The evolutionary origins of human handedness are still unknown. The study of lateralized behaviour in our closest relatives, the nonhuman primates, is useful to clarify how this trait appeared and evolved in our species. In the present study, lateralized behaviour was assessed in a population of 32 free-ranging Mexican mantled howler monkeys (Alouatta palliata mexicana) for thirteen spontaneous motor patterns, at individual and group levels, as well as the effect that age, sex and posture have on its strength and direction. The studied population of howler monkeys displayed only few significant lateral biases at the individual level with single motor patterns (Binomial tests, p≤0.05). No biases towards the use of a particular limb or side of the body were found at a population level. Therefore, even though some individuals showed significant limb/side preference with single motor patterns, no signs of task specialization, side specialization, or true handedness were found. Similarly, no effects of sex, age or posture were found on the direction or strength of lateralized behaviour. The general absence of limb/side preferences found in this population may be due to the constraints imposed by the arboreal life and/or the type of diet. Possible causal agents of the few significant individual biases found here may be the presence of handicaps and/or experience. Further research is needed in order to assess whether the lack of human-like handedness reported in this study is only specific to the studied population, a general phenomenon of the genus Alouatta or perhaps of all the Platyrrhini.

The evolutionary origins of human handedness are not yet fully understood as evidence of lateralized behavior in nonhuman primates is inconclusive. In the present study, lateralized behavior in both spontaneously occurring motor patterns and a tube task was examined in 15 white-handed gibbons (Hylobates lar). Significant side preferences at the individual level were found within all 15 studied motor patterns. However, no population-level side bias was found for any of the spontaneously occurring or task-related motor patterns and none of the gibbons were consistent in their hand preference across all motor patterns. When only considering the individuals with a significant preference, a significant majority was left-preferent for resting foot. Strength of side preference was significantly higher for the tube task than for all spontaneously occurring motor patterns. Side preferences for manipulation and resting position were significantly stronger than those for supporting hand. Additionally, the preferences for manipulation were significantly stronger than those for leading limb. In the bimanual tube task, females displayed a tendency towards a left-side bias, while males tended to display a bias to the right. Furthermore, females had a significantly stronger hand preference for supporting hand than males. No other sex differences were found. Age, posture, and kinship had no significant effect on lateralized behavior for any of the motor patterns. As in other nonhuman primates, the white-handed gibbons were only consistent in their hand preference across tasks that required similar movements. Altogether, these findings support the notion that population-level handedness may be restricted to human subjects.