Postural behavior of howler monkeys (<i>Alouatta palliata</i>, <i>A. macconnelli</i>, and <i>A. caraya</i>) during sleep: an assessment across the genus range

Abstract

Abstract. Sleep is the longest and most continuous behavioral phase in the 24 h cycle of mammals. However, selection of postures, substrates, and tree parts during sleep has not been adequately explored, as well as their evolutionary consequences. The present study investigates postural behavior, substrate, and tree part use during sleep in three howler species (A. palliata, A. macconnelli, and A. caraya) in Nicaragua, French Guiana, and Argentina. All three species were consistent in the use of a crouched ball-like sit-in posture on large, horizontal, unramified, or bifurcated substrates, and in avoiding the periphery of tree crowns. The regularities of these sleeping patterns are very likely functionally associated with protection from potential predators and extreme weather conditions, biomechanical stability, thermoregulation, and enhancement of the digestive process of hard-to-decompose plant material.

Fetal head-down posture may explain the rapid brain evolution in humans and other primates: An interpretative review

“…The frequency of sitting and other positional behaviors noticeably varies in Old World monkeys and apes [71], which is probably applicable to the New World monkeys as well. For example, Alouatta monkeys tend to sleep crouched-up, i.e., in an almost horizontal position [78], which may be the reason why they have a relatively more spacious birth canal than Nasalis and Macaca (Fig. 3), the Old World monkeys that sleep upright [72,79].…”