Predictors of orbital convergence in primates : A test of the snake 1 detection hypothesis of primate evolution 2 3 Brandon

22 Traditional explanations for the evolution of high orbital convergence and stereoscopic vision in 23 primates have focused on how stereopsis might have aided early primates in foraging or 24 locomoting in an arboreal environment. It has recently been suggested that predation risk by 25 constricting snakes was the selective force which favored the evolution of orbital convergence in 26 early primates, and that later exposure to venomous snakes favored further degrees of 27 convergence in anthropoid primates. Our study tests this snake detection hypothesis (SDH) by 28 examining whether orbital convergence among extant primates is indeed associated with the 29 shared evolutionary history with snakes or the risk that snakes pose for a given species. We 30 predicted that orbital convergence would be higher in species that: 1) have a longer history of 31 sympatry with venomous snakes, 2) are likely to encounter snakes more frequently, 3) are less 32 able to detect or deter snakes due to group size effects, and 4) are more likely to be preyed upon 33 by snakes. Results based on phylogenetically independent contrasts do not support the SDH. 34 Orbital convergence shows no relationship to the shared history with venomous snakes, 35 likelihood of encountering snakes, or group size. Moreover, those species less likely to be 36 targeted as prey by snakes show significantly higher values of orbital convergence. Although an 37 improved ability to detect camouflaged snakes, along with other cryptic stimuli, is likely a 38 consequence of increased orbital convergence, this was unlikely to have been the primary 39 selective force favoring the evolution of stereoscopic vision in primates.