Evolving through day and night: origin and diversification of activity pattern in modern primates

Activity patterns have profound implications on primates’ morphology, physiology, and behavior and have likely driven their diversification. The last common ancestor of extant primates has been traditionally considered nocturnal although this notion has been recently debated due to emerging contradictory evidence. Previous studies underestimated the role of cathemerality (i.e., the ability to remain active throughout a 24-h cycle) by simplifying primate activity to the diurnal–nocturnal dichotomy. We estimated the evolutionary trajectories of activity patterns in primates and investigated how these may have influenced their diversification rates. We used a comprehensive data set to test multiple evolutionary hypotheses, following a robust Bayesian framework by using 5000 calibrated phylogenetic trees to account for phylogenetic uncertainty. Our results support a nocturnal ancestor that has shifted to diurnality in the Simiformes, has retained nocturnality in Lorisiformes and most Lemuriformes, and shifted to cathemerality in the ancestor of Lemuridae. The diversification of activity patterns in primates seems to have mainly arose by speciation rather than shifts between activity patterns, suggesting a low flexibility of diurnal and nocturnal patterns and the key importance of cathemeral activity as transitional state to shift between more specialized activity patterns. A cathemeral activity seems to appear well before diurnality in Malagasy lemurs, suggesting an ancient origin of this trait on the island and rejecting the hypothesis of a recent transition. The present research contributes to further disentangle the adaptive role of activity patterns in primate evolution.