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Predictors of social dominance in the adult female golden hamster (Mesocricetus auratus)
Anim. Behav., 1973,21, 564-570
PREDICTORS OF SOCIAL DOMINANCE IN THE ADULT FEMALE GOLDEN HAMSTER (MESOCRICETUS AURA TUS ) By LEE C. DRICKAMER & JOHN G. VANDENBERGH Research Division, North Carolina Department of Mental Health, Raleigh, North Carolina
Abstract. Adult female golden hamsters exhibit smaller and less pigmented flank glands than do males. Nevertheless, as has been found in male hamsters, variations in these parameters of the flank glands correlate highly (rs-----0-78) with social rank attained in a group of four females. Ovariectomy and subsequent replacement with a graded series of testosterone propionate doses produces a directly related response in the flank gland and in the social rank attained in all-female groups (r,=0-72). Body weight of females also correlates directly with social status (r,=0.74), but, when body weight is held constant, social rank can still be predicted from measures of the flank gland. Oestrous-related fluctuations in aggressive behaviour of females did not alter dominance relationships. hypothesis that dominance hierarchies among female hamsters would be unstable because, every fourth day, the dominant female would be unaggressive and presumably incapable of defending its status.
In the previous paper, we have demonstrated that the social rank of a male golden hamster in a group of four males can be predicted by its body weight and the state of its flank gland (Drickamer, Vandenbergh & Colby 1973). When body weight is held constant, differences in the condition of the flank gland among individuals are significantly correlated with social dominance. The size and pigmentation of the flank gland are androgen dependent (Vandenbergh 1973) and thus, the social rank of a male can be directly related to its endogenous androgen level. Female hamsters also possess paired flank glands and, although smaller and lighter in appearance than in the male, variation in the size and pigmentation of the female gland can be measured. The size and pigmentation of the female flank gland is androgen sensitive and unaffected by estrogen (Hamilton & Montagna 1950; Vandenbergh 1973). The current study was designed to test whether variation in the female's flank gland could be used as a predictor of social rank in a manner similar to that already demonstrated in males. The female hamster possesses a regular 4-day oestrous cycle (Orsini 1961; Payne & Swanson 1970). For 3 days of this cycle the female hamster is highly aggressive when paired with either a male or female member of the species (Payne & Swanson 1970). On the fourth, or oestrous, day of the cycle the female hamster is much less aggressive and will promptly lordose when approached or stimulated by a conspecific of either sex. The concordanc~ between the oestrous cycle and aggression in the female hamster permitted us also to test the
Experiment I This study was designed to: (1) assess the importance of body weight and flank-gland size and pigmentation in predicting social status among adult female hamsters, and (2) to study the stability of social hierarchies in groups of intact female hamsters. Methods Twenty female hamsters between 120 and 200 days of age were assigned at random to five groups of four animals each, with the restriction that the maximum age difference within any group was 10 days. Each hamster was housed individually from weaning at 21 days of age until the experiment began. The behavioural observations were conducted in a 1-1 • 2.2-m plywood pen with four equal-sized compartments and doorways connecting each pen with the adjoining two pens (see Drickamer et al. 1973). Food and water were supplied ad libitum in all compartments. The hamsters and the observation pen were housed in a room maintained at 22 ~ to 26~ and 20 to 70 per cent relative humidity. The light cycle consisted of 14 hr of bright light (day) and 10 hr of dim light (night). The paired lateral sebaceous glands on the rear flanks of male and female golden hamsters are androgen dependent (Vandenbergh 1973; Drickamer et al., 1973). Ovariectomy of the 564
DRICKAMER ET AL.: SOCIAL DOMINANCE IN FEMALE GOLDEN HAMSTERS female hamster does not affect the flank gland's size or pigmentation, indicating the glands are not dependent upon ovarian hormones. By combining measures of the size and pigmentation of the flank gland, we have created a 'flank gland index' which reflects, as a bioassay, the amount of circulating androgen in the hamster (see Drickamer et al. 1973 for particulars of the gland index). Each of the five groups of four female hamsters were exposed to the same 12-day experimental procedure. On days 0, 4, and 12 the animals were weighed and the size and pigmentation of their flank glands was determined. No differences were observed in either body weight or the flank gland parameters during this 12-day period. Each day a wet-mount vaginal lavage was taken for each of the four females being tested in order to determine the stage in the oestrous cycle. Also, each day, each test female was placed with a proven male until lordosis behaviour was exhibited, or 30 s elapsed, whichever occurred first. In every instance where a female exhibited cytological oestrus she also showed behavioural oestrus when placed with a male. On day 0 the hamsters were placed in the observation pen, one per compartment with the interconnecting doors dosed. For 4 days the females were allowed to accommodate to the pen and one oestrous cycle was completed by each female. No attempt was made to synchronize oestrous cycles of the four females in any of the experimental groups. On day 5 the doors were all opened simultaneously and the hamsters were allowed to interact continuously for 8 days. During this period from
565
day 5 to day 12, two 89 observations were made daily yielding a total of sixteen 30-rain observations during the 8-day experimental period. Three hours separated the observations on a given day. During each observation period agonistic interactions among the four hamsters were scored. For each interaction the participants and the outcome of the interaction were recorded. All types of agonistic interactions were classed together, that is without respect to behavioural category (e.g. chase, attack as described by Vandenbergh 1971). All observations of flank-marking behaviour were also recorded. Results
In each of the five test groups a stable linear dominance hierarchy was formed within 4 hr. Throughout the 8 days for each test group there were no reversals in dominance status for the first and second ranking females. For third and fourth ranking animals, position reversals occurred in two of the five groups but the reversal of status did not occur on the losing female's oestrous day. The dominant females won an average of 93-5 per cent of their encounters, while fourth ranking hamsters won an average of only 6.2 per cent of their interactions (Table I). Each of the social ranks was associated with a significantly greater winning percentage (Duncan's New Multiple Range Test). In three of the five test groups the heaviest hamster was dominant. On the average, the weights of the first- to third-ranking hamsters were not significantly different from one another,
Table I. Correlates of Social Rank in Five Groups of Four Female GoldenHamsters. The Body Weight of Each of the Four Hamsters in a Group was Allowed to Vary Randomly
Social rank
Mean body weight (+1 sE)
1
[ 128 (3)
2 3
1122(5)] . 119 (3)
4
I 112 (5)
I
Mean gland index (+1 sE)
Mean percentage wins (4-1 s~)
Mean flank marks (4-1 s~)
366 (16) 310 (21)
[ 93"5(4"0) [ 44-9 (10-2)
[li 96 (16) 67 (27)
270 (32)
I 19.5(3.5)
26 (6)
246 (18)
[
6.2 ( 1.2)
7 (2)
F=
2"94
5.91
43.8
6.83
df=
3, 16
3, I6
3, 16
3, 16
<0"01
<0-01
P
0.05
<0"01
Those means not connected by the same vertical line are significantlydifferent at the P<0.05 level.
566
ANIMAL
BEHAVIOUR,
while the second- to fourth-ranking animals also were not significantly different (Table I). The F-ratio for weight by rank (F----2.94) had a probability of 0.05
21,
3
ordinate females, only 40 per cent lordosed when approached by another female even though all had shown lordosis behaviour when tested with a male. For this experiment the inter-observer reliability was r, ~-0"96 for the observations of social behaviour and r, = 0.92 for the flank gland measurements.
Experiment H Experiment I demonstrated that the flank gland index and body weight were both significant predictors of social status. In this experiment we test the notion that variations in the flank glands of females and correlated variations in their social rank can be produced by injectinggraded doses of testosterone propionate (TP) into ovariectomized females. If our hypothesis is correct, we should be able to predict the dominance hierarchy on the basis of the dose levels of TP. We would also expect the female flank gland to show a graded response in size and pigmentation when a graded series of TP doses are administered. Methods Twenty adult female golden hamsters were reared under husbandry conditions similar to those used in experiment I until they were 180 days of age, at which time they were each bilaterally ovariectomized. After a 6 to 10-week recovery period they entered the experimental sequence, The twenty females were apportioned at random into five groups of four animals each with the restriction that the maximum weight difference within any group was 10 g. Following surgical recovery each hamster was given a 3-week series of subcutaneous 0.05ml injections of TP in sesame oil. Every other day each hamster in a test group received one of three different hormone doses; (1) 0.1 mg TP; (2) 0.5 mg TP; (3) 1.0 mg TP; or (4) the vehicle only. Injections began 3 weeks prior to testing in the social pen and continued until the conclusion of the experiment. Three weeks after the injections began the hamsters were placed, one per compartment, in the observation pen of experiment I with the interconnecting doors closed. After a 3-day accommodation period all doors were opened simultaneously and the females were allowed to interact continuously for 3 days. During these 3 days the animals were observed for two 89 periods daily, totalling 3 hr of observation per test group.
DRICKAMER ET AL. : SOCIAL DOMINANCE IN FEMALE GOLDEN HAMSTERS The body weights and flank gland measurements were taken on each group before and after the 6-day period in the observation pen. Beforeand-after measurements of weight or gland indices did not differ. The techniques for recording the behavioural interactions were identical to those of experiment I. Results
In experiment I, untreated females had mean flank gland indices ranging from 246 to 366 (Table I). After 3 weeks of TP injections those females in experiment II receiving the oil vehicle had an average gland index o f 269 (-4-26). The average gland indices were 315(4- 28), 434 (4- 30) and 503 (4- 23), respectively for the hormone doses of 0-1, 0-5 and 1.0 mg TP. In each of the five test groups a linear dominance hierarchy was formed within 2 to 4 hr of opening the compartment doors. The dominant females won an average of 94.7 per cent of all their interactions, while fourth ranking hamsters won only 4.8 per cent of their encounters (Table II). An analysis o f variance and a New Multiple Range test showed that there was a significant difference between the first and second ranking females in the average percentage of fights won, while the third and fourth ranking animals did not differ significantly in this category (Table II). Of the five females receiving 1-0 mg TP, three were dominant and the other two ranked second in their respective groups. Three of the females receiving 0.5 mg TP ranked second in their groups, the remaining two being dominants.
567
All five hamsters which received 0.1 mg TP were third ranking, and all five which received the vehicle only were fourth ranking in their groups. Gland indices o f hamsters ranked first or second did not differ significantly (468 versus 430), nor was there any significant difference between the average gland indices of the third and fourth ranking females (315 versus 269). First and second ranking females did, however, have significantly larger indices than those ranking third and fourth (Table II). The dominant female flank marked significantly more than any of the subordinate hamsters in each test group (Table II). There were no significant differences in the body weights of the females at the different ranks (Table II). For this experiment the correlation between body weight and the flank gland index was r s -------0.486, P < 0 . 0 5 . Body weight was not significantly correlated with the percentage o f interactions won, rs---- --0.224, P<0.20. There was a significant positive relationship between the gland index and the percentage of encounters won r, = -k0.716, P<0.01. Inter-observer reliability was r~ = 0.94 for the behavioural observations and r, = 0.90 for the flank gland measurements in experiment II. Discussion
The relationships between body weight, the flank gland index, and the percentage of interactions won are summarized for both experiments in Table IV. From experiment I we can
Table ]I. Correlates of Social Rank in Five Groups of Ovarieetomized Female Golden Hamsters Given Testosterone Propionate Injections
Social rank
Mean body weight (-4-1 sE)
Mean gland index (4-1 SE)
1
139 (4)
[ 468 (34)
I 94.7 (1.6)
I 159 (32)
2
141 (14)
[ 430 (40)
[ 50-8(9.1)
50 (15)
3
149 (8)
! 315 (28)
[ 17-2(2.7)
10 (5)
1
!
4
147 (6)
F=
0"4
3 "43
df =
3, 16
3, 16
P
Ns
Mean percentage wins (4-1 SE)
269 (26)
0"025
[
4"8 (0"9) 69.7 3, 16 <0"01
Mean flank marks (4-1 sE)
10 (5) 15.3 3, 16 <0.01
The four hamsters in each group were matched for body weight. Those means not connected by the same vertical line are significantly different at the P <0.05 level.
568
ANIMAL BEHAVIOUR,
conclude that in the female golden hamster both body weight and the flank index are predictors of social dominance. This predictability holds not only for the dominant hamster in a group, but for all the social positions among the four hamsters. The finding in experiment I that weight is correlated with dominance is in agreement with the earlier findings in female hamsters of Payne & Swanson (1970). Since the flank gland index varies directly with injected TP it is reasonable to assume that the index reflects endogenous androgen levels and that variations in social rank result from differences in endogenous androgen. Such an assumption may be premature, however, because endogenous androgen levels have not been measured in female hamsters and because Vandenbergh (1972) found that TP had no effect on aggressive behaviour of ovariectomized females when paired with an intact female. Payne & Swanson (1971) have also shown TP to be ineffective in increasing aggression of ovariectomized females paired with males. The most likely explanation of why TP had an effect on aggressive behaviour in the current experiment but failed to be effective in Payne & Swanson's (1971) and Vandenbergh's (1972) studies is the difference in the behaviour testing conditions. We observed animals for 3 or 8 days in groups of four in a large arena in which each animal had become accustomed to at least part of the space. In both previous studies the animals were observed in pairs for either 5 or 10 min (Payne & Swanson 1971; Vandenbergh 1972) in a neutral cage. Comparisons between different tests for aggression must be made with caution because levels of aggression can differ markedly depending upon the type of testing situation. Edwards (1969), for example, showed that TP-treated, ovariectomized adult female mice showed high levels of aggression when paired with a prepubertal male mouse in the female's home cage but that very little aggression resulted when two such TP-treated females were paired in a neutral cage. In order to separate the effects of weight from the relationship between the flank gland index and dominance status we used ovariectomized, matched-weight females in experiment II. By giving graded TP doses we were able to predict the social ranks of the females in each grou]9 of four. We also were able to produce a graded effect on the size and pigmentation of the flank gland by different TP dose levels.
21,
3
Post-hoc measurements of inguinal and anal glands in the rabbit and sternal glands in marmosets have shown significant positive correlations with dominance status (Mykytowycz & Dudzinski 1966; Mykytowycz 1970; Epple 1970). In the companion paper we have demonstrated that the size and pigmentation of the male hamster's flank gland can be used to predict social status (Drickamer et al. 1973). The present finding that differences in female flank glands can also be used to predict social status in groups of female hamsters suggests a relationship between dominance, level of aggression, and an index of androgen levels in an adult female mammal. Previous workers have investigated masculinizing effects of androgens in infant female mammals (Levine & Mullins 1966; Goy 1966) but none have explored the relationships between androgens and social status of females. Edwards (1969) has shown that injecting TP into adult, ovariectomized females can or cannot increase aggression depending on the testing procedure, but the role androgens play in the social relations between females living in groups for more than a brief testing session had not been previously investigated. In both female experiments there were significant relationships between body weight and the flank gland index. These relationships differ, however, in direction (Table III). In experiment I body weight and the gland index were positively related. That is, in intact female hamsters the state of the flank gland, is in part, a function Table IH. Correlations Between Body Weight (Wt), Flank Gland Index (GI), and the Percentage of Encounters Won in Female Hamsters Under Two Different Experimental Regimes
Condition
Wt. vs GI Wt. vsyowin GI vs ~owin
Random weight TP treated
0-671 --0.486
0.748
0.775
--0.224
0.716
of the body weight of the animal. Further work will be required to ascertain the full nature of this relationship, e.g. use of a wide range of body weights (80 to 200 g) to determine over what range this relationship holds. In our experimental random weight groups' the range of body weight was only 108 to 134 g. In experiment II body weight and the gland index were inversely correlated. Thus, even though the
DRICKAMER ET AL. : SOCIAL DOMINANCE IN FEMALE GOLDEN HAMSTERS females that received the vehicle only weighed slightly more at the time of testing than those receiving high doses of TP, the females receiving vehicle only ranked fourth in each group. This suggests that the hormone may be a more important factor than body weight in determining dominance status. The explanation of the higher body weights for females receiving the vehicle only lies in the combined effects of the ovariectomy and lack of testosterone treatment on metabolism and fat deposition. In both experiments reported here the dominant hamster marked significantly more than the subordinates, although in experiment I there was no difference, on the average, between the first and second ranking females in their marking behaviour. This finding of increased flank marking related to higher social status is in agreement with the general hypothesis stated by Ralls (1971) relating dominance and marking behaviour in mammals. One of our original hypotheses was that female golden hamsters would form unstable dominance hierarchies /~s a result of changes associated with the ovarian cycle. The data demonstrate that, contrary to the hypothesis, female hamsters do form stable social hierarchies. Even when in oestrus, the dominant female retains her rank. This is accomplished by several behavioural mechanisms. First, the dominant female invariably lordosed when approached from the side or rear on her oestrous day. This lordosis posture inhibited all aggressive behaviour directed toward the dominant at this period. When approached from the front on her oestrous day, the dominant would either lordose as described, or attack and chase the subordinate, thereby maintaining her rank. Dominant females won the same percentage of encounters on oestrous and non-oestrous days. The lordosis posture was not aggressioninhibiting for all females. Subordinates were attacked by the dominant or other higher ranking females even while in the lordosis position. No female, however, lost rank on any oestrous day. A second manner in which the dominant retained her rank was through the control she exerted on the overall aggressive activity of the social group. Aggressive behaviours occurred with significantly less frequency on those days when dominant females were in oestrus as compared with aggressive interactions on days when the dominant was not in oestrous.
569
Male--Female Comparisons
In the previous paper and the present one we have examined predictors of dominance in both sexes of the golden hamster. To assess the relative levels of aggression in the two sexes we conducted a three-way analysis of variance with one measure repeated (Bruning & Kintz 1968). The two sets of non-repeated major variables were: (1) the two sexes, and (2) the two experimental conditions common to these two studies, i.e. random weight groups and groups given hormone treatment with TP. The repeated variable was time. Data from day 1 and day 3 of the observation period for each group were used to determine whether there was any decline in aggressive behaviour as the stability of the hierarchy was established. The dependent variable was the number of agonistic interactions which occurred per hour of observation per group o f four hamsters. The mean interaction rates for these various treatment combinations are presented in Table IV. Table IV. Mean Frequency of Aggressive Interactions (~:1 sn)/Hr of Observation/Test Group of Four Animals for Adult Male and Female Golden Hamsters Under Two Different Experimental Conditions
(Means are given for both Day 1 and Day 3 of the social interaction period.) Aggression/hour Day 1
Day 3
Female Random weight TP treated
407 (96) 326 (44)
84 (37) 113 (41)
Male Random weight TP treated
519 (109) 465 (24)
172 (58) 212 (32)
The analysis of variance indicated: (1) a significant drop in the level of aggression from day 1 to day 3 of the social interaction period, F = 47.57; df= 1, 16; P<0-01 ; (2) a significant sex difference in the level of aggression, F = 5.93, df---- 1, 16; P < 0 . 0 5 with males having a higher rate of aggressive interaction than females (Table IV); (3) no significant differences between the random weight groups and the hormone treatment groups, F = 0.82; df= 1, 16, P < 0 - 2 0 ; and (4) no significant two-way or three-way interaction effects among the variables tested. In both sexes tested under conditions where body weight was permitted to vary at random we found that both body weight and the flank gland index of the hamsters were significant
570
ANIMAL
BEHAVIOUR,
p r e d i c t o r s o f social status. I n females there was a significant b o d y w e i g h t - g l a n d index relationship which will require further examination. N o significant w e i g h t - g l a n d relationship was f o u n d for male hamsters. I n b o t h sexes matched-weight, g o n a d e c t o m ized animals were used to test the hypothesis t h a t exogenous a n d r o g e n was a valid p r e d i c t o r o f social status. I n b o t h sexes we were able to predict social status on the basis o f the T P doses administered. Finally, when we c o m p a r e d the levels o f aggression within unisexual social g r o u p s o f m a l e a n d female hamsters across treatments we f o u n d t h a t males showed a significantly higher aggressive interaction rate p e r h o u r o f observation. I n b o t h sexes there was a significant d r o p in the i n t e r a c t i o n rate f r o m d a y 1 to d a y 3 o f the observation period. A s the social hierarchy stabilized, r a n k was ' r e c o g n i z e d ' within the social g r o u p , a n d less active r e i n f o r c e m e n t o f r a n k was required.
Acknowledgments T h i s p a p e r was s u p p o r t e d in p a r t b y g r a n t M H - 1 6 8 7 0 f r o m the U n i t e d States Public H e a l t h Service. T h e senior a u t h o r was a N S F Postd o c t o r a l fellow at N o r t h C a r o l i n a State University on g r a n t GZ-1374 to D r D. E. Davis. W e t h a n k M r D a v i d C o l b y for his technical assistance a n d f o r reading the manuscript.
REFERENCES Bruning, J. L. & Kintz, B. L. (1968). Computational Handbook of Statistics. Glenview, Illinois: ScottForesman Inc. Drickamer, L. C., Vandenbergh, J. G. & Colby, D. R. (1973). Predictors of social dominance in the male golden hamster (Mesocricetus auratus). Anim. Behav., 21, 557-563.
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3
Edwards, D. A. (1969). Early androgen stimulation and aggressive behavior in male and female mice. PhysioL & Behav., 4, 333-338. Epple, G. (1970). Quantitative studies on scent marking in the marmoset (Callithrix jacchus). Folia PrimatoL, 13, 48-62. Goy, R. W. (1966). Role of androgens in the establishment and regulation of behavioral sex differences in mammals. J. Anim. Sci., 25, 21-35. Hamilton, J. B. & Montagna, W. (1950). The sebaceous glands of the hamster. I. Morphological effects of androgens on integumentary structures. Am. J. Anat., 86, 191-232. Levine, S. & Mullins, R. F. (1966). Hormonal influences on brain organization in infant rats. Science, N.Y., 152, 1585-1592. Mykytowycz, R. (1970). The role of skin glands in mammalian communication. In: Communication by Chemical Signals (Ed. by J. W. Johnston, D. G. Moulton and Amos Turk), pp. 327-360. New York: Appleton-Century-Crofts. Mykytowycz, R. & Dudzinski, M. L. (1966). A study of the weight of odoriferous and other glands in relation to social status and degree of sexual activity in the wild rabbit, Oryetolagus cuniculus (L.). CSIRO Wildl. Res., 11, 31-47. Orsini, M. W. (1961). The external vaginal phenomena characterizing the stages of the estrous cycle, pregnancy, pseudopregnancy, lactation and the anestrous hamster, Meseocrieetus auratus Waterhouse. Proc. Anita. Care Panel 11, 193-206. Payne, A. P. & Swanson, H. H. (1970). Agonistic behaviour between pairs of hamsters of the same and opposite sex in a neutral observation area. Behaviour, 36, 259-269. Payne, A. P. & Swanson, H. H. (1971). Hormonal control of aggressive dominance in the female hamster. PhysioL & Behav., 6, 355-357. Rails, K. (1971). Mammalian scent marking. Science, N.Y., 171, 443-449. Siegel, S. (1956). Nonparametric Statistics for the Behavioral Sciences, New York: McGraw-HiU. Vandenbergh, J. G. (1971). The effects of gonadal hormones on the aggressive behaviour of adult golden hamsters. Anita. Behav., 19, 585-590. Vandenbergh, J. G. (1973). Effects of gonadal hormones on the flank gland of the golden hamster. Hormone Res., 4, 28-33. (Received 11 January 1972; revised 20 April 1972; MS. number: M274)
PREDICTORS OF SOCIAL DOMINANCE IN THE ADULT FEMALE GOLDEN HAMSTER (MESOCRICETUS AURA TUS ) By LEE C. DRICKAMER & JOHN G. VANDENBERGH Research Division, North Carolina Department of Mental Health, Raleigh, North Carolina
Abstract. Adult female golden hamsters exhibit smaller and less pigmented flank glands than do males. Nevertheless, as has been found in male hamsters, variations in these parameters of the flank glands correlate highly (rs-----0-78) with social rank attained in a group of four females. Ovariectomy and subsequent replacement with a graded series of testosterone propionate doses produces a directly related response in the flank gland and in the social rank attained in all-female groups (r,=0-72). Body weight of females also correlates directly with social status (r,=0.74), but, when body weight is held constant, social rank can still be predicted from measures of the flank gland. Oestrous-related fluctuations in aggressive behaviour of females did not alter dominance relationships. hypothesis that dominance hierarchies among female hamsters would be unstable because, every fourth day, the dominant female would be unaggressive and presumably incapable of defending its status.
In the previous paper, we have demonstrated that the social rank of a male golden hamster in a group of four males can be predicted by its body weight and the state of its flank gland (Drickamer, Vandenbergh & Colby 1973). When body weight is held constant, differences in the condition of the flank gland among individuals are significantly correlated with social dominance. The size and pigmentation of the flank gland are androgen dependent (Vandenbergh 1973) and thus, the social rank of a male can be directly related to its endogenous androgen level. Female hamsters also possess paired flank glands and, although smaller and lighter in appearance than in the male, variation in the size and pigmentation of the female gland can be measured. The size and pigmentation of the female flank gland is androgen sensitive and unaffected by estrogen (Hamilton & Montagna 1950; Vandenbergh 1973). The current study was designed to test whether variation in the female's flank gland could be used as a predictor of social rank in a manner similar to that already demonstrated in males. The female hamster possesses a regular 4-day oestrous cycle (Orsini 1961; Payne & Swanson 1970). For 3 days of this cycle the female hamster is highly aggressive when paired with either a male or female member of the species (Payne & Swanson 1970). On the fourth, or oestrous, day of the cycle the female hamster is much less aggressive and will promptly lordose when approached or stimulated by a conspecific of either sex. The concordanc~ between the oestrous cycle and aggression in the female hamster permitted us also to test the
Experiment I This study was designed to: (1) assess the importance of body weight and flank-gland size and pigmentation in predicting social status among adult female hamsters, and (2) to study the stability of social hierarchies in groups of intact female hamsters. Methods Twenty female hamsters between 120 and 200 days of age were assigned at random to five groups of four animals each, with the restriction that the maximum age difference within any group was 10 days. Each hamster was housed individually from weaning at 21 days of age until the experiment began. The behavioural observations were conducted in a 1-1 • 2.2-m plywood pen with four equal-sized compartments and doorways connecting each pen with the adjoining two pens (see Drickamer et al. 1973). Food and water were supplied ad libitum in all compartments. The hamsters and the observation pen were housed in a room maintained at 22 ~ to 26~ and 20 to 70 per cent relative humidity. The light cycle consisted of 14 hr of bright light (day) and 10 hr of dim light (night). The paired lateral sebaceous glands on the rear flanks of male and female golden hamsters are androgen dependent (Vandenbergh 1973; Drickamer et al., 1973). Ovariectomy of the 564
DRICKAMER ET AL.: SOCIAL DOMINANCE IN FEMALE GOLDEN HAMSTERS female hamster does not affect the flank gland's size or pigmentation, indicating the glands are not dependent upon ovarian hormones. By combining measures of the size and pigmentation of the flank gland, we have created a 'flank gland index' which reflects, as a bioassay, the amount of circulating androgen in the hamster (see Drickamer et al. 1973 for particulars of the gland index). Each of the five groups of four female hamsters were exposed to the same 12-day experimental procedure. On days 0, 4, and 12 the animals were weighed and the size and pigmentation of their flank glands was determined. No differences were observed in either body weight or the flank gland parameters during this 12-day period. Each day a wet-mount vaginal lavage was taken for each of the four females being tested in order to determine the stage in the oestrous cycle. Also, each day, each test female was placed with a proven male until lordosis behaviour was exhibited, or 30 s elapsed, whichever occurred first. In every instance where a female exhibited cytological oestrus she also showed behavioural oestrus when placed with a male. On day 0 the hamsters were placed in the observation pen, one per compartment with the interconnecting doors dosed. For 4 days the females were allowed to accommodate to the pen and one oestrous cycle was completed by each female. No attempt was made to synchronize oestrous cycles of the four females in any of the experimental groups. On day 5 the doors were all opened simultaneously and the hamsters were allowed to interact continuously for 8 days. During this period from
565
day 5 to day 12, two 89 observations were made daily yielding a total of sixteen 30-rain observations during the 8-day experimental period. Three hours separated the observations on a given day. During each observation period agonistic interactions among the four hamsters were scored. For each interaction the participants and the outcome of the interaction were recorded. All types of agonistic interactions were classed together, that is without respect to behavioural category (e.g. chase, attack as described by Vandenbergh 1971). All observations of flank-marking behaviour were also recorded. Results
In each of the five test groups a stable linear dominance hierarchy was formed within 4 hr. Throughout the 8 days for each test group there were no reversals in dominance status for the first and second ranking females. For third and fourth ranking animals, position reversals occurred in two of the five groups but the reversal of status did not occur on the losing female's oestrous day. The dominant females won an average of 93-5 per cent of their encounters, while fourth ranking hamsters won an average of only 6.2 per cent of their interactions (Table I). Each of the social ranks was associated with a significantly greater winning percentage (Duncan's New Multiple Range Test). In three of the five test groups the heaviest hamster was dominant. On the average, the weights of the first- to third-ranking hamsters were not significantly different from one another,
Table I. Correlates of Social Rank in Five Groups of Four Female GoldenHamsters. The Body Weight of Each of the Four Hamsters in a Group was Allowed to Vary Randomly
Social rank
Mean body weight (+1 sE)
1
[ 128 (3)
2 3
1122(5)] . 119 (3)
4
I 112 (5)
I
Mean gland index (+1 sE)
Mean percentage wins (4-1 s~)
Mean flank marks (4-1 s~)
366 (16) 310 (21)
[ 93"5(4"0) [ 44-9 (10-2)
[li 96 (16) 67 (27)
270 (32)
I 19.5(3.5)
26 (6)
246 (18)
[
6.2 ( 1.2)
7 (2)
F=
2"94
5.91
43.8
6.83
df=
3, 16
3, I6
3, 16
3, 16
<0"01
<0-01
P
0.05
<0"01
Those means not connected by the same vertical line are significantlydifferent at the P<0.05 level.
566
ANIMAL
BEHAVIOUR,
while the second- to fourth-ranking animals also were not significantly different (Table I). The F-ratio for weight by rank (F----2.94) had a probability of 0.05
21,
3
ordinate females, only 40 per cent lordosed when approached by another female even though all had shown lordosis behaviour when tested with a male. For this experiment the inter-observer reliability was r, ~-0"96 for the observations of social behaviour and r, = 0.92 for the flank gland measurements.
Experiment H Experiment I demonstrated that the flank gland index and body weight were both significant predictors of social status. In this experiment we test the notion that variations in the flank glands of females and correlated variations in their social rank can be produced by injectinggraded doses of testosterone propionate (TP) into ovariectomized females. If our hypothesis is correct, we should be able to predict the dominance hierarchy on the basis of the dose levels of TP. We would also expect the female flank gland to show a graded response in size and pigmentation when a graded series of TP doses are administered. Methods Twenty adult female golden hamsters were reared under husbandry conditions similar to those used in experiment I until they were 180 days of age, at which time they were each bilaterally ovariectomized. After a 6 to 10-week recovery period they entered the experimental sequence, The twenty females were apportioned at random into five groups of four animals each with the restriction that the maximum weight difference within any group was 10 g. Following surgical recovery each hamster was given a 3-week series of subcutaneous 0.05ml injections of TP in sesame oil. Every other day each hamster in a test group received one of three different hormone doses; (1) 0.1 mg TP; (2) 0.5 mg TP; (3) 1.0 mg TP; or (4) the vehicle only. Injections began 3 weeks prior to testing in the social pen and continued until the conclusion of the experiment. Three weeks after the injections began the hamsters were placed, one per compartment, in the observation pen of experiment I with the interconnecting doors closed. After a 3-day accommodation period all doors were opened simultaneously and the females were allowed to interact continuously for 3 days. During these 3 days the animals were observed for two 89 periods daily, totalling 3 hr of observation per test group.
DRICKAMER ET AL. : SOCIAL DOMINANCE IN FEMALE GOLDEN HAMSTERS The body weights and flank gland measurements were taken on each group before and after the 6-day period in the observation pen. Beforeand-after measurements of weight or gland indices did not differ. The techniques for recording the behavioural interactions were identical to those of experiment I. Results
In experiment I, untreated females had mean flank gland indices ranging from 246 to 366 (Table I). After 3 weeks of TP injections those females in experiment II receiving the oil vehicle had an average gland index o f 269 (-4-26). The average gland indices were 315(4- 28), 434 (4- 30) and 503 (4- 23), respectively for the hormone doses of 0-1, 0-5 and 1.0 mg TP. In each of the five test groups a linear dominance hierarchy was formed within 2 to 4 hr of opening the compartment doors. The dominant females won an average of 94.7 per cent of all their interactions, while fourth ranking hamsters won only 4.8 per cent of their encounters (Table II). An analysis o f variance and a New Multiple Range test showed that there was a significant difference between the first and second ranking females in the average percentage of fights won, while the third and fourth ranking animals did not differ significantly in this category (Table II). Of the five females receiving 1-0 mg TP, three were dominant and the other two ranked second in their respective groups. Three of the females receiving 0.5 mg TP ranked second in their groups, the remaining two being dominants.
567
All five hamsters which received 0.1 mg TP were third ranking, and all five which received the vehicle only were fourth ranking in their groups. Gland indices o f hamsters ranked first or second did not differ significantly (468 versus 430), nor was there any significant difference between the average gland indices of the third and fourth ranking females (315 versus 269). First and second ranking females did, however, have significantly larger indices than those ranking third and fourth (Table II). The dominant female flank marked significantly more than any of the subordinate hamsters in each test group (Table II). There were no significant differences in the body weights of the females at the different ranks (Table II). For this experiment the correlation between body weight and the flank gland index was r s -------0.486, P < 0 . 0 5 . Body weight was not significantly correlated with the percentage o f interactions won, rs---- --0.224, P<0.20. There was a significant positive relationship between the gland index and the percentage of encounters won r, = -k0.716, P<0.01. Inter-observer reliability was r~ = 0.94 for the behavioural observations and r, = 0.90 for the flank gland measurements in experiment II. Discussion
The relationships between body weight, the flank gland index, and the percentage of interactions won are summarized for both experiments in Table IV. From experiment I we can
Table ]I. Correlates of Social Rank in Five Groups of Ovarieetomized Female Golden Hamsters Given Testosterone Propionate Injections
Social rank
Mean body weight (-4-1 sE)
Mean gland index (4-1 SE)
1
139 (4)
[ 468 (34)
I 94.7 (1.6)
I 159 (32)
2
141 (14)
[ 430 (40)
[ 50-8(9.1)
50 (15)
3
149 (8)
! 315 (28)
[ 17-2(2.7)
10 (5)
1
!
4
147 (6)
F=
0"4
3 "43
df =
3, 16
3, 16
P
Ns
Mean percentage wins (4-1 SE)
269 (26)
0"025
[
4"8 (0"9) 69.7 3, 16 <0"01
Mean flank marks (4-1 sE)
10 (5) 15.3 3, 16 <0.01
The four hamsters in each group were matched for body weight. Those means not connected by the same vertical line are significantly different at the P <0.05 level.
568
ANIMAL BEHAVIOUR,
conclude that in the female golden hamster both body weight and the flank index are predictors of social dominance. This predictability holds not only for the dominant hamster in a group, but for all the social positions among the four hamsters. The finding in experiment I that weight is correlated with dominance is in agreement with the earlier findings in female hamsters of Payne & Swanson (1970). Since the flank gland index varies directly with injected TP it is reasonable to assume that the index reflects endogenous androgen levels and that variations in social rank result from differences in endogenous androgen. Such an assumption may be premature, however, because endogenous androgen levels have not been measured in female hamsters and because Vandenbergh (1972) found that TP had no effect on aggressive behaviour of ovariectomized females when paired with an intact female. Payne & Swanson (1971) have also shown TP to be ineffective in increasing aggression of ovariectomized females paired with males. The most likely explanation of why TP had an effect on aggressive behaviour in the current experiment but failed to be effective in Payne & Swanson's (1971) and Vandenbergh's (1972) studies is the difference in the behaviour testing conditions. We observed animals for 3 or 8 days in groups of four in a large arena in which each animal had become accustomed to at least part of the space. In both previous studies the animals were observed in pairs for either 5 or 10 min (Payne & Swanson 1971; Vandenbergh 1972) in a neutral cage. Comparisons between different tests for aggression must be made with caution because levels of aggression can differ markedly depending upon the type of testing situation. Edwards (1969), for example, showed that TP-treated, ovariectomized adult female mice showed high levels of aggression when paired with a prepubertal male mouse in the female's home cage but that very little aggression resulted when two such TP-treated females were paired in a neutral cage. In order to separate the effects of weight from the relationship between the flank gland index and dominance status we used ovariectomized, matched-weight females in experiment II. By giving graded TP doses we were able to predict the social ranks of the females in each grou]9 of four. We also were able to produce a graded effect on the size and pigmentation of the flank gland by different TP dose levels.
21,
3
Post-hoc measurements of inguinal and anal glands in the rabbit and sternal glands in marmosets have shown significant positive correlations with dominance status (Mykytowycz & Dudzinski 1966; Mykytowycz 1970; Epple 1970). In the companion paper we have demonstrated that the size and pigmentation of the male hamster's flank gland can be used to predict social status (Drickamer et al. 1973). The present finding that differences in female flank glands can also be used to predict social status in groups of female hamsters suggests a relationship between dominance, level of aggression, and an index of androgen levels in an adult female mammal. Previous workers have investigated masculinizing effects of androgens in infant female mammals (Levine & Mullins 1966; Goy 1966) but none have explored the relationships between androgens and social status of females. Edwards (1969) has shown that injecting TP into adult, ovariectomized females can or cannot increase aggression depending on the testing procedure, but the role androgens play in the social relations between females living in groups for more than a brief testing session had not been previously investigated. In both female experiments there were significant relationships between body weight and the flank gland index. These relationships differ, however, in direction (Table III). In experiment I body weight and the gland index were positively related. That is, in intact female hamsters the state of the flank gland, is in part, a function Table IH. Correlations Between Body Weight (Wt), Flank Gland Index (GI), and the Percentage of Encounters Won in Female Hamsters Under Two Different Experimental Regimes
Condition
Wt. vs GI Wt. vsyowin GI vs ~owin
Random weight TP treated
0-671 --0.486
0.748
0.775
--0.224
0.716
of the body weight of the animal. Further work will be required to ascertain the full nature of this relationship, e.g. use of a wide range of body weights (80 to 200 g) to determine over what range this relationship holds. In our experimental random weight groups' the range of body weight was only 108 to 134 g. In experiment II body weight and the gland index were inversely correlated. Thus, even though the
DRICKAMER ET AL. : SOCIAL DOMINANCE IN FEMALE GOLDEN HAMSTERS females that received the vehicle only weighed slightly more at the time of testing than those receiving high doses of TP, the females receiving vehicle only ranked fourth in each group. This suggests that the hormone may be a more important factor than body weight in determining dominance status. The explanation of the higher body weights for females receiving the vehicle only lies in the combined effects of the ovariectomy and lack of testosterone treatment on metabolism and fat deposition. In both experiments reported here the dominant hamster marked significantly more than the subordinates, although in experiment I there was no difference, on the average, between the first and second ranking females in their marking behaviour. This finding of increased flank marking related to higher social status is in agreement with the general hypothesis stated by Ralls (1971) relating dominance and marking behaviour in mammals. One of our original hypotheses was that female golden hamsters would form unstable dominance hierarchies /~s a result of changes associated with the ovarian cycle. The data demonstrate that, contrary to the hypothesis, female hamsters do form stable social hierarchies. Even when in oestrus, the dominant female retains her rank. This is accomplished by several behavioural mechanisms. First, the dominant female invariably lordosed when approached from the side or rear on her oestrous day. This lordosis posture inhibited all aggressive behaviour directed toward the dominant at this period. When approached from the front on her oestrous day, the dominant would either lordose as described, or attack and chase the subordinate, thereby maintaining her rank. Dominant females won the same percentage of encounters on oestrous and non-oestrous days. The lordosis posture was not aggressioninhibiting for all females. Subordinates were attacked by the dominant or other higher ranking females even while in the lordosis position. No female, however, lost rank on any oestrous day. A second manner in which the dominant retained her rank was through the control she exerted on the overall aggressive activity of the social group. Aggressive behaviours occurred with significantly less frequency on those days when dominant females were in oestrus as compared with aggressive interactions on days when the dominant was not in oestrous.
569
Male--Female Comparisons
In the previous paper and the present one we have examined predictors of dominance in both sexes of the golden hamster. To assess the relative levels of aggression in the two sexes we conducted a three-way analysis of variance with one measure repeated (Bruning & Kintz 1968). The two sets of non-repeated major variables were: (1) the two sexes, and (2) the two experimental conditions common to these two studies, i.e. random weight groups and groups given hormone treatment with TP. The repeated variable was time. Data from day 1 and day 3 of the observation period for each group were used to determine whether there was any decline in aggressive behaviour as the stability of the hierarchy was established. The dependent variable was the number of agonistic interactions which occurred per hour of observation per group o f four hamsters. The mean interaction rates for these various treatment combinations are presented in Table IV. Table IV. Mean Frequency of Aggressive Interactions (~:1 sn)/Hr of Observation/Test Group of Four Animals for Adult Male and Female Golden Hamsters Under Two Different Experimental Conditions
(Means are given for both Day 1 and Day 3 of the social interaction period.) Aggression/hour Day 1
Day 3
Female Random weight TP treated
407 (96) 326 (44)
84 (37) 113 (41)
Male Random weight TP treated
519 (109) 465 (24)
172 (58) 212 (32)
The analysis of variance indicated: (1) a significant drop in the level of aggression from day 1 to day 3 of the social interaction period, F = 47.57; df= 1, 16; P<0-01 ; (2) a significant sex difference in the level of aggression, F = 5.93, df---- 1, 16; P < 0 . 0 5 with males having a higher rate of aggressive interaction than females (Table IV); (3) no significant differences between the random weight groups and the hormone treatment groups, F = 0.82; df= 1, 16, P < 0 - 2 0 ; and (4) no significant two-way or three-way interaction effects among the variables tested. In both sexes tested under conditions where body weight was permitted to vary at random we found that both body weight and the flank gland index of the hamsters were significant
570
ANIMAL
BEHAVIOUR,
p r e d i c t o r s o f social status. I n females there was a significant b o d y w e i g h t - g l a n d index relationship which will require further examination. N o significant w e i g h t - g l a n d relationship was f o u n d for male hamsters. I n b o t h sexes matched-weight, g o n a d e c t o m ized animals were used to test the hypothesis t h a t exogenous a n d r o g e n was a valid p r e d i c t o r o f social status. I n b o t h sexes we were able to predict social status on the basis o f the T P doses administered. Finally, when we c o m p a r e d the levels o f aggression within unisexual social g r o u p s o f m a l e a n d female hamsters across treatments we f o u n d t h a t males showed a significantly higher aggressive interaction rate p e r h o u r o f observation. I n b o t h sexes there was a significant d r o p in the i n t e r a c t i o n rate f r o m d a y 1 to d a y 3 o f the observation period. A s the social hierarchy stabilized, r a n k was ' r e c o g n i z e d ' within the social g r o u p , a n d less active r e i n f o r c e m e n t o f r a n k was required.
Acknowledgments T h i s p a p e r was s u p p o r t e d in p a r t b y g r a n t M H - 1 6 8 7 0 f r o m the U n i t e d States Public H e a l t h Service. T h e senior a u t h o r was a N S F Postd o c t o r a l fellow at N o r t h C a r o l i n a State University on g r a n t GZ-1374 to D r D. E. Davis. W e t h a n k M r D a v i d C o l b y for his technical assistance a n d f o r reading the manuscript.
REFERENCES Bruning, J. L. & Kintz, B. L. (1968). Computational Handbook of Statistics. Glenview, Illinois: ScottForesman Inc. Drickamer, L. C., Vandenbergh, J. G. & Colby, D. R. (1973). Predictors of social dominance in the male golden hamster (Mesocricetus auratus). Anim. Behav., 21, 557-563.
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3
Edwards, D. A. (1969). Early androgen stimulation and aggressive behavior in male and female mice. PhysioL & Behav., 4, 333-338. Epple, G. (1970). Quantitative studies on scent marking in the marmoset (Callithrix jacchus). Folia PrimatoL, 13, 48-62. Goy, R. W. (1966). Role of androgens in the establishment and regulation of behavioral sex differences in mammals. J. Anim. Sci., 25, 21-35. Hamilton, J. B. & Montagna, W. (1950). The sebaceous glands of the hamster. I. Morphological effects of androgens on integumentary structures. Am. J. Anat., 86, 191-232. Levine, S. & Mullins, R. F. (1966). Hormonal influences on brain organization in infant rats. Science, N.Y., 152, 1585-1592. Mykytowycz, R. (1970). The role of skin glands in mammalian communication. In: Communication by Chemical Signals (Ed. by J. W. Johnston, D. G. Moulton and Amos Turk), pp. 327-360. New York: Appleton-Century-Crofts. Mykytowycz, R. & Dudzinski, M. L. (1966). A study of the weight of odoriferous and other glands in relation to social status and degree of sexual activity in the wild rabbit, Oryetolagus cuniculus (L.). CSIRO Wildl. Res., 11, 31-47. Orsini, M. W. (1961). The external vaginal phenomena characterizing the stages of the estrous cycle, pregnancy, pseudopregnancy, lactation and the anestrous hamster, Meseocrieetus auratus Waterhouse. Proc. Anita. Care Panel 11, 193-206. Payne, A. P. & Swanson, H. H. (1970). Agonistic behaviour between pairs of hamsters of the same and opposite sex in a neutral observation area. Behaviour, 36, 259-269. Payne, A. P. & Swanson, H. H. (1971). Hormonal control of aggressive dominance in the female hamster. PhysioL & Behav., 6, 355-357. Rails, K. (1971). Mammalian scent marking. Science, N.Y., 171, 443-449. Siegel, S. (1956). Nonparametric Statistics for the Behavioral Sciences, New York: McGraw-HiU. Vandenbergh, J. G. (1971). The effects of gonadal hormones on the aggressive behaviour of adult golden hamsters. Anita. Behav., 19, 585-590. Vandenbergh, J. G. (1973). Effects of gonadal hormones on the flank gland of the golden hamster. Hormone Res., 4, 28-33. (Received 11 January 1972; revised 20 April 1972; MS. number: M274)