Feather pecking and monoamines - a behavioral and neurobiological approach

Abstract

Severe feather pecking (SFP) remains one of the major welfare issues in laying hens. SFP is the pecking at and pulling out of feathers, inflicting damage to the plumage and skin of the recipient. The neurobiological profile determining the vulnerability of individual hens to develop into a severe feather pecker is unknown, although brain monoamines such as serotonin (5-HT) and dopamine (DA) seem to play a role. Previous studies related lower 5-HT and DA turnover ratios to an increased risk to develop SFP.In this thesis, monoamine levels in brain areas involved in emotional regulation and motor control were compared between phenotypically and genetically selected high and low feather peckers at different ages. It was found that adult high feather peckers had higher monoaminergic activity (lower metabolite levels and/or turnover ratios) in comparison to low feather peckers, which is in contrast with results on young hens. Differences were seen in several brain areas, namely the dorsal thalamus, medial striatum, amygdala, caudocentral nidopallium, and the somatomotor arcopallium, but to a lesser extend or not in the caudolateral nidopallium and the hippocampus. To investigate the exact neurobiological mechanism behind severe feather pecking further extracellular levels of 5-HT and DA and their metabolites were measured by in vivo microdialysis. Up till now, microdialysis has only been executed in young chickens, but this thesis describes the first microdialysis study performed in adult laying hens. It was found that adult severe feather peckers had a higher baseline release of 5-HT in the caudal nidopallium, a large associative area in the chicken’s forebrain. This result could not be explained by the amount of 5-HT presynaptically stored, as both high and low SFP lines displayed a similar 5-HT release after d-fenfluramine administration. This confirms that genetic selection on SFP has altered the serotonergic system in feather pecking-phenotypes. With clear phenotypic and genotypic differences in brain areas related to emotional regulation and motor control, it can be assumed that brain deficits at a young age increase an individual’s vulnerability to stressful environmental changes, which is associated with the prevalence of SFP later in life. The cause of the inversion of neurochemical patterns in young and adult high and low feather pecking hens remains to be elucidated. Perhaps this inversion is caused by development itself. On the other hand, higher behavioral patterns (SFP and other types of allopecking) observed in the high feather pecking chickens might have influenced the monoaminergic activity since the brain influences behavior and vice versa. Altogether, this thesis demonstrates the importance of considering the impact of genetic selection and also environmental conditions on brain neurotransmission and with that, on the vulnerability of individual chickens to develop SFP. Both the serotonergic and dopaminergic systems are involved in the development of SFP. With SFP being a multifactorial problem both genotype and phenotype have to be taken into account. Furthermore, in vivo microdialysis is a valuable approach to investigate why individual laying hens start SFP. This will lead to further understanding and ultimately in the reduction of SFP.