Twilight zeitgebers, weather, and activity of nocturnal primates.

Bush babies (Galago senegalensis), slow lorises (Nycticebus coucang), and an owl monkey (Aotus trivirgatus) in outdoor enclosures usually began activity in certain illuminance ranges of dusk and accommodated to seasonal changes in sunset time. Influences of cloud cover were variable, but marked heat and cold were inhibitory. Accurate endogenous timekeeping and a reliance on the most stable zeitgeber apparently maintain activities in synchrony with the rhythmically changing environment.