Fitness-related benefits of dominance in primates.

Dominance hierarchies are thought to provide various fitness-related benefits to dominant individuals (e.g., preferential access to food or mating partners). Remarkably, however, different studies on this topic have produced contradictory results, with some showing strong positive association between rank and fitness (i.e., dominants gain benefits over subordinates), others weak associations, and some others even revealing negative associations. Here, we investigate dominance-related benefits across primate species while controlling for phylogenetic effects. We extracted data from 94 published studies, representing 25 primate species (2 lemur species, 4 New World monkeys, 16 Old World monkeys, and 3 apes), to assess how dominance affects life-history and behavior. We used standard and phylogenetic meta-analyses to analyze the benefits of dominance in primates. Dominant females had higher infant survival to first year, although we found no significant effect of dominance on female feeding success. Results for female fecundity differed between the two meta-analytical approaches, with no effect of dominance on female fecundity after controlling for phylogeny. Dominant males had a higher fecundity and mating success than subordinate males. Finally, the benefits of dominance for female fecundity were stronger in species with a longer lifespan. Our study supports the view that dominance hierarchies are a key aspect of primate societies as they indeed provide a number of fitness-related benefits to individuals.

Early maternal rejection and later social anxiety in juvenile and adult Japanese macaques.

This study investigated the relationships between early maternal style and subsequent juvenile and adult behavior of offspring in Japanese macaques (Macaca fuscata). Early maternal style had no effect on baseline behavior of offspring when adult. In contrast, early maternal style was correlated with the response of adult offspring to stressful social interactions, and particularly with their response to actual or potential aggression. Infants whose mothers encouraged more independence showing high rates of rejection were less fearful and did cope better with stressful situations when adult. Although based on correlational data, these results suggest that in macaques maternal rejection can promote offspring independence and the development of a less anxious personality.

Relationship with the mother modulates the response of yearling Japanese macaques (Macaca fuscata) to the birth of a sibling.

The authors investigated the changes induced by the birth of a sibling in the relationship of 1-year-old Japanese macaques (Macaca fuscata) with their mothers and group companions. After the birth of a sibling, mother-yearling contact, proximity, and grooming decreased dramatically. Yearlings responded to such a reduction in maternal care in either of 2 radically different ways. Yearlings either sought attention from group companions and showed no sign of depression or did not compensate for the mother's reduced availability and became depressed. The modality of response was predicted by the quality of the relationship with the mother before the sibling birth. Yearlings that had spent a larger amount of time in contact with their mothers were less likely to become depressed. Security of the attachment relationship with the mother may be the factor mediating the link between the time in contact and the yearling's response to the birth of a sibling.

Social rank and sex-biased maternal investment in captive japanese macaques: behavioural and reproductive data.

This study investigated the relationship between social rank and sex-biased maternal investment in captive Japanese macaques (Macaca fuscata) using reproductive and behavioural data. High-ranking mothers showed a significantly male-biased secondary sex ratio, spent more time in contact with and carried male infants for longer than female infants. Low-ranking mothers showed no bias in secondary sex ratio nor in the time spent in contact with male and female infants, but carried female infants for longer. No differences were observed in the interbirth intervals following male and female infants nor in the frequency and intensity of aggression received by mothers with male and female infants, either in high-ranking or in low-ranking mothers. These results show that Japanese macaque mothers can adopt flexible and rank-dependent rearing strategies.

Primate displacement activities as an ethopharmacological model of anxiety.

Using a within-subject cross-over, vehicle-controlled design, we investigated the acute effects of benzodiazepine receptor ligands with different mechanisms of action on the displacement activities (scratching, self-grooming, and body shake) of seven male macaques living in social groups. Our aim was to test the discriminative validity of displacement activities as an ethopharmacological model of anxiety. Subjects were given i.m. lorazepam (0.10, 0.20, 0.25 mg/ kg) and FG 7142 (0.1, 0.3, 1.0 mg/kg). The frequency of displacement activities was decreased by the anxiolytic lorazepam and increased by the anxiogenic FG 7142 in a dose-dependent manner. Displacement activities were apparently more sensitive to anxiolytic treatment than other behavior patterns indicative of an anxiety state (i.e., visual scanning of the social environment and fear responses directed to dominant males). These results suggest that primate displacement activities are a valid ethopharmacological model of anxiety.

Opiate receptor blockade in juvenile macaques: effect on affiliative interactions with their mothers and group companions.

The aim of this study was to test the hypothesis that, in non-human primates, the offspring-mother attachment and other social bonds within the group have in common neural mediating mechanisms involving brain opioids. The subjects were 10 juvenile macaques living in a stable social group with their mothers and other group companions. A within-subjects design, balanced for the order of drug administration (naloxone 1 mg/kg i.m. and saline), was used. In the naloxone condition, the juveniles increased their relative role in maintaining proximity with their mothers, made more grooming solicitations, and received more grooming. We found no evidence for differential effects of naloxone on the affiliative bonds the subjects had with their mothers and other group companions. The subjects' increased demands for social comfort were evenly addressed to, and responded to by, both their mothers and other group companions. This finding supports the hypothesis that, at a neural level, the endogenous opioid peptides form a common substrate for different types of social attachments in primates.

Scratching as a behavioral index of anxiety in macaque mothers.

The aim of this study was to assess the validity of the notion that anxiety influences maternal style in monkeys. To this end, we investigated the correlations between measures of the quality of mother-infant relationship and scratching, a behavior pattern that reflects a state of anxiety. The subjects were seven group-living macaque mothers and their infants observed during the first 12 weeks postpartum. Postpartum scratching by the mothers was positively correlated with both maternal possessiveness and maternal warmth, two composite measures of the quality of mother-infant relationship that reflect a possessive and attentive maternal style. Evaluation of the correlations between maternal rank and scratching before and after parturition indicated that interactions with their infants, not those with other group companions, were the main source of anxiety for the mothers. These results support the notion that anxiety influences maternal style in monkeys and suggest that studies aimed at investigating physiological and social correlates of maternal emotionality in nonhuman primates could use scratching as a useful and simple behavioral measure.

Measuring anxiety in nonhuman primates: effect of lorazepam on macaque scratching.

Lorazepam (0.2 mg/kg IM) was given to group-living female macaques to assess the effect of anxiolytic treatment on scratching, a behavior pattern referred to as a displacement activity in the primate literature. Lorazepam selectively diminished scratching behavior. The drug effect was status-dependent: especially low-ranking animals showed a marked reduction in scratching. Lorazepam exerted a direct effect on scratching, that is the effect was not due to sedation or mediated by the influence of the drug on other behaviors. These results provide pharmacological validation to the ethological finding that scratching may be a manifestation of anxiety in monkeys. In addition, they suggest to use scratching as a behavioral measure in studies investigating nonhuman primate models of anxiety.

Behavioral thermoregulation in long-tailed macaques: effect on social preference.

Environmental and behavioral data were collected on a captive group of six long-tailed macaques housed in an outdoor cage to determine whether intragroup spacing behavior would vary in a consistent manner in relation to microclimatic variables. Records were made of ambient temperature, relative humidity, and time of day. Temperature was found to influence proximity between the monkeys. Under cool conditions, the long-tailed macaques of this study increased interindividual body contact, a behavior that lowers the amount of heat lost by radiation. Behavioral thermoregulation exerted a major effect on the expression of social relationships among the individuals. Social preference thresholds that regulate interindividual body contact were dramatically lowered under cool conditions (13-24 degrees C). In contrast, affiliative thresholds for noncontact proximity did not alter as a function of thermal conditions. The effect of temperature on dyadic body contact did not seem to vary with the social characteristics of the dyad. The results support the hypothesis that certain aspects of primate social behavior may also serve some thermoregulatory function.