It will have become clear in this chapter that there is no unitary explanation for cathemerality in lemurs, but that numerous factors combine to mask the endogenous nocturnal rhythm, dominated by abiotic variables but also ranging from predation through temporal niche separation. The unpredictability of the island environment probably plays an important role in the presence of cathemerality in primates on Madagascar. The ubiquity of day-night activity across the mammals suggests that such flexibility is deeply rooted in mammalian history (Curtis and Rasmussen, 2006). Kirk (2006) suggests convergent evolution of day-night activity in many mammalian orders (also in the Lemuridae and Aotinae). A more parsimonious solution is that the ability of an essentially dark-active mammal to shift some or all of its activity into the diurnal niche may be an ancient mammalian characteristic (Curtis and Rasmussen, 2006). The ancestral lemur is assumed to have been nocturnal (Martin, 1990), but might have displayed this flexibility, facilitating the emergence millions of years ago of cathemerality and diurnality we see today. Regardless of how and when day-night activity evolved, we still have much to learn through further data collection on chronoecology, morphology, and physiology, which will increase our understanding of the underlying proximate mechanisms and the function of this activity cycle and, ultimately, its evolution.
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