Abiotic Factors Affecting the Cathemeral Activity of Eulemur fulvus in the Dry Deciduous Forest of North-Western Madagascar

Abstract

Cathemeral primates perform significant amounts of activity during the daylight and dark portions of the 24-h cycle. Most brown lemurs have been reported to be cathemeral. A previous study reported that brown lemurs in Ankarafantsika National Park (ANP) shift their activity pattern from being cathemeral in the dry season to being diurnal in the wet season. From July 2015 to March 2016, we collected data on active behaviour of brown lemurs over 46 full-day and 33 full-night observations, distributed evenly between the dry and wet seasons. This study examined the abiotic factors that potentially allow this seasonal shift from cathemeral to diurnal activity. We analysed the effects of day length (time from sunrise to sunset), nocturnal luminosity, and climatic factors on the diurnal and nocturnal activities of the brown lemurs in ANP using generalized linear mixed models. We found that the brown lemurs were cathemeral regardless of season. Their diurnal activity increased with increasing day length and at high humidity, but decreased on days of high temperature and low lunar luminosity. Their nocturnal activity increased with decreasing day length, during bright nights, at low temperature, and at low humidity. Their diurnal and nocturnal activities decreased during heavy rainfall. These data indicate that the photoperiodic variation entrained their activity rhythm. Bright nights may provide better visibility, advantageous for the detection of predators and food, although it may increase exposure to predators. The modification of activity level with temperature variation can be a behavioural thermoregulation. The high humidity during the wet season probably reduced evaporative cooling, inducing increased body temperature and stimulating water foraging as a potential response to dehydration and active heat dissipation. Investigation on physiological responses to environmental factors is needed to fully understand cathemerality in lemurs. These findings emphasize the flexibility of diurnal and nocturnal activity level of brown lemurs according to the great seasonal variation in tropical dry forests.