Owl monkey only primate to exhibit ‘early birds’ and ‘night owls’ behaviour

University of Pennsylvania anthropologists have shown that in Azaro’s owl monkeys, temperature and light are just as important environmental factors as the circadian rhythm that regulates periods of sleep and wakefulness.

The study indicates that changes in sensitivity to specific environmental stimuli may have been an essential key for evolutionary switches between diurnal and nocturnal habits in primates.

The goal was to find if masking (stimulation or inhibition of activity) was largely caused by changing environmental factors that affected the Azara’s owl monkeys’ internal timing system, or synchronized circadian rhythm.

Results showed that although regular daytime activity is represented by the output of a circadian clock, nocturnality is the result of fine-tuned masking of circadian rhythmicity by environmental light and temperature.

Throughout the year, nocturnal activity was higher during full-moon nights than during new-moon ones, and these peaks of nocturnal activity were consistently followed by mornings of low activity.

Conversely, new-moon nights were usually followed by mornings of higher diurnal activity than mornings following full-moon nights.

“If there was a biological clock that they were depending on to regulate this activity, you could expect the activity to continue even in the absence of lunar light,” said Horacio de la Iglesia at the University of Washington.

Azara’s owl monkeys are the only primates that have actually switched their activity pattern from strict nocturnality to one that also includes regular daytime activity.

“Harsher climate, food availability and the lack of predators or daytime competition have all been proposed as factors favouring evolutionary switches in primate activity patterns,” said Eduardo Fernez-Duque at Penn’s School of Art and Sciences.