The nocturnal primate niche in the New World

This integrated approach to understanding the evolution of the nocturnal monkey Aotus combines new information from field research on behavioral ecology with research results describing physiology and sensory capabilities. First, comparisons are made between the ecology of Aotus in rain forest and a sympatric same-sized diurnal monkey, Callicebus moloch. Differences were found in: (1) ranging patterns and population densities (but not home range size); (2) sleep-tree selection and regularity of exit times; (3) diet in the season of scarce resources and (4) harassment by diurnal monkeys. Next, studies of Aotus in a contrasting habitat, with diurnal raptor predators and harassing diurnal monkeys absent, suggest that Aotus has behavioral flexibility in ranging patterns, sleep-tree site selection, and day time activity. But in other parameters, such as the monogamous social system and small territory size, Aotus shows no deviation, regardless of changes in environment. Adaptations to a nocturnal lifestyle shared among the night monkey and nocturnal prosimians include a low basal metabolic rate, small body size and good vision at low light levels. Comparisons suggest that convergences between the monkey with a diurnal ancestor and the prosimians with nocturnal ancestors exist, but Aotus is intermediate between prosimians and diurnal monkeys in visual acuity in low light levels, color vision, and olfactory capabilities. Auditory acuity and reproductive biology are not different from diurnal monkeys. Within the radiation of primates found in the New World, the nocturnal niche is a successful alternative lifestyle.