Left or right: Handedness in free-ranging Hanuman langurs (Semnopithecus entellus) residing in an urban ecosystem.

Abstract

Examining manual lateralization (handedness) in nonhuman primates might be an interesting approach to gaining insight into the evolution of asymmetry in humans. Moreover, handedness could also reflect the effect of environmental alterations on the free-ranging animals who are forced to live with anthropogenic interferences. Despite addressing the handedness among monkeys and apes, only a few studies have focused on these free-ranging urban-adapted nonhuman primates, which could challenge our perception of habitat loss and deforestation. Here, we conducted 193 field-based experimental trials with two experimental tasks, one unimanual (simple reaching) and one bimanual (tube task) to explore manual lateralization in a highly human-provisioned group of free-ranging Hanuman langur (Semnopithecus entellus). Experimental outcomes revealed an asymmetrical hand-use distribution, with a bias toward the left hand. As bimanual tasks evoked a higher degree of lateralization, these tasks seem to be more suited to study manual laterality, and our results also highlight the significance of experimental tasks in establishing hand preference in langurs. Furthermore, this study also reveals that such lateralization developed with age as adults distinctly displayed their preference toward left-hand usage in contrast to juveniles and subadults who used both hands comparably. Mostly considered to be arboreal, the langurs of our study group spend a considerable amount of time with humans on the ground, thereby portraying a terrestrial tendency. Postural Origin Theory states that terrestrial animals tend to use their right hand and arboreal their left. Therefore, here the presence of group-level left-hand biasness in the adult langurs of Dakshineswar creates a dilemma in the Postural Origin Theory.