Cathemeral animals distribute their activity more or less uniformly between the dark and light phases of the 24-hour day. This activity pattern is quite widely observed among major mammal taxa, although the number of species involved is relatively low. Among primates, cathemerality is seen principally, though not uniquely, in the strepsirhine (“lower primate”) family Lemuridae; it is ubiquitous in the genus Eulemur, among species of which it seems to be a stable activity pattern with a history going back at least 8 - 12 million years. Nocturnal primates have visual systems that accentuate sensitivity to light over visual acuity, showing large cornea size, high rod:cone ratios and large numbers of photoreceptors relative to ganglion cells. Diurnal forms do the reverse, while Eulemur species are intermediate in these respects, suggesting substantial accommodation of the visual system to the cathemeral lifestyle. Studies of one Eulemur species have revealed that its cathemerality is underpinned by a robust endogenous circadian timing mechanism with a free-running periodicity of over 25 hours. The lemurs appear to be entrained to the 24-hour day by the light/dark cycle, plausibly with dusk as the primary zeitgeber. The causes of cathemerality have been sought in such environmental influences as predator avoidance, energy conservation, dietary quality, and interspecies feeding competition. All of these factors seem to be relevant to the cathemeral lifestyle, but none appears to be a crucial determinant. Nothing is known of the underlying neural mechanisms that facilitate the lemurs' accommodation to the extremes to which 24-hour activity exposes them.
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