Effects of embryonic corticosterone exposure and post-hatch handling on tonic immobility and willingness to compete in chicks

Previous experiments indicate that artificial elevation of egg corticosterone content prior to incubation may increase fear of humans, reduce the willingness to compete with conspecifics and reduce the growth rate of chicks hatching from these eggs. It can also be hypothesized that corticosterone-treated chicks might be more sensitive to the effects of stress after hatching. Accordingly, we tested the effect of embryonic corticosterone exposure (5.5 ng/ml egg) and intermittent handling after hatching on the behaviour and growth of 88 vehicle-treated (40 males and 48 females) and 88 corticosterone-treated (40 males and 48 females) chicks. The injection treatment did not have any significant effects on tonic immobility ( P ≤ 0.23), but tended to increase the male's willingness to feed in a competitive environment ( P ≤ 0.10). However, post-hatch handling increased the duration of tonic immobility in corticosterone-treated birds but not in controls ( P ≤ 0.01), as reflected in the interaction between the injection and handling treatments ( P ≤ 0.01). Intermittent handling but not corticosterone treatment reduced the willingness of male birds to eat in a competitive situation, but neither treatment affected this behaviour in females. The present results suggest that the effect of embryonic corticosterone exposure may be dependent on the hormone dosage and the environment to which birds are exposed after hatching.