On the move around the clock:: correlates and determinants of cathemeral activity in wild redfronted lemurs (Eulemur fulvus rufus)

Whether animals are active at night or during the day has profound consequences for many aspects of their behavioral ecology. Because of ecological and physiological trade-offs, most animals, including primates, are either strictly nocturnal or diurnal. However, a few primate species exhibit cathemeral activity, i.e., their activity is irregularly distributed throughout the 24-h cycle. Details and determinants of this unusual activity pattern are poorly understood because long-term 24-h observations are not feasible in the field. We therefore used small data loggers to record the activity of cathemeral redfronted lemurs (Eulemur fulvus rufus) from several neighboring groups quantitatively and continuously over a complete annual cycle in order to evaluate various proposed proximate and ultimate determinants of cathemeral activity. Activity data were examined for variation as a function of ambient temperature, time of day, lunar phase, and season. We found that cathemeral activity occurred year-round and that, on average, 3.5 times more activity occurred during the day. Total and diurnal activity increased during the long days of the austral summer. Nocturnal activity increased during the longer nights of the cool dry season. Irrespective of season, lunar phase had a significant effect on the distribution of activity across the 24-h cycle, with most nocturnal activity recorded during parts of the night with greatest brightness. These data indicate that light availability is the primary proximate determinant for the patterning of cathemeral activity. Several lines of evidence suggest that cathemerality in lemurs has evolved from nocturnal ancestors and that it represents a transitory state on the way to the diurnal niche.