Baseline cortisol levels and social behavior differ as a function of handedness in marmosets (Callithrix jacchus)

Population hand preferences are rare in nonhuman primates, but individual hand preferences are consistent over a lifetime and considered to reflect an individual's preference to use a particular hemisphere when engaged in a specific task. Previous findings in marmosets have indicated that left‐handed individuals tend to be more fearful than their right‐handed counterparts. Based on these findings, we tested the hypotheses that left‐handed marmosets are (a) more reactive to a social stressor and (b) are slower than right‐handed marmosets in acquiring a reversal learning task. We examined the hand preference of 27 male and female marmosets (ages of 4–7 years old) previously tested in a social separation task and a reversal learning task. Hand preference was determined via a simple reaching task. In the social separation task, monkeys were separated from their partner and the colony for a single 7‐hr session. Urinary cortisol levels and behavior were assessed at baseline, during the separation and 24 hr postseparation. Hand preferences were equally distributed between left (n = 10), right‐handed (n = 10), and ambidextrous (n = 7) individuals. The separation phase was associated with an increase in cortisol levels and behavioral changes that were similar across handedness groups. However, cortisol levels at baseline were positively correlated with right‐handedness, and this relationship was stronger in females than in males. In addition, the occurrence of social behaviors (pre‐ and postseparation) was positively correlated with right‐handedness in both sexes. Baseline cortisol levels did not correlate significantly with social behavior. Acquisition of the reversals was poorer in females than males but did not differ as a function of handedness. We conclude that (a) both stress reactivity and cognitive flexibility are similar across handedness groups and (b) left‐handers exhibit less social behavior and have lower basal cortisol levels than ambidextrous and right‐handed subjects. The underlying causes for these differences remain to be established.