Researchers led by a University of Pennsylvania scientist have discovered that moonlight increases the nocturnal activity of owl monkeys in South America.  The researchers say this is the first long-term study showing that heat and light can affect the nocturnality of primates in the wild.
Azara's owl monkeys were known to have regular activity patterns that included both nocturnal and diurnal (daytime) activity.  These patterns changed more frequently than a few times per year along with seasonal phenomena.  The study showed the monkeys' nocturnal activity increased in warmer months and peaked on full moon nights — but decreased during full lunar eclipses on full moon nights, which occurred three times during the study.
Lead investigator and assistant professor in U.Penn.'s Department of Anthropology Eduardo Fernández-Duque explained that many animals change their activity levels with changes in environmental conditions.  These changes are called "masking" in chronobiology.  But the species of owl monkeys is distinguishable by its increased activity during nights with better moon light.
As Fernández-Duque explained, "The behavioral outcome for these owl monkeys is nocturnal activity maximized during relatively warm, moonlit nights. While laboratory studies have pointed to the importance of masking in determining the environmental factors that cause animals to switch from nocturnal activity patterns to diurnal ones or vice versa, our study underscores the importance of masking in determining the daily activity patterns of animals living in the wild.  It also suggests that moonlight is a key adaptation for the exploitation of the nocturnal niche by primates."
"Harsher climate, food availability and the lack of predators or daytime competition have all been proposed as factors favoring evolutionary switches in primate activity patterns," said Fernández-Duque.
Team member Horacio de la Iglesia, of the Department of Biology at the University of Washington, pointed out that the changes of activity, though seasonal, aren't a built-in biological clock. He was sure that they wouldn't happen in absence of the Moon. He stated, "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. The lunar day has not been a stable force as much as the solar day to evolutionarily select for a clock. We still have to prove it in the lab, but the evidence in this paper points to a lack of a lunar biological clock."

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Researchers find moonlight influences owl monkeys' nocturnal activity