The interaction of dominance status and supplemental tryptophan on aggression in Gallus domesticus males.

In previous studies, we have found that supplemental dietary tryptophan (TRP) decreases aggression in feed-restricted male chickens (Gallus domesticus). The objective of this study was to determine if social status influences the effect which TRP has on aggression. In both experiments, Gallus males were placed on a commercial feed restriction program in which measured amounts of feed are delivered on alternate days beginning at 4 weeks of age. In the first study, birds were fed either 0.19 (control), 0.75 and 1.5% dietary TRP. In the second study, birds were fed either 0.19, 0.38, 0.75 or 1.5% dietary TRP. Dominance hierarchies were linear (p less than 0.05) in all treatment groups as indicated by Landau's indices, which ranged from 0.87 to 0.98. A significant interaction was found between dominance status and supplemental TRP for aggression, with TRP decreasing pecking more in dominant than subordinate birds. In Experiment 1, there were positive correlations (p less than 0.05) between body weight and dominance status in both the control and 0.75% TRP group (p less than 0.05) and no correlation in the 1.5% TRP group. The decrease in aggression by dominant males may have allowed subordinate birds to gain greater access to the feed. Our results indicate that dominant and subordinate males show a differential sensitivity to the effects of dietary TRP, with TRP decreasing pecking more in dominant birds.