Sociality, ecology, and relative brain size in lemurs Evan L. MacLean * , Nancy L. Barrickman, Eric M. Johnson, Christine E. Wall Department of Evolutionary Anthropology, Box 90383-0383, Duke University, Durham, NC 27708, USA article info Article history: Received 7 June 2008 Accepted 28 December 2008 Keywords: Pairbonding Group size Activity pattern Diet Brain size Sociality abstract The social brain hypothesis proposes that haplorhine primates have evolved relatively large brains for their body size primarily as an adaptation for living in complex social groups. Studies that support this hypothesis have shown a strong relationship between relative brain size and group size in these taxa. Recent reports suggest that this pattern is unique to haplorhine primates; many nonprimate taxa do not show a relationship between group size and relative brain size. Rather, pairbonded social monogamy appears to be a better predictor of a large relative brain size in many nonprimate taxa. It has been suggested that haplorhine primates may have expanded the pairbonded relationship beyond simple dyads towards the evolution of complex social groups. We examined the relationship between group size, pairbonding, and relative brain size in a sample of 19 lemurs; strepsirrhine primates that last share a common ancestor with monkeys and apes approximately 75 Ma. First, we evaluated the social brain hypothesis, which predicts that species with larger social groups will have relatively larger brains. Secondly, we tested the pairbonded hypothesis, which predicts that species with a pairbonded social organization will have relatively larger brains than non-pairbonded species. We found no relationship between group size or pairbonding and relative brain size in lemurs. We conducted two further analyses to test for possible relationships between two nonsocial variables, activity pattern and diet, and relative brain size. Both diet and activity pattern are significantly associated with relative brain size in our sample. Specifically, frugivorous species have relatively larger brains than folivorous species, and cath- emeral species have relatively larger brains than diurnal, but not nocturnal species. These findings highlight meaningful differences between Malagasy strepsirrhines and haplorhines, and between Malagasy strepsirrhines and nonprimate taxa, regarding the social and ecological factors associated with increases in relative brain size. The results suggest that factors such as foraging complexity and flexibility of activity patterns may have driven selection for increases in brain size in lemurs. Ó 2009 Elsevier Ltd. All rights reserved. Introduction Understanding the selective pressures that have influenced primate brain evolution is among the greatest challenges in bio- logical anthropology. In recent years, there has been increasing consensus that haplorhine primates evolved larger degrees of encephalization primarily in response to the demands of living in complex social groups (Dunbar, 1998). This explanation has been coined the ‘‘social brain hypothesis.’’ It asserts that species living in social groups must manage a wide variety of dynamic information relevant to social living, that these processes are cognitively demanding, and that an increase in sociality was an important causal factor in the evolution of large brains relative to body size (Kummer et al., 1997; Whiten, 2003; Dunbar and Shultz, 2007a). Evidence for the social brain hypothesis has emerged in two complementary domains. First, a wealth of observational and experimental studies have documented the exceptional abilities of primates to solve social problems (for reviews, see Byrne and Whiten, 1988; Tomasello and Call, 1997). Primates, especially apes, excel in solving problems that require complex forms of social cognition, such as imitation (Horner and Whiten, 2005), theory of mind (Hare et al., 2000, 2001), social learning (Bonnie et al., 2007), and cooperation (Melis et al., 2006). A second class of studies has examined the relationship between various neuroanatomical traits and indices of sociality among primates. Most notably, Dunbar (1992) has shown that relative neocortex volume is strongly pre- dicted by group size among haplorhine primates. This result was interpreted as reflecting an increase in cognitive demands as group size increases. Importantly, group size was implicated only as a proxy for social complexity in primates and not as the driving factor for brain evolution (Dunbar and Shultz, 2007a). Although other indices of sociality have also been shown to correlate with * Corresponding author. E-mail address: maclean@duke.edu (E.L. MacLean). Contents lists available at ScienceDirect Journal of Human Evolution journal homepage: www.elsevier.com/locate/jhevol 0047-2484/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jhevol.2008.12.005 Journal of Human Evolution 56 (2009) 471–478