Influence of mating ratio and group size on indicators of fearfulness and stress of hens and cocks

The effects of the male to female mating ratio on the heterophil to lymphocyte ratio and duration of tonic immobility (indicators of stress and fearfulness, respectively) were analyzed in two different experiments with different group sizes. In Experiment 1, four different mating ratios (1:11, 1:5, 1:3, and 1:1) and 10 different Spanish breeds of chickens were used; each breeding pen had 12 birds. In Experiment 2, two different mating ratios (1:11 and 1:1) and four breeds were analyzed; 60 birds were used. There were significant differences among mating ratios (P < 0.001) for the heterophil to lymphocyte ratio in females of both group sizes. Hens housed with a mating ratio of 1:1 had higher heterophil to lymphocyte ratios than did hens housed with a 1:11 mating ratio in the group with 12 birds (0.53 vs. 0.35) or that with 60 birds (0.76 vs. 0.37). Hens in a mating ratio of 1:1 had significant heterophilia and lymphopenia (P < 0.001). The 1:5 and 1:3 male to female mating ratios did not differ significantly from the 1:11 mating ratio. The effect of the group size was significant when the mating ratio was 1:1 but not when it was 1:11. There was no significant difference among mating ratios in terms of tonic immobility for hens. Cocks housed with a mating ratio of 1:1 and a group size of 60 birds showed shorter duration of tonic immobility (P < 0.05) than did cocks housed with a 1:11 mating ratio (203 vs. 296 s). The results suggest that very high mating ratios should not be used in conservation populations because it increases physiological and psychological stress responses.