Effect of gonadal steroids on the worm-running behavior of domestic chickens
Brooke Reed, Sylvie Cloutier, Catherine Ulibarri and Ruth C. Newberry
Center for the Study of Animal Well-being, Integrative Physiology and Neuroscience (IPN),
Washington State University, Pullman, WA 99164-6520
Worm-running is behavior in which a chick runs carrying a worm-like object while flock mates follow and attempt to grab the object from its beak. This behavior has previously been found to be increased by an acute dose of testosterone given shortly before testing. We hypothesized that testosterone injected at hatch have long-term organizational effects on worm-running behavior. We predicted that pullets injected with testosterone at hatch would interact more with a worm-like object (a twisted piece of green paper) than control-, dihydrotestosterone- (DHT), or estrogen-injected birds. At 8, 9, 11, 12, 15, and 16 weeks of age, we scored worm-running behavior in 32 groups of 2-9 female White Leghorn pullets injected at hatch with testosterone, DHT, estradiol, or vehicle (safflower oil control). Interaction with the ‘worm’, including pecking, tugging, and carrying the ‘worm’ in the beak while running, walking or standing, was scored using 1-0 scan sampling at 10-s intervals during 5-min tests. The amount of interaction with the ‘worm’ decreased with age (P < 0.0001) and did not differ between hormone treatments (P < 0.05). Worm-running was more prevalent in larger than smaller groups (P=0.0009), especially at older ages (P = 0.03). Our results suggest that testosterone injection at hatch did not have long-term effects on worm-running behavior. The effect of age and group size on worm-running is consistent with the interpretation that worm-running with a non-food item is a form of play rather than representing dominance-related competitive behavior.