Spider monkeys (Ateles geoffroyi) are vocally active throughout the night in a human-modified habitat.

Diurnal primates show periods of wakefulness at night because of ecological factors, but the degree to which they shift the timing of their activities in response to anthropogenic factors is poorly understood. We aimed to examine whether Geoffroy's spider monkeys (Ateles geoffroyi) are active during the night and evaluate the effects of climatic and anthropogenic factors on their nocturnal activity. We conducted 66 half-night behavioral observations on wild spider monkeys at a known sleeping site in the human-modified habitat of Los Árboles Tulum, Quintana Roo, Mexico between August 2018 and July 2019. We used generalized linear mixed models to test the effects of moonlight intensity, heat index, anthropogenic light, and anthropogenic noise on vocal and non-vocal activity during the night. Diurnal spider monkeys showed some level of activity at all hours of the night within their sleeping site. We found no effect of the predictor variables on the amount of time spider monkeys were non-vocally active, but the number of vocalizations was positively affected by moonlight intensity, heat index, and anthropogenic noise, and negatively by anthropogenic light. Increased moonlight intensity, perceived temperature and noise may disrupt sleeping patterns, leading to more vocal activity, whereas spider monkeys may emit whinnies when they cannot see one another, potentially explaining the negative relationship between the number of whinnies and anthropogenic light. Our findings contribute to the view that diurnal primates may be more flexible in their activity patterns than previously thought and that spider monkeys can adjust to permanent low-level anthropogenic impact and live alongside humans in modified habitats.