Living in the Dark: Exploring the Factors Driving Nocturnal Activity in Three Lemur Species

Many animal species have adapted to being active both during the day and night (i.e., cathemeral) as an ecological response to the varying environmental conditions that occur along the day-night cycle. Understanding the extent of nocturnal activities of cathemeral species and the factors that may influence such patterns can, thus, provide critical insights into the mechanisms shaping animal behavior and evolutionary adaptation. With a focus on three lemur species (<i>Eulemur rubriventer</i>, <i>E. rufifrons</i>, and <i>Varecia variegata editorum</i>) in a Malagasy tropical rainforest, we investigated how nocturnal luminosity (NLI), temperature, and rainfall influenced their active state and distance traveled at night compared to the day. We monitored them across 24-h cycles from May 2023 to September 2024 using GPS collars. We found that both <i>Eulemur</i> species displayed distinct cathemeral activity patterns with longer distances traveled at nighttime than during the day; however, there was no difference in range sizes between day and night for <i>E. rufifrons</i>. <i>Varecia variegata editorum</i> individuals displayed fewer active states at night, shorter nocturnal travel distances, and a smaller nighttime range than during the day. We also found that NLI, temperature, and rainfall influenced the amount of nocturnal activity and the distances traveled at night, but the influencing factors varied across the three species. These findings highlight the dynamic nature of lemur activity patterns, highlighting the potential adaptive role of cathemerality in response to changing environments. They also point out the critical role of astronomical variations (i.e., moonlight conditions) and climatic variables in shaping animal ecology. More broadly, our findings provide important insights for the conservation planning of endangered species, as nocturnal activity patterns may influence their vulnerability to nighttime hunting, habitat disturbance, and the availability of light penetrating the canopy, especially as environmental changes and habitat loss continue to threaten animal survival.