Selection on adult female body size in rhesus macaques Gregory E. Blomquist a, * , Jean E. Turnquist b a Department of Anthropology, University of Missouri, 107 Swallow Hall, Columbia, MO 65203, USA b Department of Anatomy and Neurobiology, Medical Sciences Campus, University of Puerto Rico, PO Box 365067, San Juan, PR 00936-5067, USA article info Article history: Received 23 February 2010 Accepted 5 June 2010 Keywords: Adult mortality Cayo Santiago Female life history Body size Population density Macaca mulatta abstract Body size is a critical descriptor of animal biology with many ecological, behavioral, and physiological correlates. Size differences among species or between populations are often explained by adaptive scenarios invoking the action of selection, although studies of selection in action on primate body size, or other phenotypic traits, are very rare. We document directional selection for larger skull and postcranial size in the skeletons of female semi-free ranging rhesus macaques (Macaca mulatta) from Cayo Santiago, born between 1957 and 1982. Larger females live to later ages and consequently give birth to more offspring. Despite selection for larger size, there are indications of a trend toward generally smaller size in the same birth cohorts. This trend is provisionally attributed to increasing population density, though other environmental factors may play a role. Small selection differentials and low heritabilities also limit the genetic response to selection. Alternative explanations for increasing adult size in the skull and postcranium, such as continued adult growth or pathological bone deposition, do not adequately explain the observed age-related trends and are inconsistent with longitudinal studies of adult skeletal change. Ó 2010 Elsevier Ltd. All rights reserved. Introduction Body size is an important descriptor of organisms, having many morphological, physiological, life history, and ecological correlates (Gould, 1977; Peters, 1983; Schmidt-Nielsen, 1984; Calder, 1984; Jungers, 1985; Harvey, 1990; Charnov, 1993; Brown and West, 2000; Ruff, 2002; van Bergen and Phillips, 2005). Interspecific and intraspecific size differences and scaling relationships are explained through the past action of natural or sexual selection. However, the macro- and microevolutionary scale of body size adaptation can be remarkably different (Kozlowski and Weiner, 1997; Gordon, 2006a,b). Currently, studies of within-population variation doc- umenting phenotypic selection in action on wild or free ranging groups are exceedingly rare for primates (e.g., DeGusta et al., 2003; Lawler et al., 2005), though other vertebrate taxa have been more thoroughly investigated (Endler, 1986; Kingsolver et al., 2001). Most morphological traits, including body size, are thought to be maintained near optimum by selection against extreme values (i.e., stabilizing selection) where the fitness benefit/cost ratio of a particular size is maximized (Preziosi and Fairbairn, 2000). However, many cases of directional selection in the wild are known and have been related to changing environmental conditions that likely alter the costs or benefits of being a particular size (Garant et al., 2004; Pelletier et al., 2007; Ozgul et al., 2009). An alterna- tive is that selection is primarily directional on morphological traits, but direction of favored change fluctuates frequently with environmental changes (Grant and Grant, 2002). Measuring patterns of selection on primate body size can help clarify debates on the socioecology and evolution of sexual dimor- phism or body size in general. The majority of hypotheses about primate and hominin dimorphism have been generated by inter- specific comparisons, which, while valuable, cannot measure the mechanisms suggested to lead to sexual dimorphism (Blanckenhorn, 2005). Similarly, female hominin energetics, and presumably fitness, have been intimately linked to body size (Aiello and Key, 2002; Aiello and Wells, 2002), such that the energetic demands of female body size increase in Homo are thought to have been offset by cooperative care of infants or sexual division of labor. While intriguing, we currently lack any demonstration that female body size variation within primate populations has any effect on fitness. Explanations for dimorphism typically invoke sexual selection. Strong dimorphism is generally expected when mate competition in a highly polygynous mating system favors body size increase in males, although other factors are likely involved (Plavcan, 2001; Lindenfors, 2002; Clutton-Brock, 2004). Importantly, body size increase in females has been modeled as a correlated response due to selection only on males and a genetic correlation between the sexes (Lande, 1980; Smith and Cheverud, 2002; Gordon, 2006a). However, there are many potential advantages to large * Corresponding author. E-mail addresses: blomquistg@missouri.edu (G.E. Blomquist), jean.turnquist@ upr.edu (J.E. Turnquist). Contents lists available at ScienceDirect Journal of Human Evolution journal homepage: www.elsevier.com/locate/jhevol 0047-2484/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jhevol.2010.05.010 Journal of Human Evolution 60 (2011) 677e683