Thermoregulation in the only nocturnal simian: the night monkey Aotus trivirgatus

The night monkey, a tropical monkey, is the only nocturnal simian; its thermoregulation was studied for comparison with other nocturnal or diurnal primates and other tropical mammals. Resting metabolic rate was 2.6 W (closed-circuit method) and 2.8 W (open-circuit method), 24 and 18% below the value predicted from body mass. The thermoneutral zone was very narrow; the lower critical temperature (LCT) was 28 degrees C and the upper critical temperature (UCT) was 30 degrees C. The body temperature (Tb) was at its minimum (38 degrees C) at an ambient temperature (Ta) of 25 degrees C, thus below the LCT. At low Ta, the increase in metabolic rate (MR) was smaller than predicted by the Scholander model, since MR intersected to a Ta 13 degrees C above Tb when extrapolated to MR = 0; this was attributed to a decrease of body surface area by behavior. The thermal conductance at the LCT was low: 2.3 W . m-2 . degrees C-1. Above the UCT, panting was the major avenue of heat loss. The response pattern of nocturnal habits, low resting metabolic rate, low thermal conductance, and panting in the night monkey, unique among simians, is found in many other mammals of tropical and hot desert habitats; it may be considered as an alternative adaptation to the thermal environment.