A Re-Evaluation of the Role of Vision in the Activity and Communication of Nocturnal Primates

Abstract

This paper examines the importance of vision in the lives of nocturnal primates in comparison to diurnal and cathemeral species. Vision is the major sense in all primates and there is evidence that the eyesight of nocturnal species is more acute and variable than has previously been recognized. Case studies of the behaviour of a galago and a loris in open woodland habitats in relation to ambient light show that Galago moholi males are more likely to travel between clumps of vegetation along the ground when the moon is up, and during periods of twilight, whereas they retreat to more continuous vegetation and travel less when the moon sets. This is interpreted as a strategy for avoiding predators that hunt on the ground when it is dark. The travel distances of Loris lydekkerianus are not affected by moonlight but this species reduces its choice of food items from more mobile prey to mainly ants when the moon sets, indicating the importance of light when searching for high-energy supplements to its staple diet. Evidence is presented for the first time to indicate key aspects of nocturnal vision that would benefit from further research. It is suggested that the light and dark facial markings of many species convey information about species and individual identity when animals approach each other at night. Differences in the colour of the reflective eye-shine, and behavioural responses displayed when exposed to white torchlight, point to different kinds of nocturnal vision that are suited to each niche, including the possibility of some degree of colour discrimination. The ability of even specialist nocturnal species to see well in broad daylight demonstrates an inherent flexibility that would enable movement into diurnal niches. The major differences in the sensitivity and perceptual anatomy of diurnal lemurs compared to diurnal anthropoids, and the emergence of cathemerality in lemurs, is interpreted as a reflection of evolution from different ancestral stocks in very different ecosystems, and not a recent shift towards diurnality due to human disturbance.

Cathemerality and Lunar Periodicity of Activity Rhythms in Owl Monkeys of the Argentinian Chaco

“…Conversely, when days are very warm, all of the diurnal activity is concentrated during the early hours in the morning and the late hours in the evening. An apparent behavioural adjustment to cold days has also been reported in A. a. boliviensis and Galago moholi [Bearder et al, 2006]. For example, although A. a. boliviensis is nocturnal in Bolivia Braza, 1987, 1993], some anecdotal evidence suggests that during unusually cold weather (6-7 ° C) owl monkeys foraged during the day [Mann, 1956].…”

Living together in the night: abundance and habitat use of sympatric and allopatric populations of slow lorises and tarsiers

“…All levels of vegetation were scanned with a Petzl Myo Zoom 4.5 V headlamp, with a mix of red and white filters. Red filters were preferred, as studies indicate that red lights are less disturbing and increase animal observation time (Ne ka ris 2003, Bearder et al 2006, Nekaris & Nijman 2007.…”

“…Detection of most nocturnal primates depends on their eye shine, which is produced by a reflective layer of tissue behind the retina known as the tapetum lucidum (Bearder et al 2006). Slow lorises possess this layer of tissue (Fleagle 1999), unlike tarsiers (Schwartz 2003).…”

Densities, distribution and detectability of a small nocturnal primate (Javan slow loris Nycticebus javanicus) in a montane rainforest

“…The activity patterns of slow lorises and indeed other lorisiformes are affected by the amount of lunar light, with lower levels of activity during bright moonlit nights (Bearder et al 2006). This difference in activity may affect detectability.…”