More eggs but less social and more fearful? Differences in behavioral traits in relation to the phylogenetic background and productivity level in laying hens

Different lines of laying hens have undergone a strong selection pressure for productivity traits, which has been proposed as a potential cause of undesirable side effects like behavioural disorders. One reason for such behavioral changes might be due to energy trade-offs, as high productive laying hens may conserve energy expenditure on activity in order to sustain the high level of egg production. However, beside the level of egg production, also phylogenetic origin has potential implications on differences in fearfulness, stress reactivity and sociality in layers. In this study, we compared four purebred layer lines differing in egg laying productivity and phylogenetic origin on their fearfulness and sociality prior to the start of laying. We used 77 chickens in a 2 x 2 group design, with two layer lines with white egg shell color and two lines with brown color which differed in laying performance (~200 versus ~300 egg / y, respectively, within colors). We measured fearfulness and sociality using five different behavioral tests (visual cliff, social approach, novel-object, mirror approach and tonic immobility). The data from the tests were analyzed using a principle component analysis revealing that our data are best explained by two components representing the dimension social behavior (PC1) and fear (PC2), respectively, which were further analyzed by a linear model (LM). The two high productive lines showed a lower social motivation (lower values on PC1social, LM: p < 0.0001) than the two less productive lines and the white hens had a higher social motivation than brown hens (higher values on PC1social, LM: p = 0.0004). Our results indicated that phylogeny and level of egg production affected the two dimensions of behavior differently with sociality but not fearfulness likely linked to the level of egg productivity. The decreased engagement in social behavior is consistent with the notion of energy trade-offs due to intense selection on egg productivity. The unintentional impacts of selection on behavior deserves further research, especially with regard to potential effects on welfare.