Primate handedness: Inadequate analysis, invalid conclusions

Laterality of manual function in foraging and positional behavior in wild indri (Indri indri)

This research is based on the idea that some prosimian species are good models in which to test certain postulates of the "postural origins" theory proposed by MacNeilage and colleagues [Behavioral and Brain Sciences 10:247-303, 1987] to explain the evolution of hand preference within the order Primates. We investigated manual laterality in 16 wild indris (eight males and eight females, living in four social groups) in their habitat, the Madagascan tropical rain forest. Data were collected on two spontaneous behaviors: "branch-reach," an action that occurs during foraging, and "higher support," a posture typical of clingers and leapers. A total of seven subjects were significantly lateralized for branch-reach (two showed a right preference, and five showed a left preference). Four subjects were significantly lateralized for higher support, and all of them showed a right-hand preference. Most of the indris we studied showed no preference. Our research suggests that indri are at "level 1 of laterality" in the classification framework proposed by McGrew and Marchant [Yearbook of Physical Anthropology 40:201-232, 1997]. The data presented here are not discordant with the "postural origins" theory, as lateralized subjects are often in the direction predicted by MacNeilage and colleagues [Behavioral and Brain Sciences 10:247-303, 1987], but they are the minority.

A Meta-Analysis of Primate Hand Preferences, Particularly for Reaching

P. F. MacNeilage, M. G. Studdert-Kennedy, and B. Lindblom (1987) proposed a progression for handedness in primates that was supposed to account for the evolution of a right bias in human handedness. To test this proposal, the authors performed meta-analyses on 62 studies that provided individual data (representing 31 species: 9 prosimians, 6 New World monkeys, 10 Old World monkeys, 2 lesser apes, and 4 greater apes), of the 118 studies of primate handedness published since 1987. Although evidence of a population-level left-handed bias for prosimians and Old World monkeys supports P. F. MacNeilage et al., the data from apes, New World monkeys, and individual species of prosimians and New World monkeys do not. Something other than primate handedness may have been the evolutionary precursor of the right bias in hand-use distribution among hominids.