Evolution of eye size and shape in primates Callum F. Ross a, * ,1 , E. Christopher Kirk b,1,2 a Organismal Biology & Anatomy, University of Chicago, Chicago, IL 60637, USA b Department of Anthropology, University of Texas at Austin, Austin, TX 78712, USA Received 3 May 2006; accepted 26 September 2006 Abstract Strepsirrhine and haplorhine primates exhibit highly derived features of the visual system that distinguish them from most other mammals. Comparative data link the evolution of these visual specializations to the sequential acquisition of nocturnal visual predation in the primate stem lineage and diurnal visual predation in the anthropoid stem lineage. However, it is unclear to what extent these shifts in primate visual ecology were accompanied by changes in eye size and shape. Here we investigate the evolution of primate eye morphology using a comparative study of a large sample of mammalian eyes. Our analysis shows that primates differ from other mammals in having large eyes relative to body size and that anthropoids exhibit unusually small corneas relative to eye size and body size. The large eyes of basal primates probably evolved to improve visual acuity while maintaining high sensitivity in a nocturnal context. The reduced corneal sizes of anthropoids reflect reductions in the size of the dioptric apparatus as a means of increasing posterior nodal distance to improve visual acuity. These data support the conclusion that the origin of anthropoids was associated with a change in eye shape to improve visual acuity in the context of a diurnal predatory habitus. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Cornea; Primate origins; Anthropoidea; Visual system; Vision Introduction Primates are distinguished from other mammals by a suite of derived features of the visual system, including forward- facing eyes and a wide binocular visual field, relatively high visual acuity, and a proliferation of functional areas in the neo- cortex devoted to processing visual information (Allman, 1977, 1999; Kaas, 2002; Heesy, 2004; Ross and Kay, 2004; Martin and Ross, 2005). Although primate visual adaptations have historically been linked to an arboreal lifestyle (Elliot Smith, 1924; Le Gros Clark, 1959; Crompton, 1995), compar- ative data suggest that nocturnal visual predation had a major selective influence on the early evolution of the primate visual system (Cartmill, 1972; Heesy, 2003, 2004; Ravosa and Savakova, 2004; Ross et al., 2006; Ross and Martin, 2007). In this context, convergence of the orbital and optic axes in stem primates would have improved image quality at low light levels, increased image brightness, and facilitated stereoscopic depth judgements for manual prey capture (Cartmill, 1972; Allman, 1977; Pettigrew, 1978). Anthropoid primates (monkeys, apes, and humans) further differ from other mammals in demonstrating extreme morpho- logical specializations for high visual acuity (Ross, 2000; Kirk and Kay, 2004). High acuity in anthropoids is the product of key derived features of the anthropoid eye and retina, including an all-cone retinal fovea, a macula lutea, and a UV-screening lens. Although retinal foveae and short-wavelength ocular filters are found in some nonanthropoid species, this complex of acuity-enhancing features is unique to anthropoid primates among all vertebrates (Kirk and Kay, 2004; Ross, 2004). Adap- tive explanations for the evolution of high acuity in anthropoids emphasize the importance of a transition to diurnality in the anthropoid stem lineage (Cartmill, 1980; Ross, 1996, 2000; Kirk, 2004, 2006a,b; Kirk and Kay, 2004). Furthermore, * Corresponding author. Tel.: þ1 773 834 7858; fax: þ1 773 702 0037. E-mail addresses: rossc@uchicago.edu (C.F. Ross), eckirk@mail.utexas. edu (E.C. Kirk). 1 The authors contributed equally to this work. 2 Tel.: þ1 512 471 0056; fax: þ1 512 471 6535. 0047-2484/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jhevol.2006.09.006 Journal of Human Evolution 52 (2007) 294e313