Crowding, reproduction, and maternal behavior in the golden hamster

BEHAVIORAL BIOLOGY 23, 477--486 (1978)

Crowding, Reproduction, and Maternal Behavior in the Golden Hamster MILTON

DIAMOND 1 AND MARIAN MAST

Department of Reproductive Biology, University of Hawaii, School of Medicine, Honolulu, Hawaii 96822 H a m s t e r s , h o u s e d in groups c o m p o s e d of one, two, four, or six females, with two males per group, were o b s e r v e d for a period o f 60 days. T h e effect of such groupings on reproductive s u c c e s s was a s s e s s e d by determining interference with p r e g n a n c y , lactation, maternal behavior, and litter survival. While there was little interference with the o n s e t or course of pregnancy, a m a r k e d effect of group size on n u r s i n g behavior and litter survival occurred. T h e s e effects increased over the c o u r s e of the e x p e r i m e n t so that second litters were less likely to be nursed and to survive in the larger groups. T h e results appeared to be attributable to a breakd o w n in maternal behavior as well as to the killing of pups by nonlactating female h a m s t e r s . It is significant that the o u t c o m e of this experiment, reduced litter survival, parallels the effects that h a v e been reported for other rodents, since different m e c h a n i s m s are involved in the case of the hamster.

High population density (crowding) has been shown by both laboratory and field studies to produce pregnancy failure and aberrant sexual behavior in various rodent species (Brain, 1971; Calhoun, 1949, 1961, 1962a, b; Louch, 1956, 1958; Christian, 1961, 1963; Christian and Davis, 1964). For mice this reproductive decline has been generally attributed to alteration of the pituitary-gonadal axis due to stressful conditions that provoke high levels of ACTH release and a decrease in crucial gonadotropins and sex steroids (Brain, 1971; Christian and Davis, 1964; Christian et al., 1965). The stress of crowding appears to be due to excessively high rates of social interaction and particularly to encounters associated with the establishment and maintenance of dominance. Behaviorally, crowding has been associated with decreases in sexual and maternal behavior (Brain, 1971; Calhoun, 1949, 1961; Christian, 1961). The latter effects, coupled with reduction in successful pregnancy, appear to underly the self-limitation in populations of rats and mice (Christian, 1975). 1 W e would like to e x p r e s s appreciation to Dr. T h o m a s A b r u n z o for his assistance with the observations of the h a m s t e r s . This s t u d y was supported by Grant No. H D 05179 of the National Institutes of Health a n d in part by the Ford F o u n d a t i o n Grant 660-0202A. 477 0091-6773/78/0234-0477502.00/0 Copyright (~) 1978 by Academic Press, Inc. All rights of reproduction in any form reserved.

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The effect of crowding on reproductive efficiency and litter survival in the hamster has received little detailed attention although the unique endocrine and behavioral characteristics of this species would appear to make such a study especially valuable, particularly in probing the generality of the ACTH mechanism. The female hamster is a highly aggressive animal and may thus be better able to withstand aggression stress. Although both reduced fighting (Brain, 1972) and poor reproductive success (Louch, 1956; Yaron et al., 1963) have been reported in grouped female hamsters, these appear to be unrelated to increases in ACTH or to changes in sex steroids (Brain, 1972). Therefore, if group housing results in reproductive failure in the hamster it must be on the basis of a mechanism different than that which is generally accepted as underlying reproductive decline in the mouse. In their natural habitat hamsters are primarily solitary animals (EiblEibesfeldt, 1953) unlikely to be in close contact with female conspecifics during pregnancy and lactation but sometimes tolerating the presence of the male (Dieterlen, 1959). In contrast to the more common rodents, the female hamster is larger and more aggressive than the male (Dieterlen, 1959; Lawlor, 1963; Payne and Swanson, 1970). This generally high level of aggressive behavior in the female may be particularly important during reproduction. An analysis of the endocrine basis of aggression in the cycling female hamster has indicated a possible role of progesterone in her maintainance of dominance over the male and her success in encounters with other females (Payne and Swanson, 1971, 1972). The high level of progersterone secretion found during pregnancy in the hamsters and the sharp drop just before parturition and low levels during lactation (Lukaszewska and Greenwald, 1970; Leavitt and Blaha, 1970) might be expected to affect the female's aggressive behavior, increasing her aggression during pregnancy and making her particularly vulnerable to attack during parturition and lactation. Descriptions of the hamster female's aggressive behavior during these periods have generally indicated increased aggression during pregnancy (Rowell, 1961; Yaron et al., 1963). Leonard (1970) reported an increase in aggression during pregnancy but no drop at parturition, as might be expected if progesterone level was the major determinant. Wise (1974) reported increased aggression in late pregnancy (progesterone levels high) and especially during lactation (when progesterone is low) but not during pseudopregnancy (when progersterone is high). The relationship between progesterone levels and aggression was thus called into question. Wise instead suggested that prolactin, which is elevated during late pregnancy and parturition but not during pseudopregnancy, may play a role in the increased aggression found. Marques and Valenstein (1977), however, found no increase in aggression during pregnancy or postpartum in male-female hamster pairs when they were housed together for extended periods. Since Wise (1974) used a brief test session

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with individually housed animals, the difference may be due to the duration of grouping. It seems likely that a highly aggressive female might be particularly stressed by grouping during pregnancy or postpartum, but the precise manner in which this aggressive tendency may interact with the effects of chronic grouping are unclear. Marques and Valenstein (1977) reported a very low rate of litter survival among heterosexually paired hamsters despite an absence of increased aggression. In the single study which has so far examined the effect of varying sized groups on reproductive success in the hamster, Yaron et al. (1963), handling animals similar to us, found relatively little effect on litter survival when a single male was left with the female for mating, pregnancy, and lactation. When more than one female was present, however, few litters were delivered and none survived. Their study, unfortunately, presents insufficient detail to assess the mechanism of pregnancy failure or to determine the nature of the reported reduction in maternal care. Quite possibly the litter losses were due primarily to pup killing by the other hamsters. Unlike the rat and mouse, the virgin or pregnant hamster has been reported to cannibalize pups on first introduction although they may subsequently assume a nursing posture (Richards, 1966; Siegel and Greenwald, 1975). The male hamster, on the other hand, does not cannibalize pups and instead typically exhibits parenting behavior (Marques and Valenstein, 1977). A general review of maternal behavior in rats, hamsters, and mice has appeared (Noirot, 1972). The present experiment was designed to determine the effects on pregnancy, sexual behavior, and litter survival of housing hamsters in groups of varying sizes. Continuous and spaced observations of the reproductive condition and behavior of the hamsters were made to suggest the manner in which these factors might influence procreation and maternal behavior.

SUBJECTS AND PROCEDURE Mature female hamsters, all having completed one pregnancy, were randomly assigned to groups containing either one, two, four, or six females, with two males per cage. Females were 5 months old and males were 3 to 5 months of age at the onset of the study. All groups were tested for 60 days. All groups were housed in wire cages (11 cm high x 60 cm long x 30 cm wide) with wood-chip bedding. All cages were provided with food and water a d lib. Cages were cleaned once per week with a minimum of disturbance to the animals. Daily over the 60-day period, each group was observed for 2 hrs continuously. These observations were made under red-light illumination sometime between 8 AM and 12 noon, during the dark portion of their reversed light cycle (dark: 6 AM to 6 PM). During this time females were checked for vaginal indications of their estrous cycle or pregnancy. Rec-

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ord was made of lordosis and mating if observed during this period. Those females who were expected to be in estrus, either on the basis of their vaginal cycle or because they showed the typical estrous vaginal discharge, but were not observed mating were also checked for sperm. Body weights were taken periodically to help determine pregnancy. Blood, if present in the vagina, was noted as indicating resorption. When no litter was found on the expected date of delivery but blood was found in the cage or in large amounts in the vagina coupled with a weight loss, it was taken to indicate that parturition had occurred and that the pups had been cannibalized. When litters were found, the number of pups was noted and the mothers were observed to determine whether maternal care (nest building, retrieval, and nursing) occurred. Unusual behavior patterns such as sharing litters and stealing of pups were noted. If not cannibalized, litters were left with the mothers until the next day and then removed. Those females with two or more surviving pups on the day following parturition were noted as having surviving litters. At any time during the first 46 days, those animals (mostly males) who were fatally injured were replaced. During the final 2 weeks no new animals were added since, as the observation period advanced, it b e c a m e apparent that new animals were invariably killed. Throughout the experiment, records were kept of pregnancy, resorption, vaginal bleeding, littering, nursing, killing of young, and individual body weight. The results were analyzed by use of nonparametric statistical tests (Siegel, 1956). After 60 days of observation, all animals were sacrificed, and adrenal and gonadal weights were recorded. The uteri of the females which were pregnant at the time were examined for embryos and resorption sites. R ES U LTS

The results of this study are summarized in Table 1 and Fig. 1. Most obvious and notable is that all females, regardless of group size, mated and bore litters. The most marked effects of the treatment differences appeared postpartum in regard to those litters nursed and surviving. These measures show a marked relationship between an increase in group size and failure to nurse (H = 12.82, P < 0.01) as well as the probability of litter destruction (H = 10.72, P < 0.02). In the more crowded groups (four or six females per cage) as the study progressed litters were frequently destroyed so quickly after delivery that they were not seen by the experimenter. Observation of the females whose litters were destroyed and their cages suggested that often no attempt was made to build a nest, to gather and protect the young, or to nurse. In addition to the intergroup differences in nursing and destruction of litters, there was a definite increase in severity of effect with time. Of all second litters, only 37% were nursed compared to 65% of first litters (X2 =

6 4 2 2

1 2 4 6

6 4 2 2

1 2 4 6

1 0 1 1

12 8 4 4

Total 6 ' s at start of e x p e r i m e n t

6 8 8 12

Total $ ' s

Total ~? deaths

a M a l e s killed after t h e first 46 d a y s were n o t replaced.

Number of groups

N u m b e r of ~ ' s p e r cage

Males

N u m b e r of groups

N u m b e r of 2 ' s p e r cage

Females

3.0 2.88 3.13 2.83

Mean pregnancies per 2

11 7 2 3

0 .12 .12 .33

83% 75% 87% 67%

Percentage p r e g n a n t at sacrifice

8 6 3 3

Total deaths

M e a n litters resorbed per 2

Total c~'s at e n d of e x p e r i m e n t ~

2.17 2.13 2.13 1.83

M e a n litters per ~

TABLE l Survival and Reproduction by G r o u p Size for F e m a l e s and Males

17% 65% 76% 100% P < 0.02

Percentage litters destroyed

1.33 1.5 1.5 1.5

M e a n d e a t h s p e r cage

92% 59% 28% 22% P < 0.01

Percentage litters nursed

482

DIAMOND AND MAST I00']

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GROUP SIZE AND REPRODUCTIVE SUCCESS

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The relationship between group size and littering, nursing, and litter survival.

5.8, P < 0.02), and the few females in multi-female group cages w h o delivered third litters within the 60-day period did not nurse at all. In a one-female cage in which a third litter was born nursing occurred normally. Destruction of litters occurred m o r e frequently as the duration of crowding was extended. Of first litters 62% were destroyed as c o m p a r e d with 77% of second litters (X2 = 4.2, P < 0.05). This overall increase basically reflects a higher rate of destruction a m o n g two-female and four-female cages. In the one-female cages the p e r c e n t a g e of litters destroyed remained low throughout the 60-day duration, and in the sixfemale cages all litters were destroyed f r o m the start (Fig. 1). While all females bore a litter initially, a t e n d e n c y toward a reduction of subsequent littering a m o n g the four-female and six-female groups compared to the smaller groups was seen (Fisher's exact test, P -- 0.10). Only five females had a second litter by the time of sacrifice at D a y 60 and the third litter data were too incomplete to determine whether this trend would have continued. The n u m b e r of days from introduction to the group until first delivery of a litter was similar in all but the six-female group. In the smaller groups all females but one conceived within the first two ovulatory cycles and carried the litter to term. In the six-female group, h o w e v e r , a delay in initial conception and successful pregnancy was noted. T h r e e of the 12 females delivered first litters which were not conceived until their fifth or sixth estrous cycles. As vaginal bleeding did not occur in the period before their first litters were born, resorption was not an obvious reason for reproductiv.e failure. W h e t h e r failure was due to interference with copulation, anovulation, or failure to implant is not known. Fetal resorption occurred only in the multiple-female groups, but its o c c u r r e n c e was rare in all groups (Table 1). Vaginal bleeding did not occur during any first p r e g n a n c y but was present in 21% of the second pregnancies and 34% of the third pregnancies. This a p p e a r e d to be m o r e a function of the duration

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of the crowding rather than its magnitude, since no systematic relationship to group size was found. A comparison of the number of normally developing fetuses as opposed to resorption sites among the large number of females who were in their third pregnancy at time of sacrifice indicated no significant group differences. Body weight change was difficult to evaluate in the females because of the large number of pregnancies at sacrifice. Over the course of the experiment only one female lost weight. This weight loss was due to injury. The pregnant females all gained weight. Evaluation of change in body weight of the males is complicated by the many replacements. Only two males lost weight and these were both from one-female cages. A comparison of adrenal and ovarian weights for the females in the various groups did not reveal meaningful relationships. Valid comparisons were difficult since at the time of sacrifice varying numbers were pregnant and at varying stages in pregnancy. Among the males, no significant group differences in adrenal weight were found; however, the large number of replacements made during the experiment make interpretation of these data difficult. The number of males killed by cagemates among the various sized groups was quite variable and seemingly independent of group size (Table 1). In some cages no males were killed while in others as many as five males were killed consecutively. While many more females than males were in use, only three females were killed during the experiment. However, females were frequently observed fighting and found injured. The female deaths all occurred within the first 10 days of being grouped and no more than one death occurred in each condition; in the two-female groups there were no deaths. It seems likely that the data are distorted minimally from replacements of these females (Table 1). While specifically looked for, and readily detected when present, copulatory mounting or other sexual behaviors were rarely seen. When observed no grossly abnormal activities were discernable. Our observation time extended for approximately 15% of the total time the animals are usually active (2 of 12 hr daily). DISCUSSION

It is significant that in the hamster crowding induces a relatively small effect on observed mating, conception, and implantation initially and only a rare increase in resorption. These results are in marked contrast to the great effect on reproduction postpartum. Grouping hamsters interferes markedly with maternal care and litter survival. These results seem to support the possibility that group housing during parturition and the early lactation period might be especially stressful even to the typically highly aggressive hamster. It is interesting to note that although sporadic fighting continued throughout the 60-day period, no females were killed after the

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first 10 days. This result is in agreement with previous findings (Brain, 1972; Wise, 1974) of higher levels of aggression among singly housed females than among those chronically group housed. It seems likely that some degree of dominance hierachy was achieved among the females early in the experiment. The hamster, however, is normally solitary, and the establishment of stable hierachies seems unlikely. The continued bursts of fighting that were observed may have been an indication of this instability. Much evidence indicates that it is not fighting per se but defeat and changes in dominance which are especially stressful (Calhoun, 1949; Christian and Davis, 1964). This reduction in female deaths with time contrasts with the continued killing of males found throughout the experiment in some groups. The males in the present experiment were relatively unimportant in terms of attacks on the females or on the young. They appeared to be primarily engaged in behavior to avoid the more aggressive females. These findings are congruent with the report by Marques and Valenstein (1977) of typical female dominance in malefemale pairs and of considerable variability on the part of the females in aggressiveness toward their male partners. The nonaggressive behavior of the males toward the pups is consistent with previous findings reporting no cannibalizing by males (Marques and Valenstein, 1976). The quality of maternal behavior appeared deficient in the larger groups and group size clearly affected nursing behavior in the present study. These findings support the Yaron et al. (1963) suggestion that defective maternal care resulted from housing in large groups. However, the degree to which pup killing can be directly attributed to the mothers themselves remains inconclusive. In the larger groups the mother was often seen running back and forth with the pups scattered and uncared for. Under these circumstances the pups were subject to predation by other adults or might eventually be killed by the specific mother of the litter, possibly because they seemed weak and sickly. The reason for the high level of activity on the part of the mother is unknown. Mothers were seen to cannibalize their own pups only on occasion and those of other females more often. The direct observations of pup deaths, however, were not sufficiently extensive to determine the percent of killing done by the mothers. The apparent cannibalization of whole litters often shortly after parturition would require that continual observation be made at the time of delivery to clarify the roles of maternal behavior and cannibalizing in the high rate of litter destruction. Group housing of hamsters during parturition and the postpartum period is clearly associated with high rates of litter destruction. This effect appears to be attributable, at least in part, to alteration in normal maternal care. These results extend the generality of previous findings of reproductive failure and defective maternal behavior in crowded rodents and

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indicate that these effects will o c c u r in a species in which crowding is not associated with increased A C T H or reduced sex steroid secretion (Brain, 1972). This suggests that results similar to those found in mice may be produced by various mechanisms. It is n o t e w o r t h y that unlike the rat and mouse (Calhoun, 1962a; L o u c h , 1956) the hamster under c r o w d e d conditions does not seem to exhibit aberrant sexual behaviors. The level of density which produces deleterious effects may be relative to a species' typical tendency toward colonial or solitary living. C r o w d e d living conditions have been reported to be associated with increased human infant mortality (Winsborough, 1965). If we assume high population density is a source of stress among humans, then we may expect a variety of problems of childbirth and abnormalities of the newborn (Davids e t a l . , 1961). Galle e t al. (1962) have reported that in humans high population density is associated with increased fertility coupled with reduced effectiveness of parental care. Thus, the self-corrective tendency w h e r e b y high population density per se reduces fertility among many rodents appears not to be the case in humans. In this respect the human m a y be more similar to the hamster, where little interference is found with the progress or duration of pregnancy but much disruption in postnatal care. The human, like the hamster, may be without strong behavioral and physiological mechanisms which automatically regulate pregnancy relative to population density but may have a sensitivity in maternal mechanisms.

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