Social Influences on Group Membership in Propithecus verreauxi verreauxi

Variation in Female Leverage: The Influence of Kinship and Market Effects on the Extent of Female Power Over Males in Verreaux’s Sifaka

Female mammals employ reproductive strategies (e.g., internal gestation) that result in power asymmetries specific to intersexual dyads. Because the number of eggs available for fertilization at any given time for most mammals is quite limited, having a fertilizable egg is potentially an important source of economic power for females. Control over mating opportunities is a source of intersexual leverage for female Verreaux’s sifaka (Propithecus verreauxi). We examined economic factors thought to influence the value of mating opportunities, and, thus, the extent of female leverage: kinship and market effects. Using a longitudinal dataset of agonistic interactions collected during focal animal sampling of all adult individuals in 10 social groups from 2008 to 2019, we tested the effects of relatedness, female parity, reproductive season, and adult sex ratio (population and group) on (1) the direction of submissive signaling and (2) which sex won a contested resource. While 96% of the acts of submission were directed from males toward females, females only won a third of their conflicts with males. Thus, our study has implications for evolutionary explanations of female-biased power. If female power evolved due to their greater need for food and other resources, then intersexual conflicts would be expected to result in males more consistently relinquishing control of resources. As expected, males were more likely to chatter submissively toward successful mothers, during the mating season, and when the sex ratio was male-biased. Although females generally had less power to win a conflict when their fertilizable egg was less valuable (when they were nulliparous or unsuccessful mothers or when interacting with male kin) and with an increasing female-bias in the sex ratio, this ability to win additionally was influenced by which sex initiated the conflict. Our study demonstrates that female leverage can be influenced by the supply and demand for mating opportunities, but evoking submission does not translate into winning a resource. Indeed, intersexual power is dynamic, contextual, and dependent on the individuals in the dyad.

Social groups constrain the spatiotemporal dynamics of wild sifaka gut microbiomes

Primates acquire gut microbiota from conspecifics through direct social contact and shared environmental exposures. Host behaviour is a prominent force in structuring gut microbial communities, yet the extent to which group or individual-level forces shape the long-term dynamics of gut microbiota is poorly understood. We investigated the effects of three aspects of host sociality (social groupings, dyadic interactions, and individual dispersal between groups) on gut microbiome composition and plasticity in 58 wild Verreaux's sifaka (Propithecus verreauxi) from six social groups. Over the course of three dry seasons in a 5-year period, the six social groups maintained distinct gut microbial signatures, with the taxonomic composition of individual communities changing in tandem among coresiding group members. Samples collected from group members during each season were more similar than samples collected from single individuals across different years. In addition, new immigrants and individuals with less stable social ties exhibited elevated rates of microbiome turnover across seasons. Our results suggest that permanent social groupings shape the changing composition of commensal and mutualistic gut microbial communities and thus may be important drivers of health and resilience in wild primate populations.

Genetic consequences of social structure in the golden-crowned sifaka

Many species are structured in social groups (SGs) where individuals exhibit complex mating strategies. Yet, most population genetic studies ignore SGs either treating them as small random-mating units or focusing on a higher hierarchical level (the population). Empirical studies acknowledging SGs have found an overall excess of heterozygotes within SGs and usually invoke inbreeding avoidance strategies to explain this finding. However, there is a lack of null models against which ecological theories can be tested and inbreeding avoidance quantified. Here, we investigate inbreeding (deviation from random mating) in an endangered forest-dwelling pair-living lemur species (Propithecus tattersalli). In particular, we measure the inbreeding coefficient (F_IS) in empirical data at different scales: SGs, sampling sites and forest patches. We observe high excess of heterozygotes within SGs. The magnitude of this excess is highly dependent on the sampling scheme: while offspring are characterised by a high excess of heterozygotes (F_IS < 0), the reproductive pair does not show dramatic departures from Hardy-Weinberg expectations. Moreover, the heterozygosity excess disappears at larger geographic scales (sites and forests). We use a modelling framework that incorporates details of the sifaka mating system but does not include active inbreeding avoidance mechanisms. The simulated data show that, although apparent "random mating" or even inbreeding may occur at the "population" level, outbreeding is maintained within SGs. Altogether our results suggest that social structure leads to high levels of outbreeding without the need for active inbreeding avoidance mechanisms. Thus, demonstrating and measuring the existence of active inbreeding avoidance mechanisms may be more difficult than usually assumed.

Patterns of male dispersal in Verreaux's sifaka (Propithecus verreauxi) at Kirindy Mitea National Park

Males of many group-living mammals emigrate from their social groups to improve mating opportunities. To help mitigate the social and locational costs of dispersal and to increase reproductive benefits, they may immigrate seasonally, immigrate alongside a partner, and/or replace the alpha male (versus entering a group as a subordinate). Verreaux's sifaka (Propithecus verreauxi) are highly seasonally breeding primates characterized by male-biased dispersal. We hypothesized that (i) males time immigrations to pursue immediate mating opportunities, (ii) entrances with partners more often result in alpha male replacement, and (iii) male competitive ability affects immigration strategy and alpha male tenure length. To assess these proximate aspects of male dispersal, we examined 7 years of demographic, morphological, and behavioral data for five social groups of Verreaux's sifaka in the Kirindy Mitea National Park in western Madagascar. Contrary to expectations and studies of sifaka dispersal in other populations, we detected no strong seasonal pattern in immigrations. Immigrations occurred individually and with partners, and a trend existed for partners to increase the likelihood of replacing an alpha male. Pronounced activity of the sternal scent gland (a proxy for testosterone and prior dominance status), but not body mass, canine size, or potential correlates of leaping ability, significantly influenced immigration strategy. The absence of a seasonal immigration pattern suggests that fluid group boundaries may allow mating success without establishment in a social group before the mating season. Our results also suggest that male immigration strategies are affected by age, prior dominance status, and testosterone levels but not morphological indicators of individual competitive ability. Coalitions may be used to improve competitive ability. Finally, differences in seasonal immigration patterns and length of alpha male vacancies observed at Kirindy Mitea may be due to the relatively low population density. Am. J. Primatol. 79:e22455, 2017. © 2015 Wiley Periodicals, Inc.

Social status influences responses to unfamiliar conspecifics in a cooperatively breeding fish

In group living animals, individuals may visit other groups. The costs and benefits of such visits for the members of a group will depend on the attributes and intentions of the visitor, and the social status of responding group members. Using wild groups of the cooperatively breeding cichlid fish (Neolamprologus pulcher), we compared group member responses to unfamiliar ‘visiting’ conspecifics in control groups and in experimentally manipulated groups from which a subordinate the same size and sex as the visitor was removed. High-ranking fish were less aggressive towards visitors in removal groups than in control groups; low-ranking subordinates were more aggressive in the removal treatment. High-ranking females and subordinates the same size and sex as the visitor responded most aggressively toward the visitor in control groups. These results suggest that visitors are perceived as potential group joiners, and that such visits impose different costs and benefits on current group members.

Effects of the distribution of female primates on the number of males

The spatiotemporal distribution of females is thought to drive variation in mating systems, and hence plays a central role in understanding animal behavior, ecology and evolution. Previous research has focused on investigating the links between female spatiotemporal distribution and the number of males in haplorhine primates. However, important questions remain concerning the importance of spatial cohesion, the generality of the pattern across haplorhine and strepsirrhine primates, and the consistency of previous findings given phylogenetic uncertainty. To address these issues, we examined how the spatiotemporal distribution of females influences the number of males in primate groups using an expanded comparative dataset and recent advances in bayesian phylogenetic and statistical methods. Specifically, we investigated the effect of female distributional factors (female number, spatial cohesion, estrous synchrony, breeding season duration and breeding seasonality) on the number of males in primate groups. Using bayesian approaches to control for uncertainty in phylogeny and the model of trait evolution, we found that the number of females exerted a strong influence on the number of males in primate groups. In a multiple regression model that controlled for female number, we found support for temporal effects, particularly involving female estrous synchrony: the number of males increases when females are more synchronously receptive. Similarly, the number of males increases in species with shorter birth seasons, suggesting that greater breeding seasonality makes defense of females more difficult for male primates. When comparing primate suborders, we found only weak evidence for differences in traits between haplorhines and strepsirrhines, and including suborder in the statistical models did not affect our conclusions or give compelling evidence for different effects in haplorhines and strepsirrhines. Collectively, these results demonstrate that male monopolization is driven primarily by the number of females in groups, and secondarily by synchrony of female reproduction within groups.

Structure of social networks in a passerine bird: consequences for sexual selection and the evolution of mating strategies

The social environment is a critical determinant of fitness and, in many taxa, is shaped by an individual's behavioral discrimination among social contexts, suggesting that animals can actively influence the selection they experience. In competition to attract females, males may modify sexual selection by choosing social environments in which they are more attractive relative to rivals. Across the population, such behaviors should influence sexual selection patterns by altering the relationship between male mating success and sexual ornament elaboration. Here we use network analysis to examine patterns of male social behavior in relation to plumage ornamentation and mating success in a free-living population of house finches. During the nonbreeding season, less elaborate males changed associations with distinct social groups more frequently, compared to more elaborate males that showed greater fidelity to a single social group. By the onset of pair formation, socially labile males effectively increased their attractiveness relative to other males in the same flocks. Consequently, males that frequently moved between social groups had greater pairing success than less social individuals with equivalent sexual ornamentation. We discuss these results in relation to conditional mating tactics and the role of social behavior in evolutionary change by sexual selection.

Stranger to familiar: wild strepsirhines manage xenophobia by playing

The power of play in limiting xenophobia is a well-known phenomenon in humans. Yet, the evidence in social animals remains meager. Here, we aim to determine whether play promotes social tolerance toward strangers in one of the most basal group of primates, the strepsirhines. We observed two groups of wild lemurs (Propithecus verreauxi, Verreaux's sifaka) during the mating season. Data were also collected on nine visiting, outgroup males. We compared the distribution of play, grooming, and aggressive interactions across three conditions: OUT (resident/outgroup interactions), IN (resident/resident interactions in presence of outgroups) and BL-IN (baseline of resident/resident interactions in absence of outgroups). Play frequency between males was higher in OUT than in IN and BL-IN conditions; whereas, grooming was more frequent in IN than in OUT and BL-IN conditions. Aggression rates between resident and outgroup males were significantly higher than those between residents. However, aggressions between resident and outgroup males significantly decreased after the first play session and became comparable with resident-resident aggression levels. The presence of strangers in a well-established group implies the onset of novel social circumstances, which sifaka males cope with by two different tactics: grooming with ingroup males and playing with outgroup ones. The grooming peak, concurrently with the visit of outgroups, probably represents a social shield adopted by resident males to make their pre-existing affiliation more evident to the stranger “audience”. Being mostly restricted to unfamiliar males, adult play in sifaka appears to have a role in managing new social situations more than in maintaining old relationships. In particular, our results indicate not only that play is the interface between strangers but also that it has a specific function in reducing xenophobia. In conclusion, play appears to be an ice-breaker mechanism in the critical process that “upgrades” an individual from stranger to familiar.

Grooming patterns in Verreaux's sifaka

Lemur grooming has received very little attention in the literature. Nevertheless, allogrooming in lemurs has been suggested to be fundamentally different from the grooming of anthropoids. One reason is that lemurs generally rely on oral rather than manual grooming. Lemur allogrooming has also been suggested to serve less of a social function than has been attributed to anthropoid grooming. I analyzed the allogrooming behaviors of 29 Verreaux's sifaka (Propithecus verreauxi) living in five social groups in the Kirindy Forest of Madagascar. Based upon 1,586 observation hours, I found that sifaka, like anthropoids, spend very little time mutual grooming (2+/-3%). Half of all allogrooming involved parts of the body that could have been easily groomed by the recipient, such as the limbs. Even though ectoparasite loads are expected to be greater during the rainy season, allogrooming did not increase during the rainy season. Allogrooming rates were influenced by both rank and sex, and increased by 50-100% during the mating season. The results of this study suggest that allogrooming in Verreaux's sifaka plays an important social function, even though it is performed with a toothcomb.

Even adult sex ratios in lemurs: Potential costs and benefits of subordinate males in Verreaux's sifaka (Propithecus verreauxi) in the Kirindy Forest CFPF, Madagascar

Optimal group size and composition are determined by both the costs and benefits of group living for the group's members. Verreaux's sifakas (Propithecus verreauxi), a diurnal lemur, form multimale multifemale groups with the tendency toward even adult sex ratios despite a small average number of females per group. The unexpected presence of multiple adult males may be explained by tolerance of other group members if subordinate males provide benefits to the group that outweigh the costs associated with their presence. Results based on both demographic data collected over a 13-year period and behavioral observations suggest that subordinate males provide no benefits in terms of infant survival and defense against group takeover by outside males. Although groups with more males are more likely to win intergroup encounters, subordinate males do not participate in these encounters more often than expected. Subordinate males are not costly to other group members in terms of direct intragroup feeding competition, but aggression rates between dominant and immigrated subordinate males increase in the mating season. Even though subordinate males provide very few benefits to the group, they are not very costly either and thus may be tolerated by resident females and dominant males. This tolerance may help to partially explain the tendency towards their unusual adult sex ratio.

Chest staining variation as a signal of testosterone levels in male Verreaux's Sifaka

Male Verreaux's sifaka (Propithecus verreauxi) exhibit variation in the staining of chest hair in association with the activity of the sternal gland. Scent-marking behavior and social relationships have been shown to vary with the state of chest staining. Research on other mammals suggests that sternal gland activity is modulated by testosterone. The goal of this study was to examine the relationship among fecal testosterone, testes mass, and chest staining in sifaka. I predicted that for sifaka (1) testosterone and testes mass are positively correlated, (2) stained males have higher testosterone and larger testes than clean-chested males, (3) testes mass is greater during the mating season than the birth season, and (4) testes mass is comparable to species with multimale mating systems. Eighty fecal samples were collected from eight adult males in five social groups in the Kirindy Mitea National Park of Madagascar during the mating season and analyzed using enzyme immunoassays. Males were captured during the mating season and the subsequent birth season to measure body mass and testes size and to document chest staining. Stained males had significantly higher fT and larger testes mass than clean-chested males. Testes mass was significantly greater during the mating season than the birth season for all males. However, the stained males exhibited less testes mass reduction during birth season than clean-chested males. These results suggest that the activity of the sternal gland is regulated by testosterone and that sexual selection has shaped male morphological variation in Verreaux's sifaka.