Squirrel monkey dominance and social behavior as related to group size and group structure

Adult male and female squirrel monkeys were tested in nonsocial adaptation and pairwise and triad social situations differing in sex composition. Social behaviors, nonsocial behaviors, and dominance hierarchies were observed during social testing. Dominance hierarchies were similar in groups differing in size and social structure. Nonsocial behaviors decreased in females and submissive animals paired with males or dominant monkeys. Aggressiveness between females decreased and the beginnings of coalitions between females were observed in the presence of a male. The social behavior patterns, but not dominance hierarchies, are consistent with behaviors observed in larger groups of squirrel monkeys.     

Rank,relationships, and responses to intruders among adult male vervet monkeys

We examined the influences of dyadic relationships among captive adult male vervet monkeys (Cercopithecus aethiops sabaeus) on behavior directed toward caged “intruder” males placed inside subjects' enclosures. Subjects were all 9 adult male residents from three stable social groups, each of which contained 3 adult males, at least 3 adult females, and their immature offspring. Every male was observed in two 3-hour sessions, each time with one of the 2 other adult males from his group. Observation sessions consisted of six consecutive 30-min stages in which group composition and the presence of the intruder were manipulated. All groups exhibited a stable, linear male dominance hierarchy prior to and throughout the study. In each group, there was one pair of males, when together, in which each member exhibited higher rates of intruder-directed approach and aggressive behaviors than when either animal was paired with the third male of his social group. Such pairs were also distinguished by high levels of within-pair agonistic interactions. The higher-ranking member of each dyad was the most aggressive male toward the intruder in his social group, although only one of these animals was the dominant male of his group. Mutual facilitation of aggression against intruding males is interpreted as cooperative behavior benefitting both males by increasing the likelihood of repelling a potential competitor for resident females. Such cooperation provides further evidence in nonhuman primates for cohesive male-male dyads between animals whose social interactions are characterized by agonism. © 1993 Wiley-Liss, Inc.     

Effects of estradiol benzoate, estrone, and propionates of testosterone or dihydrotestosterone on sexual and related behaviors of ovariectomized rhesus monkeys

Ovariectomized adult rhesus monkeys were injected daily for 10 days with either 1 mg of dihydrotestosterone propionate (DHTP), 1 mg of testosterone propionate (TP), 10 μg of estradiol benzoate (EB), or 500 μg of estrone (El). On the 5th and 10th days of treatment, females received two 24-min behavioral tests with each of two adult males. All females received every hormonal treatment during the course of the study, with the order of treatments counterbalanced. Prior to the initiation of an hormonal treatment, each subject received two tests with no hormone treatment (NORX). Three behaviors related to female proceptivity were recorded. Treatment with DHTP had no influence on any aspect of proceptivity measured, in comparison to the NORX condition, whereas El or TP treatment augmented the frequencies of two of the proceptive behaviors and EB increased all three. The response of the male toward the female was influenced by the female's hormonal condition. Treatment with TP or DHTP did not increase the frequency of male contact or the mount rate in comparison to the NORX condition, whereas EB or El treatment did. In addition, DHTP was the only steroid which failed to increase the percentage of tests with intromission or ejaculation when compared to NORX. Female receptivity, as measured by acceptance or rejection of male contacts, was not different for the NORX-, TP-, EB-, or El-treated conditions. DHTP treatment, however, reduced female receptivity in comparison to all other conditions. Treatment with DHTP or TP resulted in an increase in the frequency of female yawning behavior, whereas neither estrogen treatment showed any effect on this behavior. The influences of TP on female proceptive and male sexual behavior were never duplicated or even approximated by treatment of females with the nonaromatizable DHTP. Nor was there any evidence that TP inhibited female receptivity below the level characteristic of NORX females, as was true for DHTP.     

Social organization of captive monandrous squirrel monkey groups (Saimiri sciureus).

1 adult male and 4 adult female squirrel monkeys were observed together as a group, isolated from all other monkeys. 3 of the 4 females were deafened for a previous experiment. Deafening, however, had no apparent, permanent effect on social behavior. Social dominance hierarchy was evaluated in a variety of situations. The results were compared with those of a similar set of observations on the females prior to the introduction of the male. Before the male was introduced, the dyadic interactions involving food stealing, body grasping, and sexual behaviors were indicative of a female linear rank order. After the male was introduced, the rank order among the females generally remained intact, with the male becoming the highest ranking member in the group. The noteworthy exception to the stability involved the highest ranking female, whose position in the hierarchy was threatened. Heterosexual interactions predominated. Homosexual behavior was also observed, although appreciably reduced in frequency as compared to the all-female group situation. A similar rank order hierarchy was observed in a second group of squirrel monkeys comprised of 1 adult male and 4 adult females. None of these monkeys was deaf.     

The influence of rank on affect perception by pigtailed macaques (Macaca nemestrina)

Previous studies of affect perception in nonhuman primates have involved young animals and/or manipulations of early experience. Based upon data suggesting that middle-ranked monkeys in small social groups show patterns of behavior different from their low- or high-ranked counterparts, the current study examined the role of social rank in affect perception by normally reared, adult female pigtailed macaques. Employing color videotapes as the presentation medium, we observed animals as they watched unfamiliar animals display social (aggressive or submissive) or nonsocial behavior. Virtually all threats were recorded as the subjects watched the submissive presentations, and most submissive signals occurred while watching aggressive behavior tapes. Middle-ranked monkeys were most attentive during all presentations, and levels of disturbance behavior were related to rank of the watcher and type of presentation observed. The results suggest that dominance status is an influence in the process of affect perception.     

Dominance index: A simple measure of relative dominance status in primates

A simple measure of relative dominance status (cardinal rank) is described which we have termed the dominance index. Like more familiar techniques for assessing rank order, it is based on the direction of aggressive and submissive behaviors between all possible paired combinations of animals in a social group. Using data from five groups of female rhesus monkeys, it reliably produced the same ordinal ranks as fight interaction matrices. There was also good agreement with the cardinal ranks produced by two additional measures of dominance and with those produced by observer ratings. The dominance index can be calculated when fights have not actually occurred and is largely independent of the frequency of agonistic interactions. It has, therefore, wide application and can estimate dominance during brief sampling periods (one hour) and also in stable groups when agonistic interactions are low. Its application is described in experiments in which the male in a group of females was changed and the hormonal status of the females was altered. Estrogen increased female dominance status relative to other females.     

Quantitative analysis of small group interaction in rhesus monkeys

The effect of role relations on social interaction in small groups of rhesus monkeys was examined quantitatively, through use of an interval scale of social behavior. Concurrent pair interactions were tabulated for all possible combinations in 24 triad groups: Six groups containing three males; six groups containing three females; six groups of one male, two females; and six groups of two males and one female. A specific behavior which involves participation of all three animals at once, the incitement interaction, was found to quantitatively alter each monkey's reaction to other social behaviors displayed at the same time. The interaction demonstrates the monkeys' attention to and manipulation of role relations. In all triad types one could quantitatively document the fact that the most subordinate monkey established a role with the most dominant monkey which allowed it some freedom to behave less submissively when interacting with the middle ranking monkey. This general phenomenon in role relations may contribute to group cohesiveness.     

An analysis of dyadic interactions of male Japanese monkeys (Macaca fuscata fuscata) in a cage-room observation

This is a laboratory observation (Home-cage and observation room) for individual and social behavior of four male Japanese monkeys. The monkeys were placed in a dyadic and trio situation with the purpose to investigate the relationships between aggressive and affinitive factors which regulated their social behaviour. Total amounts of attacks and receiving groom showed a consistent order for four monkeys through the pairwise conditions, which was presumed to indicate a dominance order. Grooming and mounting behavior, however, were influenced by the particular dyadic interactions, probably social attraction, rather than dominance order itself. The presence of the third animal facilitated such a social preference between animals positioned closely in the rank order as well as elicited aggression from the dominant animals.     

Dominance, cortisol, and behavior in small groups of female cynomolgus monkeys (Macaca fascicularis)

The relationships among social rank, basal cortisol concentrations, and social behavior were assessed in adult female cynomolgus monkeys (Macaca fascicularis). Subjects were 157 unrelated, reproductively intact animals housed in 30 small groups. Rank determinations were made monthly. Blood samples were collected on two occasions, 4.5 and 7.5 months following initial group formation. Regular behavioral observations were conducted on a subset of animals over a period of 4 weeks, 9 months following group formation. Analyses revealed that serum cortisol values were significantly correlated across the two sampling periods, with no significant change in absolute values. While social rank was positively correlated across both samples, there was no relationship between rank and cortisol. However, dominant and subordinate animals did differ in the rates of performance of aggressive and submissive behaviors. These data suggest that social rank does not influence baseline serum cortisol in adult female cynomolgus monkeys, despite stability in measures of rank and cortisol and the presence of the usual behavioral differences between dominants and subordinates.     

Competition for limited environmental resources on the social dominance model in laboratory mice

Asymmetry of social rank in the competition for food and female was studied using the social dominance model with only two male mice. Marking activity was recorded as a useful indicator of the social status. Social rank was determined by asymmetry in aggressive behavior. A food test was presented for 10 min daily within 5 days of the experiment, whereas a sexual test was performed only on the 5th day for 30 min. Marking behavior was estimated twice: before the first interaction and on the 4th day of the experiment. The competition for food was accompanied by active attacks, escapes, vertical defense postures, and sniffing. The level of aggression, sniffing, and food activity was higher in dominant than submissive males. Time course of aggressive, defensive, and sniffing behaviors was characterized by maximum scores in the period of formation of social hierarchy; however, the rate of food activity in this period was low and increased only to the 4th day. Introduction of a receptive female into the male group with the stable social hierarchy stimulated the intermale aggression, defensive and sniffing behaviors. Dominant males were characterized by a greater number of victories over and sniffing contacts with both male and female. Marking activity was also more intense in dominants. Thus, significant unidirectional rank differences in agonistic, sniffing, food, sexual, and marking behaviors were shown on the social dominance model with the minimum number of partners.     

Genealogical and cross-genealogical dominance relations in a group of free-ranging rhesus monkeys (Macaca mulatta) on Cayo Santiago

Observations of dominance relations in a large group of rhesus monkeys on Cayo Santiago were carried out over a period of 25 months. Dyadic interactions in which an aggressive gesture in one individual was followed by a submissive gesture in the other were recorded as fights and considered reliable indices of dominance. The analysis revealed the following characteristics: (1) Maternal dominance over female offspring; (2) maternal dominance over male offspring up the age six years at which time the son leaves his natal group or remains in the group and rises in rank over his mother; (3) dominance of older brothers over younger male siblings until the age of five years at which time the younger brother rises in rank; (4) rank reversal between sisters when the younger sister reaches the age of three to four years; (5) brother-sister relative rank dependency on age until the male sibling reaches three to four years at which time he rises in rank; (6) linear dominance relations in the crossgenealogical dominance hierarchy; and (7) linear, but unstable, dominance relations in the adult male hierarchy. With few exceptions, the pattern of genealogical, cross-genealogical, and adult male dominance relations in the group under study was consistent with data reported for a small social group (group F) on Cayo Santiago and for Japanese macaques.     

Close bonds and social rank similarities favor non-random mating in captive stump-tailed macaques (Macaca arctoides)

Assortative mating is non-random mating by the mutual choice of phenotypes or behavioral types. In polygynandrous species, competition for mating by social rank can lead to assortative mating. However, although not an individual trait, social bonds also influence mating opportunities resembling assortative mating. Stump-tailed macaques form long-term close bonds and organize in a linear dominance–subordination hierarchy. Therefore, we studied whether the strength of the social bond and rank closeness influenced mating decisions and increased mating opportunities, particularly for low- and middle-ranking animals. Firstly, we observed whether females directed proceptive behavior to close-bonded or adjacent rank males. Secondly, we measured whether successful copulations were related to the strength of social bonds and close ranking. Thirdly, to ensure that copulations owed mainly to the aforementioned factors, we also evaluated whether sexual coercion was unrelated to social bonds and rank similarities. Finally, we assessed whether close bonds mediated agonistic support to females. The study subjects were 12 adult female and 11 male captive stump-tailed macaques. We monitored daily females' reproductive status by vaginal cytology. Sexual behavior was recorded by all occurrences sampling and scan sampling to collect the agonistic and affiliative instances required to calculate social ranks, social bond strength, and agonistic support. The results indicated that the probability of females displaying proceptivity increased during the follicular phase toward close-bonded and high-ranking males. Copulation chances increased with male–female social bonds and rank closeness. Forced copulation decreased in close-bonded individuals, while agonistic support increased in close-ranking strong-bonded animals. In conclusion, close social bonds and similar social rank result in non-random mating in stump-tailed macaques.     

Social dominance determines estrous entrainment among female hamsters

In order to study the phenomenon of estrous entrainment and its relation to social behavior two studies were conducted with female hamsters. In the first experiment single females were placed in close proximity, but not in direct contact with three other females whose estrous cycles were already synchronous. The single female was either in phase (control) or out of phase (experimental) with the synchronous females. In the second experiment two females at the same (control) or different (experimental) points in their estrous cycles lived together until entrainment occurred. Their social interactions during the initial 30 min in this shared habitat were observed and a dominance hierarchy established. Estrous stage was determined daily for each animal by vaginal lavage. The major findings of these studies are (1) estrous entrainment occurred in all female hamsters whether living in close proximity (Expt I), or living together (Expt II); (2) regardless of the stage of estrous at the start of cohabitation, hamsters' cycles may be either accelerated or decelerated depending on whether the partner's stage is ahead or behind their own; (3) social interaction appears to accelerate the entrainment process, and (4) when examined in light of social dominance, the behaviorally submissive animals entrained to the behaviorally dominant animals.     

Relation between the dominance rank of female rhesus monkeys and their access to males

Systematic observations were made on 12 measures of the sexual, aggressive, and social interactions of 24 male–female pairs of rhesus monkeys in six social groups, each consisting of one male and four ovariectomized females tested in a large room. Each female in a group was treated in turn first with estradiol alone and then with estradiol and progesterone in combination. When hormone-treated, the female was also observed during pair tests with the male in the same large observation room (four males, eight females, 240 group tests, 240 pair tests). The dominance ranks of females during group tests were determined post hoc by means of the dominance index [Zumpe & Michael, American Journal of Primatology 10:291–300, 1986]. In all six groups, the most dominant female virtually monopolized the male, and the subordinate females' interactions with the male, assessed during pair tests, were almost completely suppressed during group tests. This “dominant female effect” was a robust phenomenon that depended solely on female dominance rank. It was independent of the identity and hormonal status of females and of the social preferences of males as expressed in pair tests. These findings demonstrate the existence of female mate competition in an Old World primate.     

Dominance and its Behavioral Measures in a Captive Group of Bonobos (Pan paniscus)

We investigated the existence of a social dominance hierarchy in the captive group of six adult bonobos at the Planckendael Zoo. We quantified the pattern of dyadic exchange of a number of behaviors to examine to what extent each behavior fits a linear rank order model. Following de Waal (1989), we distinguish three types of dominance: agonistic dominance, competitive ability and formal dominance. Fleeing upon aggression is a good measure of agonistic dominance. The agonistic dominance hierarchy in the study group shows significant and strong linearity. The rank order was: 1. female (22 yr), 2. female (15 yr)., 3. male (23 yr.), 4. female (15 yr.), 5. male (9 yr.), 6. male (10 yr.). As in the wild, the females occupy high ranks. There is prominent but nonexclusive female agonistic dominance. Teeth-baring does not fulfil the criteria of a formal submission signal. Peering is a request for tolerance of proximity. Since its direction within dyads is consistent with that of fleeing interactions, it is a useful additional measure to determine agonistic ranks in bonobos. In competitive situations, the females acquire more food than other group members do. The rank obtained from access to food resources differs from the agonistic rank due to female intrasexual social tolerance, expressed in food sharing. We typify the dominance styles in the group as female intrasexual tolerance and male challenging of rank differences. The agonistic rank order correlates significantly with age and has a strong predictive value for other social behaviors.     

Social organization and social behavior in two subspecies of squirrel monkeys (Saimiri sciureus).

Social organization and social behavior were examined in two subspecies of squirrel monkeys which differ markedly in the degree of sexual dimorphism. The Bolivian squirrel monkeys, the subspecies with greater sexual dimorphism, manifested a sexually segregated form of social organization, while the social organization of the Guyanese monkeys was sexually integrated. Dominance relationships were found to reflect these patterns of sexual segregation or integration; in the Bolivian social groups separate linear dominance hierarchies were established within each sex while the Guyanese monkeys established a single linear hierarchy which included both males and females. Relationships between males and females in the two subspecies appear to be regulated by two distinct mechanisms, dominance in the Guyanese monkeys and sexual segregation in the Bolivians.     

The reproductive correlates of social hierarchy in laboratory male mice

In laboratory male mice the effects of social hierarchy on hormonal and spermatogenic testicular function, accessory organs and testicular weights, sexual behaviour have been investigated using an experimental model of social hierarchy, which is characterised by a minimal size (two male mice) and 5 days period of social interactions. The social rank of the partners was detected by asymmetry in aggressive behaviour. Using the experimental condition, when the both partners have no preferences for exclusive use of area we demonstrated that there were no rank differences in the number of mounts and testicular testosterone content. Nevertheless a rank asymmetry in the male sniffing behaviour towards a receptive female, weights of the testes, seminal vesicles, epididymes and the number of epididymal sperm was kept up in a stable social group. Social dominance was found to affect negatively on testicular testosterone increase in response to introduction of a receptive female and sexual attractiveness of male to a receptive female in both dominant and subordinate males. The results obtained demonstrate the impact of social hierarchy on reproduction in laboratory male mice, particular in respect of spermatogenesis and the testicular testosterone in response to a receptive female.     

Subspecies differences in hormonal and behavioral responses after group formation in squirrel monkeys

The behavioral and hormonal responses of squirrel monkeys of the Bolivian and Guyanese subspecies were compared after a group formation procedure. Two groups of each subspecies, consisting of five females and three males (later reduced to two) were observed daily during the week of group formation and for nine weeks following removal of a single male from each group. Measures of plasma cortisol were examined in the females after the initial group formation and after the groups were reduced by one male. The levels of plasma testosterone were assessed in all the males during the initial week of group formation. Linear dominance hierarchies were determined both within and across the sexes in both subspecies. The frequency and directionality of low-level aggressive interactions indicated that females of the Bolivian subspecies were dominant to the males, while the males of the Guyanese subspecies ranked over all the females. Additionally, the Bolivian squirrel monkey females showed an elevation of plasma cortisol on the day of group formation, which declined 48 hr later, then reelevated after the groups were reduced by one male and declined gradually over a nine-week period. Guyanese females showed little change in cortisol levels during both periods. This suggests fundamental hormonal, as well as behavioral, differences between the two subspecies. The change in plasma testosterone levels in the males during the initial week of group formation was positively correlated with social status. Furthermore, differences in the dynamics within individual groups for each subspecies were reflected by the levels of plasma cortisol of the female members.     

The changes in the dominance hierarchy over time of a complete field-captured colony of Cryptomys hottentotus hottentotus

The common mole-rat, Cryptomys h. hottentotus , is a social subterranean rodent occurring in colonies in which one female and one to three males are involved in reproduction and the remaining colony members are non-reproductive. Within each sex the reproductive animals are usually the largest and most dominant animals.
The dominance hierarchy amongst a field-captured colony was linear ( h = 0.95, calculated from Landau's linearity index) soon after capture. The non-reproductive females were ranked low in the dominance hierarchy; many were subordinate to non-reproductive males. The order of capture of mole-rats was not related to the position in the dominance hierarchy. The hierarchy became non-linear ( h = 0.56) after six months in captivity during which two juvenile animals became adult. The breakdown in the hierarchy may result from the lack of opportunity in captivity for animals to disperse and establish satellite colonies, or from colony members becoming co-dominant in the hierarchy as a result of a rise in rank by young animals.
Dominant mole-rats are involved in a greater proportion of interactive behaviours than subordinates. Popularity studies show that females tend to be more popular animals than males. The largest reproductive male was the least popular animal in the first study, whereas a beta male was the least popular animal in the second study period. The reproductive female was the most popular in both periods.     

Agonistic behavior and dominance relationships in female Phayre's leaf monkeys -- preliminary results

Socioecological theory suggests a link between the strength of competition for food/safety, rates of agonism, structure of dominance hierarchies, and dispersal among group-living females. This study presents preliminary data on agonistic behavior and dominance relationships for female Phayre's leaf monkeys (Trachypithecus phayrei), a species in which females routinely disperse. Behavioral observations were conducted on two groups (four adult females, and five adult females plus two juvenile females, respectively) at Phu Khieo Wildlife Sanctuary, northeast Thailand. Rates of agonistic behavior were analyzed from focal continuous recordings, while dominance hierarchies were constructed from all agonistic behaviors (focal and ad libitum sampling). Overall, female-female agonistic behaviors (aggression, submission, and displacements) occurred at a rate of < 0.25 interactions per hour. Agonistic interactions involving food occurred more frequently than expected based on feeding time. Females in both groups exhibited linear dominance hierarchies with some reversals, and possibly an age-inversed hierarchical structure in the larger group. The results fit well with previous results for colobine monkeys regarding frequency of interactions, displacements predominating agonistic behavior, and the possibility of an age-inversed hierarchy. The results contradict the suggested link between linearity of hierarchies and female philopatry. Future studies should consider the notion that female dispersal may coexist with linear dominance hierarchies.