My primary research interest lies in the interacting biological and behavioral strategies of males and females in animal social systems. I'm particularly interested in the implications of these interactions for the evolution of reproductive strategies and complex societies. More broadly, I am interested in the social behavior of human and nonhuman animals and the costs, benefits, and evolution of sociality. My taxonomic focus is Papio baboons, and I am currently involved in two field projects, one on hamadryas baboons (Papio hamadryas hamadryas) at the Filoha field site in Ethiopia and one on chacma baboons (Papio hamadryas ursinus) in the Cape Peninsula of South Africa.
Hamadryas baboons (Papio hamadryas hamadryas) have been characterized as the most male-dominated primate species, one in which social organization is largely if not exclusively controlled by males. Such a social system lies at the far extreme of known variation in sex roles among primates, and begs explanation. As a means to elucidate the role of female behavior in hamadryas social organization, I have focused my research over the past 15 years on the behavior and ecology of wild hamadryas baboons at the Filoha field site in Ethiopia.
Hamadryas baboon social organization centers on one-male units (OMUs), or 'harems', in which a single 'leader male' maintains exclusive sexual access to a small group of females (Kummer 1968). The cohesive force maintaining the OMU structure is the behavior of the leader males, who condition females to remain nearby by threatening them and biting them on the nape of the neck (Swedell & Schreier 2006; Swedell et al 2011). Despite this coercion, hamadryas females do appear to pursue their own social and sexual agenda. For example, females appear to develop and maintain social relationships with other females (Swedell 2002, 2006); females may be able to influence the outcome of male takeovers of one-male units (Bachman and Kummer 1980; Swedell 2000, 2006); and female reproductive behavior may include strategies specifically oriented towards preventing infanticide by males (Swedell and Tesfaye 2003; Swedell and Saunders 2006).
My current research with my collaborators focuses on further exploration of the interaction of biology and behavior in hamadryas and other baboons. For example, having found that social relationships among hamadryas females appear to be more important than previously thought, I would like to ascertain whether these social relationships are influenced by biological kinship. In addition, further characterization of both male and female strategies in the hamadryas social system hinges on a determination of the leader male's share of paternity of the infants born into his one-male unit. Hamadryas leader males make strenuous efforts to maintain exclusive access to the females in their one-male units, but females do copulate with other males surreptitiously. We do not yet know, however, how often these surreptitious copulations lead to conception.
Currently, Mathew Pines, Julian Saunders and I are focusing on the role of follower males in hamadryas social organization. Within the unusual multi-tiered social system of hamadryas baboons, OMUs coalesce into larger groupings called clans and bands. Present in clans and bands but not associated with individual OMUs are solitary males, who move throughout the band and interact with other solitary males and juveniles. Some OMUs also include one or more follower males, who consistently associate with a particular OMU. Within this system, it seems fairly certain that most males eventually gain females and become leaders; however, it is not clear why, on the path to that role, some males become solitary and others become followers. It is also unclear why leader males tolerate followers in their OMUs. Whether leaders and followers are close relatives is not known, though it has been speculated that they are half siblings and that they both benefit from this relationship via inclusive fitness. Using a combination of behavioral, genetic, and hormonal data, we are comparing the social and sexual strategies of follower males with those of solitary males in order to (1) document the life history trajectory of males before, during, and after they are follower males, (2) characterize the leader-follower relationship, (3) determine the explanatory paradigm behind the dichotomy between solitary and follower males and the relative costs and benefits of each, and (4) assess why leader males tolerate the presence of follower males in their OMUs. Preliminary results from this research include Pines et al (2011) and Pines and Swedell (2011).
As a complement to my work on hamadryas baboons, I am also expanding my research program to include chacma baboons of the Cape Peninsula of South Africa. There, I am collaborating with South African colleagues and students to investigate the impact of social structure, social behavior, and human disturbance on stress, reproduction, and sexual strategies in chacma baboons. My PhD student Shahrina Chowdhury has recently finished her behavioral data collection focusing on stress and sociality in females, and my post-doctoral associate Steffen Foerster is following up on this research with a project that aims to elucidate the proximate mechanisms underlying the link between sociality and fitness in baboons (cf. work by Silk and colleagues).
For more information on my current research and collaborators, please see the Filoha Hamadryas Project website and the Cape Peninsula Baboon Research Unit websites.
