Sociality in Callithrix penicillata: II. Individual Strategies During Intergroup Encounters

Intergroup encounters in pair-living primates: Comparative analysis and a case study of pair-living and monogamous owl monkeys (Aotus azarae) of Argentina

The function of intergroup encounters (IGEs) may differ substantially among species of different group sizes and social organizations. Research in group-living primates has shown that the behavioral responses during IGEs can vary widely from affiliative to neutral or aggressive interactions; still, little is known about IGEs in pair-living taxa. We conducted a systematic literature review to find relevant studies on the functions of IGEs in pair-living nonhuman primates that could inform analyses of IGE data (n = 242 IGEs, 21 groups and 10 solitary individuals, 1997-2020) from wild owl monkeys, a pair-living, monogamous primate with extensive biparental care. We identified 1315 studies published between 1965 and 2021; only 13 of them (n = 10 species) contained raw data on the number of IGEs. Our review of those studies showed that IGEs are common, but highly variable in their nature and characteristics in pair-living primates. To examine the non-mutually exclusive hypotheses of resource-, and mate defense, and infanticide avoidance we analyzed data from the Owl Monkey Project 27-year long database to build first an a priori model set. To incorporate prior knowledge from the literature review, we conducted our analyses as a consecutive series of binomial logistic regressions. All IGEs including all biologically relevant parameters (N = 156) were codified into three different behavioral categories (Reaction, Agonism, and Physical Aggression). The analysis showed that owl monkeys regularly engaged in IGEs, most of which were agonistic. They showed more reaction when infants were present, but reactions were less physically aggressive when infants and pregnant females were involved. Overall, our results lend more support for the infant and mate defense hypotheses than they do for the resource defense one.

Electrocutions in free-living black-tufted marmosets (Callithrix penicillata) in anthropogenic environments in the Federal District and surrounding areas, Brazil

Shrinking natural habitats exposes some non-human primates to the risk of accidents associated with electrical transmission lines. We examined dead marmosets (Callithrix penicillata) collected in the region from January 2015 to April 2018 to determine the animals' cause of death and for electrocuted animals we examined the locations the animals had died as well as the configuration of the power lines at these sites. We also recorded the sex of the animal, the body region affected, and characteristics of the injuries. We diagnosed electrocutions in 11% (n = 34) of the marmosets studied. Most of the affected animals were male (n = 22) with single or double sites of injury on the limbs. Animals were injured in urban (n = 26) and peri-urban (n = 8) areas on lower-voltage alternate current lines, and we detected no seasonality or hotspots of electrocution. Our findings suggest that movement along transmission lines composed of bundled conductors is a major factor in electrocutions of marmosets in the Federal District and surrounding areas. The planning of electrical power grid infrastructure should consider arboreal primates to prevent electrocutions.

Intergroup encounters in Verreaux's sifakas (Propithecus verreauxi): who fights and why?

ABSTRACT

Individuals living in groups have to achieve collective action for successful territorial defense. Because conflicts between neighboring groups always involve risks and costs, individuals must base their decision to participate in a given conflict on an evaluation of the trade-off between potential costs and benefits. Since group members may differ in motivation to engage in group encounters, they exhibit different levels of participation in conflicts. In this study, we investigated factors influencing participation in intergroup encounters in Verreaux's sifakas (Propithecus verreauxi), a group-living primate from Madagascar. Over a period of 12 months, we studied eight adjacent sifaka groups in Kirindy Forest. We observed 71 encounters between known neighboring groups in which adult females and males participated equally as often. No individual participated in every encounter, and non-participation occurred more often in larger groups. Females participated less often in encounters when they had dependent infants, presumably to reduce the risk of infanticide. Male participation was influenced by social status: dominant males participated in most encounters, whereas males with fewer opportunities to reproduce participated less often, hence male participation is influenced by the incentive of maintaining access to females. The number of actively participating individuals in the opponent group positively influenced the participation in both sexes. Thus, sifakas seem to decide joining a given encounter opportunistically, most likely based on a combination of individual incentives and the actual circumstance of each encounter, suggesting that the complexity in intergroup relationships appears to be the product of decisions made by each individual group member.

SIGNIFICANCE STATEMENT

Cooperation among group-living animals is often challenged by collective action problems resulting from individual differences in interests in contributing to collective behaviors. Intergroup encounters involve distinguished costs and benefits for each individual despite being in the same social group. Therefore, encounters between groups offer a good opportunity to investigate individual participation in collective action. In this study, we investigate the influence of different incentives on individual participation in intergroup encounters in wild Malagasy primate, Verreaux's sifakas. We propose a novel approach that takes into account the variable circumstances of each conflict, such as the number of individuals fighting in both groups as a predictor for participation. We believe that our study not only provides novel data on wild sifakas, but it also offers new perspectives for the interpretation of intergroup relationships in other taxa.

Female participation in collective group defense in black howler monkeys (Alouatta pigra)

Many group-living animals actively defend a home range against neighboring groups. In many of these societies, males are the primary participants during group defense, while female participation ranges from seldom to frequent. Among howler monkeys (Alouatta spp.), loud calls (i.e., howling) are often used in the context of intergroup spacing as a form of cooperative group defense. Males initiate and lead these howling bouts, but females occasionally participate as well. During a 28-month study, I examined social and ecological factors influencing the participation of adult females in naturally occurring howling bouts of five multimale-multifemale groups of black howler monkeys (A. pigra) at Palenque National Park, Mexico. I calculated the percentage of time each female participated during howling bouts for which the participation of all resident females could be recorded ≥80% of the time (N = 287). At least one female was observed to participate in 53% of the vocal displays. Female participation was significantly greater during howling bouts that were part of visual intergroup encounters compared to spontaneous calls or calls in response to nearby calls when there was no visual contact with rival groups. Female calling behavior was not influenced by the presence of infants vulnerable to infanticide or by the proximity to food resources. Nonetheless, in four of the five study groups, one female called significantly more than the other resident female(s), suggesting that these females played a special role within the group's social dynamics, not previously recognized for this species.

Reporting social behaviours of mixed-species troops formed by Callithrix jacchus and Callithrix penicillata (Primate, Callitrichidae)

In New World primates, mixed-species troops have been reported. Here, we analysed the performance of affiliative and agonistic behaviours of Callithrix jacchus and Callithrix penicillata living in mixed groups. For this purpose, we recorded the interaction of the individuals from two groups located in Bauru city, in the state of São Paulo (Brazil). Our data show that in both groups, affiliative behaviours appeared more frequently than agonistic ones. We concluded that there is cohesion inside the mixed-species troops observed. We suggest that a deeper knowledge about the social behaviour of mixed-species troop species certainly may be useful in projects linked with the management of the impact caused by them.