Visual System of the Only Nocturnal Anthropoid, Aotus: The Owl Monkey

Abstract

Diurnal haplorhines exhibit derived traits for high-acuity photopic vision, unique across mammals, and many other vertebrates. Yet, sometime in the mid-Miocene (12–15 MYA), the ancestor of owl monkeys shifted to a nocturnal activity pattern, which requires different visual morphology, including fundamental changes in retinal anatomy, that conflicts with the demands of a diurnal system. As one of only two night-active haplorhines, Aotus offers a unique opportunity to investigate how a visually oriented primate has adapted to low light environments. In this chapter, we synthesize data from anatomy, neuroscience, psychophysics, genetics, and behavioral ecology to review the key adaptations of the owl monkey visual system for dim light vision. First, we review Aotus evolutionary history and highlight the dramatic variation available in light environments at night. We next describe aspects of the visual system that allow nocturnal animals to experience “nighttime” differently than diurnally adapted humans. Finally, we situate Aotus within a broader comparative framework by contrasting their visual features with those observed in other primarily night-active primates (tarsiers, lemurs, and lorisiforms). Even compared to nocturnal strepsirrhines, owl monkeys exhibit a number of derived traits for high acuity vision in dim light. Moreover, although both owl monkeys and tarsiers transitioned to nocturnality from a diurnal haplorhine ancestor with high visual acuity, it appears that the two species followed very different paths in their evolution of dim light vision.