Uniquely among higher primates, night monkeys are typically nocturnal, although cathemeral activity (mixed day and night activity) has been reported for some populations. Several lines of evidence indicate that they have become secondarily adapted for nocturnal life, following divergence from a diurnal ancestor. During the daytime, they typically sleep in tree hollows. Their basal metabolism is relatively low and this is reflected in quite sluggish movement and limited ranging during the nocturnal phase of activity. Olfactory marking is performed with urine and with marking glands. There is a small marking gland on the chest and a diffuse glandular area on the underside of the tail base. Night monkeys also perform "urine washing," in which the palms of the hands and the soles of the feet are impregnated with urine that is then deposited during locomotion.
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