harem size was very large between years. Mean harem size was significantly higher in the years when the sex ratio was higher. This indicates that harem size increases with the number of mares relative to the stallions in the herd. Berger (1986) reported that stallions in the Great Basin acquire or enlarge harems in three basic ways: appropriation of wandering mares, deposition of resident stallions through escalated aggression and fighting and cooperation and alliances. In the present study, aggression or fighting between stallions for mares, or active herding behavior toward mares were rarely observed in reformations or new formations of harem groups early in the breeding season of each year. Some mares which did not remain with any particular stallion seemed to spontaneously select partner stallions or harem groups. Some mares which left their harem groups in the previous winter were confronted with a choice to select either the same stallion as in the previous year or the other stallion early in the breeding season. Some young mares which left their native harem groups and reached sexual maturity faced the same problems. Those mares may determine their partner stallions according to a certain selective criteria in such cases. Mares should try to select potential stallions who have the ability to keep a stable consort relation with them, because the reproductive success of mares in stable harem groups is higher than that of mares in unstable ones (Berger, 1983a; Kaseda et al., 1995). It is possible that mares may select their consort partner stallions by evaluating them according to their relative abilities such as dominance (Feist and McCuliough, 1976; Berger, 1977; Miller, 1981) in some cases, aggressive behavior (Miller, 1981) or attentiveness to their mares (Rutberg, 1990). A stallion's abilities may change with advancing age or with the stallion's physical strength and experience. Change in harem size due to the stallion's age or change in sex ratio is considered the result of a relative evaluation of stallions by mares according to such selective criteria.
4.2. Relationships between harem size and reproductive success of stallions Previous authors have stated that stable consort relations between harem stallions and mares are usually kept for several months to years and almost all matings are exclusively done between them (Feist and McCullough, 1975; Klingel, 1982). Meanwhile, Miller (1981) reported that the band stallion bred in 18 of the 21 observed matings and the other three mares were bred by males from other bands. However, paternity tests with blood types were not done in these previous studies. In the present study, 15 of the 99 foals born in the stable harem groups were sired by other harem stallions. The present result supports, by paternity tests with blood types, the previous observations that some mares in the stable harem groups were mated by other harem stallions (Miller, 1981; McCort, 1984).
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Reproductive contributions over a lifetime are the best measure of a stallion's reproductive success (Berger, 1986). Changes in harem size and the number of foals sired over the lifetimes of three stallions were observed in the present study. Two stallions produced 24 and 25 foals for 10 and 11 years of reproductive lifespan, respectively. Another one produced only five foals in 6 years of reproductive lifespan. Berger (1986) reported that reproductive success was affected by age, body weight, fighting abilities, home-range locations and reproductive lifespan, and that the differences in reproductive success among six stallions between the ages of 7 and 11 years were great. In the present study, the differences in number of mares with which a stallion bad consort relations and number of foals sired were significant among seven stallions between the ages of 6 to 9 years. These differences in reproductive success are probably due to factors other than age, home-range or reproductive lifespan as these three factors were controlled in this comparison. A stallion's reproductive success is observed to relate positively to the number of mares in his harem (Klingel, 1982; Stevens, 1990). In this population too, there was a significant and positive correlation between the total number of mares with which the harem stallion had stable consort relations and the total number of foals sired throughout the observation period for seven stallions between the ages of 6 to 9 years. The results indicate that the larger the harem and the longer the tenure a stallion has, the higher his reproductive success. It is especially important for stallions to attract as many mares as possible and to keep stable consort relations with them as long as possible (Stevens, 1990). The number of foals sired increased with harem size under size 6, but not over size 6. This indicates that the reproductive success of stallions may be restricted if harem size is too large. The number of foals sired by the harem stallions and born in the harem were very few in spite of many harem mares over harem size 7, and some of the foals born in the harem were sired by other harem stallions. If a particular stallion monopolizes too many mares, it is highly possible that he could not sire so many offspring because he could not always prevent his rival stallions from mating with his mares in wild or feral circumstances. A certain optimum harem size may exist for the reproductive success of wild or feral equids.
Acknowledgements The authors wish to thank the Japan Equine Affairs Association for their support of the present study. This investigation was supported in part by a Grant-in-Aids for Research (No. 03660283) from the Ministry of Education, Science, and Culture in Japan.
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Berger, J., 1983b. Predation, sex ratios, and male competition in equids (Mammalia: Pressodactyla). J. Zool. Lond., 201: 205-216, Berger, J., 1986. Wild Horses of the Great Basin. Social Competition and Population Size. The University of Chicago Press, Chicago, pp. 89-95, 128-147, 196-231. Duncan, P., Feh, C., Gleize, J.C., Malkas, P. and Scott, A.M., 1984. Reduction of inbreeding in a natural herd of horses. Anim. Behav., 32: 520-527. Feist, J.D. and McCullough, D.R., 1975. Reproduction in feral horses. J. Reprod. Fert. Suppl., 23: 13-18. Feist, J.D. and McCuUough, D.R., 1976. Behaviour patterns and communication in feral horses. Z. Tierpsychol., 41: 337-371. Kaseda, Y., 1981. The structure of the groups of Misaki horses in Toi Cape. Jpn. J. Zootech. Sci., 52: 227-235. Kaseda, Y., 1983. Seasonal changes in the home range and the size of harem groups of Misaki horses. Jpn. J. Zootech. Sci., 54: 254-262. Kaseda, Y., 1991. Some factors affecting on the population dynamics of two herds in Misaki feral horses. Anim. Sci. Technol. (Jpn), 62:1171-1178. Kaseda, Y., Nozawa, K. and Mogi, K., 1982. Sire-foal relationships between harem stallions and foals in Misaki horses. Jpn. J. Zootech. Sci., 53: 822-830. Kaseda, Y., Khalil, A.M. and Ogawa, H., 1995. Harem stability and reproductive success of Misaki feral mares. Equine Vet, J., 27: 368-372. Klingel, H., 1969. Reproduction in the Plains zebra, Equus Burdielli Boehmi: Behaviour and ecological factors. J. Reprod. Fert. Suppl., 6: 339-345. Klingel, H., 1975. Social organization and reproduction in equids. J. Reprod. Fert. Suppl., 23: 7-11. Klingel, H., 1979. A comparison of the social organization of the equids. In: R.H. Denniston (Editor), Symposium on the Ecology and Behaviour of Wild and Feral Equids. University of Wyoming, Laramie, pp. 113-123. Klingel, H., 1982. Social organization of feral horses. J. Reprod. Fert. Suppl., 32: 89-95. McCort, W.D., 1984. Behavior of feral horses and ponies. J. Anita. Sci., 58: 493-499. Miller, R., 1979a. Interband dominance in feral horses. Z. Tierpsychol., 51: 41-47. Miller, R., 1981. Male aggression, dominance and breeding behavior in Red Desert feral horses. Z. Tierpsychol., 57:340-351. Penzhorn, B.L., 1984. A long-term study of social organization and behavior of Cape Mountain Zebras Equus zebra zebra. Z. Tierpsychol., 64: 97-146. Rutberg, A.T., 1990. Inter-group transfer in Assateague pony mares. Anim. Behav., 40: 945-952. Salter, R.E. and Hudson, R.J., 1982. Social organization of feral horses in western Canada. Appl. Anim. Ethol., 8: 207-223. Stevens, E.F., 1990. Instability of harems of feral horses in relation to season and presence of subordinate stallions. Behav., 112: 149-16l. Waring, G.H., 1983. Horse Behavior. The Behavioral Traits and Adaptations of Domestic and Wild Horses, Including Ponies. Noyes Publications, New Jersey, USA, pp. 142-147.