Maternal care and selection for low mortality influence mineralocorticoid receptor levels and behavior in laying hens
Heerkens, Jasper L.T.
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Feather pecking and cannibalism are major welfare problems in commercial laying hen husbandry. Breed differences in feather pecking indicate a genetic component. Further, early-life conditions play an important role in the development of these behaviors. A selection experiment was initiated selecting on low mortality in group housing for four generations. This resulted in a low mortality line (LML) and an unselected control line (CTL). Previous studies have shown that chickens from the low mortality line show a number of behavioral and physiological differences compared with their control counterparts, including altered whole-blood serotonin levels, plasma corticosterone levels and open field behavior. Here, behavioral differences between selection lines were investigated further and it was tested whether interactions between genetic selection and maternal care determined on mineralocorticoid receptor (MR) levels, which could have implications for sociality, fearfulness and thus welfare. Brains from the second generation of LML and CTL, reared either with or without maternal care, were collected at 40 weeks of age and immunocytochemically stained for MR. No differences were seen between selection lines in MR receptor expression. Brooding hen presence lead to MR lateralization in the nidopallium caudolaterale + posteriosis amydalopallii, and tended to increased MR levels in the left hemisphere in the nidopallium caudolaterale + posteriosis amydalopallii. No differences were seen in the hippocampus. At 1-4 weeks of age, behavior of fourth generation LML birds and CTL birds was studied in an open-field test, T-maze test, holeboard test and in a voluntary approach test. No line differences in sociality and fearfulness were found in the open-field test. The holeboard tests revealed no line differences in working memory and reference memory. T-maze results showed increased sociality at 12-16 days of age: the LML spent more time with conspecifics (LML:181±38s; CTL:0±0s) in the T-maze and showed a learning curve in time needed to find conspecifics in the T-maze (p<0.001 for cubic trend), whereas the control line did not show any learning. At 26 days the LML were less fearful as they had a shorter latency to approach a familiar human in the voluntary approach test [LML: (mean ± standard error of the mean) 17±6s CTR: 84±14s]. Thus, brooding hen presence during the rearing period leads to increased MR expression in the left hemisphere and increased MR receptor lateralization in later life. Selection for low cannibalism in laying hens leads to increased sociality and reduced fearfulness at young age.