Ecological and Anthropogenic Correlates of Activity Patterns in Eulemur

The ultimate determinants of cathemerality, i.e., activity spread over the 24-h cycle, in primates have been linked to various ecological factors. Owing to the fast rate of habitat modification, it is imperative to know whether and how this behavioral flexibility responds to anthropogenic disturbance. The true lemurs (Eulemur clade) constitute a valuable case to study these potential effects, as all species studied so far exhibit cathemerality. Here we explored the effects of anthropogenic disturbance on activity patterns of Eulemur while controlling for ecological factors proposed as determinants of activity shifts. We first performed a meta-analysis using 13 long-term studies conducted over the last three decades on various populations of Eulemur. We fitted a beta regression using the proportion of diurnality (the activity taking place between sunrise and sunset) as the response variable and seven climatic, ecological, and anthropogenic disturbance variables at each site as predictors. We also present a validation with original data using year-round, 24-h activity of collared brown lemurs (Eulemur collaris) in forest fragments with different levels of disturbance in southeastern Madagascar. Diurnality was prevalent at most sites. Seasonality, proportions of leaves in the diet, and group size were all found to be significant predictors of the proportion of diurnal activity. After controlling for socioecological factors in the model, overall anthropogenic disturbance emerged as a negative predictor of diurnality. Our validation suggests that the lemurs in the more disturbed area exhibited more nocturnal activity than those in the less disturbed area. It is unclear whether the plasticity observed might allow populations of Eulemur to persist in disturbed areas longer than lemurs with less flexible activity patterns.