Preliminary study on hand preference in captive northern white-cheeked gibbons (Nomascus leucogenys)

The evolution and biological correlates of hand preferences in anthropoid primates

The evolution of human right-handedness has been intensively debated for decades. Manual lateralization patterns in non-human primates have the potential to elucidate evolutionary determinants of human handedness, but restricted species samples and inconsistent methodologies have so far limited comparative phylogenetic studies. By combining original data with published literature reports, we assembled data on hand preferences for standardized object manipulation in 1786 individuals from 38 species of anthropoid primates, including monkeys, apes, and humans. Based on that, we employ quantitative phylogenetic methods to test prevalent hypotheses on the roles of ecology, brain size, and tool use in primate handedness evolution. We confirm that human right-handedness represents an unparalleled extreme among anthropoids and found taxa displaying population-level handedness to be rare. Species-level direction of manual lateralization was largely uniform among non-human primates and did not strongly correlate with any of the selected biological predictors, nor with phylogeny. In contrast, we recovered highly variable patterns of hand preference strength, which show signatures of both ecology and phylogeny. In particular, terrestrial primates tend to display weaker hand preferences than arboreal species. These results challenge popular ideas on primate handedness evolution, including the postural origins hypothesis. Furthermore, they point to a potential adaptive benefit of disparate lateralization strength in primates, a measure of hand preference that has often been overlooked in the past. Finally, our data show that human lateralization patterns do not align with trends found among other anthropoids, suggesting that unique selective pressures gave rise to the unusual hand preferences of our species.

Hand preferences in coordinated bimanual tasks in non-human primates: A systematic review and meta-analysis

The evolutionary significance of hand preferences among non-human primates and humans has been studied for decades with the aim of determining the origins of the population-level tendency. In this study, a meta-analysis was conducted to statistically integrate data on hand preferences in non-human primates performing the tube task and other bimanual tasks to determine the presence and direction of manual laterality. Significant individual-level lateralization was obtained for these bimanual tasks. In nonhuman primates, 82% of the animals analysed showed right or left-hand preference performing the tube task, this figure being 90% for other bimanual tasks. In contrast with humans, no asymmetry was found at the population level. Additionally, population-level preferences were not found in either of the tasks, although a strong manual preference was found when performing the tube task and other bimanual tasks. Species was studied as a variable moderator throughout the meta-analysis. These results highlight the importance of standardized testing methodologies across species and institutions to obtain comparable data and fill the gaps in the taxonomy.

Captive gibbons (Hylobatidae) use different referential cues in an object-choice task: insights into lesser ape cognition and manual laterality

BACKGROUND

Utilization of visual referential cues by non-human primates is a subject of constant scientific interest. However, only few primate species, mostly great apes, have been studied thoroughly in that regard, rendering the understanding of phylogenetic influences on the underlying cognitive patterns difficult.

METHODS

We tested six species of captive gibbons in an object-choice task (n = 11) for their ability to interpret two different pointing gestures, a combination of body orientation and gaze direction as well as glancing as referential cues. Hand preferences were tested in the object-choice task and in a bimanual tube task (n = 18).

RESULTS

We found positive responses to all signals except for the glancing cue at the individual as well as at the group level. The gibbons' success rates partially exceed results reported for great apes in comparable tests and appear to be similarly influenced by prior exposure to human communicative cues. Hand preferences exhibited by the gibbons in the object-choice task as well as in a bimanual tube task suggest that crested gibbons (Nomascus sp.) are strongly lateralized at individual but not at population level for tasks involving object manipulation.

DISCUSSION

Based on the available data, it can be assumed that the cognitive foundations to utilize different visual cues essential to human communication are conserved in extant hominoids and can be traced back at least to the common ancestor of great and lesser apes. However, future studies have to further investigate how the social environment of gibbons influences their ability to exploit referential signals. Gibbons' manual laterality patterns appear to differ in several aspects from the situation found in great apes. While not extensive enough to allow for general conclusions about the evolution of hand preferences in gibbons or apes in general, our results add to the expanding knowledge on manual lateralization in the Hylobatidae.