Activity of nocturnal primates: influences of twilight zeitgebers and weather.

The time of onset of activity of bush babies (Galago senegalensis), slow lorises (Nycticebus coucang), and an owl monkey (Aotus trivirgatus) living in outdoor enclosures usually fell in certain illuminance ranges of dusk and kept pace with seasonal progressions of sunset time. Influences of cloud cover were variable, but excessive heat and cold were inhibitory. Accurate endogenous timekeeping and reliance on the most stable zeitgeber apparently maintain activities in synchrony with the rhythmically changing environment.