Arboreal locomotion and trophic security at the dawn of Euprimate vision

Abstract

Primate vision is thought to have evolved in connection with life in the trees. However, several inter-related origins theories—those addressing possible co-evolution with size, predation, diet, daylight, locomotion, and groups—also provide reasonable explanations of their distinct cranial-visual morphology. We hypothesized that demand for high-speed arboreal grasp-landing facilitated predation avoidance thereby reducing the need for lateral facing orbits. To test this proposed influence, in the context of a likely very multi-causal adaptive landscape, we consolidated published data on extant primate species including body mass, daily path length, arboreality, insectivory, frugivory, activity period, leaping, swinging, and group size. Phylogenetically controlled regressions, on three different taxonomic subsets of the primate order, highlight size and environmental influences as the most compelling factors explaining higher orbital convergence [OC]. Moreover, activity period and group size (in anthropoids) as well as arboreality and body mass (in non-anthropoids) associated convincingly with higher OC. After considering size and path length, suspensory (and to a lesser extent leaping-based) grasp-landed locomotion co-varied with OC, primarily in anthropoids. Nocturnality had negative, and leaping mixed, associations with OC—thus, with the exception of those relating to nocturnal-locomotion, all adaptive origins theories considered were at least partially corroborated. The conflicting associations of OC with leaping, is largely attributable to the exceptionally small (and more taxonomically contentious) members of the order. But prospects otherwise remain bright for our two freshly illuminating theories of grasp-swing, in anthropoids, and rear attack risk reduction [RARR], more generally, as they provide compelling alternatives to sized based models (e.g. predation deterrence and allometric scaling) in explaining deep divisions in the primate order.