Abstract
Investigations on manual laterality in non-human primates can help clarify human evolutionary origins of hand preference and cerebral cognition. Although body posture can influence primate hand preference, investigations on how posture affects hylobatid manual laterality are still in their infancy. This study focused on how spontaneous bipedal behavioral tasks affect hand preference in Hylobatidae. Ten captive northern white-cheeked gibbons (Nomascus leucogenys) were chosen as focal subjects. Unimanual grooming during sitting posture and supported bipedal posture were applied as behavioral tasks. The gibbons displayed a modest tendency on left-hand preference during sitting posture and right-hand preference during supported bipedal posture, although no group-level hand preference was detected for either posture. From the sitting to supported bipedal posture, 70% of individuals displayed different degrees of right-side deviation trends. The strength of manual laterality in the supported bipedal posture was higher than that in the sitting posture. We found significant sex differences in manual laterality during supported bipedal posture but not during sitting posture. Thus, to a certain degree, bipedal posture in N. leucogenys facilitates stronger hand preference, elicits a rightward trend in manual laterality, and produces sex-specific hand preference.
Highlights
Hand preference was viewed as unique to humans (Corballis, 2002; Marchant & McGrew, 1998; Porac & Coren, 1981)
The postural origin hypothesis states that (1) arboreal primates prefer using the right hand to support the body in trees and the left hand for manual tasks, whereas (2) more terrestrial primates prefer using the right hand during manual tasks (MacNeilage et al, 1987; MacNeilage, 1991)
The present study focused on how spontaneous bipedal behavioral tasks influence hand preference in northern whitecheeked gibbons (Nomascus leucogenys)
Summary
Hand preference was viewed as unique to humans (Corballis, 2002; Marchant & McGrew, 1998; Porac & Coren, 1981). Research on how posture affects hylobatid hand preference is still in its infancy, with only one relevant study addressing this topic using experimental bipedal reaching tasks (Olson et al, 1990) (Table 1). We chose unimanual grooming during sitting posture and supported bipedal posture as the measured tasks because these behaviors are common in primates, especially arboreal species, facilitating interspecies comparison. As bipedal posture is reported to induce a right-side shift in hand preference in great apes (Braccini et al, 2010; Hopkins, 1993), we predicted that the direction of hand preference would generally present a rightward trend from the sitting to supported bipedal posture
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