Cool, safe, thirsty and hungry: determinants of cathemerality in the brown lemurs in north-western Madagascar

抄録

Despite the contrasting niches during daytime and nighttime, few primates called cathemeral exhibit significant activity distributed over the 24-h. The main hypotheses suggested as the adaptive significance of cathemerality were thermoregulation strategy to avoid heat/cold stress, antipredator strategy to avoid diurnal raptor, feeding strategy to fill energy requirement by feeding fibrous food over 24-h during periods of fruits scarcity. Few studies tested together these hypotheses and found that feeding strategy appeared to be the strongest factors determinants of cathemerality in Eulemur (Donati et al 2011 Animal behav). However, this was a controversial hypothesis for cathemeral lemurs, Hapalemur, that could digest fiber (Eppley et al 2017 Behav Ecol Sociobiol). A study featuring Eulemur did not support also this hypothesis (Curtis et al. 1999 Amer J Primatol). We previously found that cathemerality in dry forest in Ankarafantsika is probably a strategy for maintaining water-balance on days of high ambient temperatures and dry condition (Razanaparany and Sato 2020 Folia Primatol). Therefore, we tested the hypotheses postulated as the adaptive significance of cathemerality in the brown lemurs in dry forest in northwestern Madagascar. We followed two groups of (Common brown lemur) Eulemur fulvus all-day and all-night, over 9 months, equality distributed two seasons. As brown lemurs were hypothesized to shift into nocturnal during defoliation periods to avoid diurnal raptors, we determined the canopy openness and use the data as a proxy of exposure to raptors. We assessed the fiber intake, water intake from food and metabolizable energy intake. We found that the diurnal activity level of the brown lemurs decreased on days of lower humidity, higher ambient temperatures, and closer canopy cover by leaves. Their nocturnal activity level increased with their energy intakes and during defoliation periods. Our results supported that cathemerality is probably thermoregulation to cope with the heat stress, an anti-predator strategy to avoid the diurnal raptors. As canopy cover and humidity are likely linked to the water availability, these results also suggest that cathemerality is probably a strategy to cope with dry/heat stresses in the daytime and ensuring the energy intake during the night. Water accessibility would explain the increase of their diurnal activity during hot daytime and the decrease of nocturnal activity level and nocturnal energy intake at night in the wet season.