Tonic immobility and effects of early stress on chickens (Gallus gallus)

Abstract: Tonic immobility, TI, is an innate fear response in many vertebrate animals, induced by brief physical restraint. It is a widespread theory that chickens (Gallus gallus) reared under stressful conditions react stronger in tonic immobility tests, i.e. stay still and lay still for a longer period of time, than individuals reared under more stress-free conditions. In our study we attempted to see if stressful conditions early in life (temporary isolation from the flock on a daily basis) had any effect on how the individuals handled the stressful and fear-evoking experience of tonic immobility tests. A total of 77 chickens of the HyLine strain of White Leghorn were used. Three sets of tests were performed; first at the hatchery facility at Linköpings universitet; second after a stressful experience – the moving to the Wood-Gush facility at Vreta jordbruksgymnasium; and a random sample with about half of the birds the third time, also at Vreta. The results were inconclusive, but pointed more in the direction of the early-in-life stress having no effect on TI tests rather than the other way around.