The Lateral Balancing of Handedness Tested in Slowly and Rapidly Moving Lorisidae

Abstract

The handedness of primates may follow either from learning mechanisms, especially operant conditioning, or from a functional asymmetry of hemispheres. However, in contrast to handedness in man, the lateral preference of hand application in non-human primates may show significant temporal fluctuations. This has been evidenced by long-term studies in Cercopithecidae (Cole 1957, Warren et al. 1967, Ettlinger and Moffet 1964, Beck and Barton 1972, Lehman 1978, Flowers 1975, Todt and Götz 1980) and Callithricidae (Rothe 1973). In accordance with the common finding that these fluctuations occur particularly prominently (1) in young individuals, and (2) during test situations easy to settle and familiar to the animals, we suggest a hypothesis providing both an explanation of lateral fluctuations and a basis for further experiments. There is a central component that promotes a lateral balancing of handedness: After successful conditioning of a particular hand (skill 1), this component generates lateral variation of handedness, specifically without the precondition of a previous unsuccessful application (= frustration of the conditioned hand). Consequently, the component guarantees that not only one, but both hands can be trained correspondingly on the long run (skill 2; Fig. 1).