Cathemerality in a small, folivorous primate: proximate control of diel activity in Hapalemur meridionalis

摘要

A non-adherence to a strict diurnal or nocturnal activity cycle is prevalent among mammals, including taxa of Lemuridae, but rare among other primates. While non-mutually exclusive ecological hypotheses attempted to explain the evolution of this activity, termed cathemerality, as either an old or a recent phenomenon, the scarcity of systematic data collected over 24 h limits our potential to explore its proximate and ultimate determinants. Among strepsirrhines, systematic studies involving only two lemurid genera (Eulemur and Lemur) have recorded this activity pattern, while fewer quantitative observations are available for other taxa. If cathemerality could be shown in most members of Lemuridae despite their different ecological adaptations, this would support the hypothesis that this trait is basal and appeared early during lemurid evolution. Here, we investigated whether the folivorous southern bamboo lemur (Hapalemur meridionalis) exhibits cathemeral activity, and determined which environmental factors influence its pattern. We deployed ten archival tags across four social groups to continuously record activity data over a 15-month period. This allowed us to generate a diurnalocturnal (DN) ratio for each 24-h period and assess their diel activity. Our data suggest that southern bamboo lemurs are cathemeral within Mandena; while climatic factors showed no influence, nocturnal activity increased with greater nocturnal luminance. Despite contrasting dietary niches, visual morphologies, and body sizes between Hapalemur, Eulemur, and Lemur, all three exhibit cathemerality and lunarphilia. The close phylogenetic proximity of these lemurids supports this flexible activity pattern as an ancestral trait that likely dates to the origin of the Lemuridae radiation.