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Field observations demonstrate clear differences in locomotion and feeding postures between spider monkeys (Ateles) and howling monkeys (Alouatta). When feeding, Ateles employs sitting postures approximately half the time, and a variety of suspensory postures using the tail the other half. Ateles moves quadrupedally during 52% of locomotion, by tail-arm suspension 25%, and various mixed support-suspensory modes the remainder. Tail-arm suspension is practiced more rapidly on thinner supports, and on more negatively inclined supports than is quadrupedal movement. Howlers do not locomote by tail-arm suspension: movement is almost entirely quadrupedal and is slower than that of spider monkeys. The positional behavior of spider monkeys fits closely recent views of major adaptive changes in hominoid evolution emphasizing brachiation and speed during travel. Howler locomotion and also tissue composition appear related to diet and digestive mechanisms.
John G. H. Cant (Fri,) studied this question.