Adaptive significance of primate binocular vision in grasping and locomotion



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Forward facing eyes, which provide a wide field of binocular vision and precise depth perception, are one of the diagnostic features of crown primates. However, the adaptive significance of this feature remains contentious. Competing primate origins hypotheses variously propose that (1) foraging for fruit, (2) feeding on insects, and (3) leaping locomotion were responsible for the evolution of diagnostic primate traits, including forward facing eyes and a wide binocular visual field. This dissertation uses an experimental approach to improve our understanding of the evolution of this diagnostic primate trait. To evaluate which primate origins hypotheses provide a viable explanation for the evolution of primates’ forward facing orbits, the importance of binocular depth cues for the three tasks invoked by the hypotheses was evaluated experimentally in two species of lemurs considered reasonable living analogues of the earliest primates. Performance at fruit foraging, insect predation and leaping were evaluated when the animals had use of their full binocular visual field and when their binocular visual field was restricted using a helmet-mounted blinder. Restriction of the binocular field had no effect on fruit foraging performance. However, the restricted binocular field condition significantly decreased performance in both leaping and insect predation. Subjects leapt more slowly, restricted leaps to shorter distances and were more likely to exhibit adverse landings when their binocular field was restricted. Subjects were also significantly less likely to successfully contact and retrieve insect prey when their binocular field was restricted. Differences in behavioral variables also suggest that insect predation is a more visually demanding task than fruit foraging. These results support the role of both leaping and insect predation, but not fruit foraging, in contributing to the selective pressures that led to the evolution of forward facing eyes and a wide binocular field in crown primates.



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