Gorilla and Homo Skeletons: Comparative skeletons from a macaque monkey (not pictured- on page 82), gorilla and man illustrate the physiological progression from four-footedness through arm-swinging to two-legged walking. The monkey's arms are somewhat shorter than its legs; it can walk with its hands palm down, an impossible feat for a gorilla. On the other hand, the monkey's down-turned shoulder sockets prohibit brachiation in the true sense of
Male baboon: Safe from predators, a male baboon adopts the posture it may keep all night long. Its ischial callosities, or callous pads, bear its weight and permit it to sleep sitting up even on slender branches.