Looking back on why we look forward: A special research series by Adam Glover, Part 3

The first set of hypotheses proposed to explain the anteriorly directed orbits and orbital convergence of primates were proposed in the early 1900’s, and have since become known collectively as making up the “Arboreal theory.” The Arboreal theory suggests that the specialization of the visual system and the evolution of modern eye anatomy were adaptations to the occupation of an arboreal niche where olfaction would not be as valuable and where the need to judge distances between branches before leaping/ locomoting acrobatically through a network of fine arboreal supports would favor bringing the eyes closer together toward the front of the face (Elliot Smith, 1912; Jones, 1916; Le Gros Clark, 1959).

However, observations of non-primate mammals which survive and reproduce in an arboreal niche without anteriorly facing eyes eventually brought this theory under scrutiny. In 1972 a new model of orbit evolution was proposed called the Visual Predation Hypothesis (Cartmill, 1972). It took issue with the fact that, as was just alluded to, at least nine other orders of mammals include arboreal species and many of them possess divergent orbits. Additionally, it noted that various non-primate, non-arboreal mammals possess orbital convergence similar to that of modern strepsirhine primates. These observations contradict the assumption that if leaping through an arboreal environment selects for the overlap of two visual fields (which by definition is what orbital convergence provides), then arboreal mammals will possess more convergent orbits than their terrestrial and semi-arboreal relatives; quantitative measurements of orbital orientation in both primate and non-primate arboreal mammals also contrast this assertion (Fig.1). Orbital convergence is not found to be significantly less in the terrestrial or semi-arboreal species than in arboreal species (Cartmill, 1970). Orbital convergence is significantly pronounced, however, in predators outside of the primates which use stereoptic integration of the two visual fields to improve the accuracy of their strikes. It is this observation that characterizes the Visual Predation hypothesis; citing the fact that other animals with forward facing eyes are predominantly predators, and that these predators have significantly convergent orbits, the Visual Predation hypothesis proposes that it was the need to stalk and grab prey (insects) at a close range that pushed the first primates towards the development of convergent orbits (Cartmill, 1972).

Figure 1. Skull length (cm) versus orbital convergence (degrees) was measured in different families of arboreal mammals to determine whether arboreality is correlated with orbital convergence. The measurements in the figure above are representative of : (A) lorisiform prosimians, (B) feloid carnivores, (C) didelphids (dashed line) and diprodont marsupials, (D) procyonid carnivores and (E) sciurids. Stars indicate a species within one of these families that is carnivorous; such a distinction is made because, outside of the primates, pronounced orbital convergence is found most often in predators. White symbols represent terrestrial mammals and stippled symbols represent semi-arboreal mammals. Least squares regressions of orbital convergence on skull length for lemurs (upper line) and haplorhine primates (lower line) are also provided for comparison. Orbital convergence was not found to be significantly less in the terrestrial or semi-arboreal species of these families, as would be expected if arboreality is what selected for orbital convergence (Cartmill, 1970).