Adaptationism and the anthropoid postcranium: selection does not govern the length of the radial neck

The length of the radial neck has been assumed to vary in living and extinct primates in accordance with its role as a moment arm during flexion by the m. biceps brachii. We here use a simple developmental approach to investigate whether or not this trait does, in fact, vary in such a manner. We find, instead, that virtually all variation in radial neck length is explicable as a simple correlate of overall body size, and that there is no evidence to conclude that selection has separately modified radial neck length in response to differing locomotor patterns. Further implications for the interpretation of mammalian skeletal morphology are briefly discussed.