Hand preference depends on posture in common marmosets

Postural conditions are known to affect hand use in many primate species. It remains to be examined how posture and task differences modulate, or interact with, hand use in common marmosets (Callithrix jacchus). Three experimental conditions were introduced to assess the occurrence of preferential hand use as a function of posture and size of aperture for the subjects to grasp and retrieve the food item bi- or unimanually: ground level condition for requiring quadrupedal posture with enough space for food reaching with both hands, large hole (4 cm in diameter) condition for requiring upright posture with available space for both hands, and small hole (2 cm in diameter) condition for requiring upright posture with available space for only one hand. While the distribution of hand preference did not significantly differ among the three conditions at the group level, eight out of twelve marmosets did not change hands when identical upright postures were required in large and small hole conditions. Some marmosets simultaneously used both hands when both hands were free to reach the food items; however, the number of left hand users increased when the marmosets were forced to use either hand to pass through the reduced hole size. Significant correlations in hand use between the upright posture conditions were observed, whereas no correlation was observed between the different posture conditions. These results suggest that, although preferential hand use was not found at group level, posture, but not size of aperture, has effects on hand use in individual common marmosets.