The Evolution of Social Intelligence in Macaques

Abstract

Primates lead complex social lives, and this complexity is thought to drive the evolution of social intelligence, including social cognitive abilities that are thought to be especially sophisticated in humans. However, it remains unclear (1) how different social styles promote the emergence of these abilities, and (2) how social cognition alters naturalistic patterns of social behavior. My PhD research examines the social cognition and social behavior of two closely related primate species that have different social styles—despotic rhesus macaques (Macaca mulatta) and socially tolerant Barbary macaques (Macaca sylvanus), to characterize the processes shaping social cognition and test several major hypotheses about the evolutionary contexts that support the emergence of complex cognition. The first focus of my dissertation is to characterize complex social cognition in primates. In Chapters 2 and 3, I assess whether rhesus and Barbary macaques––like humans––have a sophisticated understanding of others’ gaze, that accounts for their line-of-sight. In Chapter 4, I further assess whether rhesus macaques are sensitive to gaze as a communicative cue, which has been proposed to have been especially important during human evolution (Csibra & Gergely, 2011). Finally, I review evidence for different social cognitive abilities across primates in Chapter 6, and argue that human-unique social cognitive abilities may be linked to cooperation. The second focus of my dissertation is to test the role of social style in promoting sophisticated social cognition. In particular, while much work on primate social cognition has suggested that high rates of social competition drive social cognitive evolution (Byrne & Whiten, 1989; Dunbar, 1998), other work has argued that tolerance promotes sophisticated social cognition––and that despotic primate species are actually constrained to utilizing sophisticated social abilities within competitive contexts (Hare, 2001; Lyons & Santos, 2006). Chapters 2, 3, and 5 test these hypotheses, by assessing whether despotic rhesus macaques can flexibly use a sophisticated understanding of others’ gaze even in a non-competitive context (Chapter 2), testing whether tolerant Barbary macaques or rhesus macaques show greater skill in this task (Chapter 3), and examining the link between social behavior and cognition (Chapter 5). The final focus of my thesis is to examine how social cognition predicts natural social behavior. The social intelligence hypothesis posits that primate sociality selected for cognitive skills which allow individuals to respond adaptively to others’ behavior (Byrne & Whiten, 1989; Hare, 2017; Humphrey, 1976; Jolly, 1966), yet has not been empirically tested despite intense theoretical interest. In Chapter 5, I utilize a novel approach that combines both experimental assessments of cognitive abilities with naturalistic social behavior upon Barbary macaques. I then contrast three influential hypotheses about the relationship between intelligence and behavior: (1) does social intelligence enable interaction with a larger number of individuals (Dunbar 1998); (2) does social intelligence facilitate strong social bonds or other prosocial behavior (Cheney, Seyfarth, and Smuts 1986; Hare 2017; Dunbar and Shultz 2007); and (3) does social intelligence facilitate sophisticated competitive behavior? (Byrne and Whiten 1989). Overall, this dissertation examines the evolution of social intelligence in macaques, by contrasting cognition and behavior across two closely related species that vary in social tolerance. This will provide new insights into how complex social intelligence evolves across species, including humans.