Diurnal variations in plasma testosterone in a male nocturnal primate, the owl monkey (Aotus trivirgatus).

Six adult male owl monkeys were kept in conditions of controlled lighting (12 h white light alternating with 12 h of very dim reg light) and blood samples were taken at different times of day, once a week, for 10 weeks. Plasma testosterone levels correlated with phases of the lighting cycle: highest levels (mean +/- s.e.m.) occurred in the light (24.8 +/- 5.3 ng/ml) and lowest levels during periods of darkness (4.7 +/- 1.2 ng/ml). The owl monkey is nocturnal, and these daily changes in circulating testosterone are the reverse of those reported for some diurnal primates, although the time of the high and low levels in relation to activity patterns is similar.