Stress in chickens Effects of domestication and early experience on behaviour and welfare

Sammanfattning: The domestication is the process where animals have adapted to human conditions. A prerequisite for domestication is tame behaviour towards humans and subsequently, selection for other desirable traits took place which led to changes in several behavioural and physiological parameters. The domestication of chickens (Gallus gallus) was initiated around 8000 years ago, and today we see clear phenotypic and genotypic alterations when comparing domestic breeds with the wild ancestor. Our modern domestic production breeds have been selected for, and still undergo heavy selection for high meat and egg yields. Beside obvious morphological changes, studies investigating behavioural differences between the ancestral Red Junglefowl and domestic breeds show, for example, differences in fearfulness, foraging strategies, exploratory behaviour and sociality. The modern production environments are intense and already from hatch chicks are exposed to harsh conditions. From an animal welfare perspective, the production environment contain many stressful aspects. The shift in stressor types between wild and captive conditions have likely contributed to alterations in stress tolerance when comparing domestic breeds to the wild ancestors. Previous research on mammalian models have underlined that early-life stressor exposure can induce negative consequences both immediately and in adulthood, but can also affect the offspring in a transgenerational fashion. The effects observed are for example disturbance of normal brain development, a hyper-reactive hypothalamic-pituitary-adrenal (HPA) -axis, decreased immune function and increased risk of developing cognitive dysfunction and abnormal behaviour. In chickens, the long-term effects of stress during the chick phase and during puberty are not well-investigated, and further, conflicting data has been presented on the hatchling HPA-axis reactivity.In this thesis, results from four projects are presented, which all concern stress and welfare at different ages in chickens. The development of the HPA-axis and how chicks respond to early stress both on the short- and long term was investigated. Furthermore, an experiment on the effects of stress exposure a different ages during puberty was conducted, in search of particularly stress-sensitive periods. Two papers address domestication effects on the stress response.In paper I, the results show that the HPA-axis is fully functioning at hatch, resulting in elevated corticosterone levels at exposure to stressful conditions. Breed differences indicate domestication effects on the reactivity and development of the HPA-axis; the Red Junglefowl displayed a lower corticosterone baseline and a lower stress response on day one, compared to a domestic breed, whereas the results were the opposite on day 23. Similar results were seen in paper IV, conducted on adult birds, where the Red Junglefowl had a more pronounced reaction to acute stress but a faster recovery period, both with respect to behaviour and physiology. In commercial hatcheries, chick are exposed to multiple potential stressors on their first day of life. In paper II, chicks who had experienced the potentially stressful environment in a commercial hatchery was compared to a quietly treated control group. The hatchery managed birds displayed a reduced growth pattern and tendencies towards altered vigilance and reduced locomotion was seen as an effect of stress in adulthood.In paper III, it was demonstrated that one week of stress exposure at different ages during the chick phase and puberty affect long-term behaviour and physiology, however depending on age of stress exposure, different parameters were affected. The early stress also induced transgenerational effects, most clearly on HPA-axis reactivity, and showed some overlapping differentially expressed genes.In summary, domestication has altered the acute stress coping behaviours as well as the development and reactivity of the HPA-axis both in young and adult birds. Furthermore, puberty can be regarded as an equally stress sensitive period as the chick stage and affect various behaviours, stress physiology and gene expression. The outcome can vary depending on timing and nature of stressor.