To pair or not to pair: Sources of social variability with white‐faced saki monkeys (Pithecia pithecia) as a case study

Abstract

Intraspecific variability in social systems is gaining increased recognition in primatology. Many primate species display variability in pair-living social organizations through incorporating extra adults into the group. While numerous models exist to explain primate pair-living, our tools to assess how and why variation in this trait occurs are currently limited. Here I outline an approach which: (i) utilizes conceptual models to identify the selective forces driving pair-living; (ii) outlines novel possible causes for variability in social organization; and (iii) conducts a holistic species-level analysis of social behavior to determine the factors contributing to variation in pair-living. A case study on white-faced sakis (Pithecia pithecia) is used to exemplify this approach. This species lives in either male-female pairs or groups incorporating "extra" adult males and/or females. Various conceptual models of pair-living suggest that high same-sex aggression toward extra-group individuals is a key component of the white-faced saki social system. Variable pair-living in white-faced sakis likely represents alternative strategies to achieve competency in this competition, in which animals experience conflicting selection pressures between achieving successful group defense and maintaining sole reproductive access to mates. Additionally, independent decisions by individuals may generate social variation by preventing other animals from adopting a social organization that maximizes fitness. White-faced saki inter-individual relationships and demographic patterns also lend conciliatory support to this conclusion. By utilizing both model-level and species-level approaches, with a consideration for potential sources of variation, researchers can gain insight into the factors generating variation in pair-living social organizations. Am. J. Primatol. 78:561-572, 2016.

A Saki Saga: Dynamic and Disruptive Relationships among Pithecia aequatorialis in Ecuador

“…On the other hand, if extra males are indeed beneficial for more effectively defending resources, females or infants against outside individuals [Thompson, 2015], it is unclear why the males would have jointly abandoned a well-established territory, two reproductive-age females and a newborn infant, without being challenged. Although the males ranged partially outside of the group's territory, they also ranged within it and they returned to this territory and to the resident females twice.…”

“…Joint mate and/or territorial defense may be especially important if floaters (i.e. solitary individuals without territories of their own) are prevalent in the area [Port and Johnstone, 2013;Thompson, 2015]. Indeed, a study of Azara's owl monkeys (Aotus azarai) revealed that encounters with floaters can lead to high turnover rates among pair-mates [Fernandez-Duque and Huck, 2013].…”

“…If strong intrasexual aggression is typical for some socially monogamous primates, under what conditions might pairs tolerate additional adults? Extra-group males may be able to immigrate into an established group by providing a service to the resident male, such as assistance with parental care, mate defense or territorial defense [Davies, 1992;Savini et al, 2009;Thompson, 2015]. Joint mate and/or territorial defense may be especially important if floaters (i.e.…”

Delayed Dispersal and Immigration in Equatorial Sakis (Pithecia aequatorialis): Factors in the Transition from Pair- to Group-Living

“…Saki monkeys are considered to be pair-living primates that also occasionally live in groups with multiple adults of one or both sexes [reviewed in Norconk and Setz, 2013;Van Belle et al, 2015;Thompson, 2016]. Sakis have been reported to retain adult offspring [Thompson et al, 2010;Norconk, 2011] and to accept putatively nonrelated adult immigrants into the group [Porter et al, 2015].…”

“Monogamy” in Primates: Variability, Trends, and Synthesis: Introduction to special issue on Primate Monogamy

“…The extent of intraspecific variability or phenotypic plasticity in these species, traditionally defined as monogamous, has emerged as a key issue [Thompson, 2015; Garber et al, this issue; Diaz-Munoz, this issue] in the study of monogamy. A prominent example has been variability in callitrichid (marmoset and tamarin) groups in the wild, which have been described as monogamous [Kleiman 1977, 1979; Leutenegger 1980; Baker et al 1993], polyandrous [Goldizen 1987; Sussman and Garber 1987], and “flexible” [Goldizen 1988; Caine 1993].…”