Sex difference in squirrel monkeys handedness for unimanual and bimanual coordinated tasks

Whether nonhuman primates exhibit population-level manual bias remains controversial. There is a growing body of evidence showing consistent individual lateralization and population-level handedness particularly for complex manual behaviours such as bimanual coordinated actions in both monkeys and apes. Whereas the few published studies on hand preferences in squirrel monkeys, Saimiri sciureus, have focused only on unimanual behaviours and generally failed to elicit population-level handedness, we report in this paper the first set of data on hand use for a bimanual coordinated behaviour elicited by the manipulation of an artificial tube in a sample of 37 captive-born squirrel monkeys. The task consisted of holding an opaque tube with one hand and removing the food inside the tube with the other hand. For comparison, hand use data were collected from 38 subjects for unimanual reaching for food from an experimenter when the squirrel monkeys were clinging vertically on the door of the cage. For bimanual coordinated actions, we found a nonsignificant trend towards left-handedness at a population level, especially in males, whereas unimanual reaching when clinging elicited a significant predominance of right-handedness, especially in females. These results are discussed within comparative approaches across primate species and within different theoretical frameworks about the determining factors of handedness in nonhuman primates