Summary
Swedish researchers investigated the early post-hatch ontogeny (maturation in the early stages of life) of the hypothalamic-pituitary-adrenal axis (HPA-axis) reactivity to stress in chickens. They examined three different breeds of chickens: ancestral Red Junglefowl, the White Leghorn research strain SLU13, and the commercial layer hybrid Dekalb White. They measured post hatch corticosterone (CORT) levels (on days 1,9,16 and 23) and conducted behavioral stress tests (on days 1 and 23).
The chickens’ HPA axis responded to acute stress immediately after hatching, leading to a significant increase in CORT levels after 10 minutes of physical restraint, irrespective of breed. Additionally, there was a decline in baseline CORT levels with age, signifying ongoing post-hatch maturation of the stress response system. Behavioral changes in response to stress were evident, highlighting distinctions between domesticated breeds and wild ancestors, implying the impact of domestication on HPA axis development and reactivity.
A limitation of the study is the relatively low sample sizes in the HPA-axis reactivity test, ranging from 33 to 40 chickens per group. A strength is the consistency among rearing conditions, ensuring the reduction of potential confounding variables in the study. These findings can lead to improvements in the welfare and management of chickens in commercial hatcheries where chickens are routinely exposed to stressors.
Domestication is thought to increase stress tolerance. The connection between stressor exposure, glucocorticoids and behavioural responses has been studied in adults, where domestication effects are evident. Early stress exposure may induce detrimental effects both in short-and long term. Previous research has reported a lack of glucocorticoid response in newly hatched chickens (Gallus gallus), whereas others have found opposite results. Hence it remains unclear whether the HPA-axis is functional from hatch, and if domestication has affected the early post-hatch ontogeny of the stress response. Our aims were to investigate the early ontogeny of the HPA-axis and characterize behavioural and hormonal stress responses in ancestral Red Junglefowl and in two domestic layer strains. Plasma corticosteone and behavioural responses before and after physical restraint was measured on day one, nine, 16 and 23 post hatch. The results showed significant increases of corticosterone after stress in all three breeds at all the different ages. The HPA-response decreased with age and was lower in Red Junglefowl. Behavioural responses also decreased with age, and tended to be stronger in Red Junglefowl. In summary, the HPA-axis is reactive from day one, and domestication may have affected its development and reactivity, alongside with related behaviour responses.