Domestication effects on behavioural and hormonal responses to acute stress in chickens
Introduction
Domesticated animals have evolved for thousands of years in a relatively protected environment compared to the original natural habitat, with reduced predator pressure, protection from disease and regular food supply. However, other potential stressors have replaced the natural ones, such as crowding [1], [2], human presence and handling [3], [4], [5], [6], artificial light [7] and lack of out-door space. The stressors affecting domestic animals in the human-controlled environment are therefore different than for the wild ancestors, and the ability and strategy to cope with various types of stressors may have been modified during domestication [8].
It has been suggested that modifications in the stress response system may have been a driving force behind the numerous changes in morphology, physiology and behaviour seen in domesticates [9], [10]. In the present study, we compare stress recovery patterns between the Red Junglefowl (RJF), which is the main ancestor of all domestic chicken breeds [11], [12] and a domesticated egg-laying strain, the White Leghorn (WL). RJF is known to show more fearful behaviour in a range of test situations [13], and several behaviour differences have also been described in foraging strategies and exploration patterns [14], [15], [16].
A central aspect of stress is the endocrinological response of the individual. Steroid hormones pass the blood brain barriers and act on the brain by modulation of gene expression [17] or through non genomic pathways such as modification of GABA receptor function [18]. Various studies have compared hypothalamic–pituitary–adrenal (HPA) axis reactivity of wild and domesticated animals and they suggest that attenuated HPA-axis reactivity in domesticates is common in many species including mice [19], rats [20], pigs [21], guinea pigs [10] silver foxes [22], mallard ducks [23] and salmonids [24]. Furthermore, activation of the HPA-axis suppresses the hypothalamic–pituitary–gonadal (HPG) axis in most animals through several mechanisms [25]. For instance, corticotropin releasing factor (CRF) inhibits secretion of gonadotropin releasing hormone (GnRH), while glucocorticoids suppress secretion of pituitary luteinizing hormone (LH), as well as progesterone and oestrogens from ovaries [26]. In general stress limits the efficiency of reproduction and causes economic loss to the farm. However, in contrast to the HPA-axis, the influences of domestication on the stress related function of the HPG-axis have not been explored thoroughly in any species. Some acute events, in addition to physical restraint, have been shown to induce a stress response in chickens, for example heat stress [27], [28], [29] and unpredictable light regimes [30]. Restraint stress however is a well-established method for triggering a stress response in chickens (see for example [29], [31], [32]) where also the HPA-axis recovery has been investigated [31].
We used detailed behavioural comparisons of the behaviour of domestic chickens and their ancestors during recovery from an acute stress experience, combined with a uniquely broad assessment of changes in an array of steroid hormones. This allowed us, for the first time, to assess the effects of domestication on behavioural and endocrine stress responses in chickens. In particular, we focused on the recovery process and the return to normal levels of both behaviour and concentrations of the steroids, using a similar approach as in earlier research on stress recovery [33], [34], [35], [36], [37], [38], [39], [40].
Hence, the aims of this study were to integrate a comprehensive picture of behavioural and endocrine (steroid) responses during recovery from an acute stress experience in chickens, and to compare the response between domesticated chickens and their wild ancestors.
Section snippets
Ethical statement
The project was approved by the Linköping Council for Ethical Licensing of Animal Experiments; ethical permit no 122-10.
Birds and housing
In total 71 birds, of which 25 males and 46 females were tested. Half the birds were domesticated White Leghorn (WL) and the other half were ancestral Red Junglefowl (RJF). The RJF population in the current study originates from a wild caught group of animals from Thailand, maintained in our research facilities for about 10 generations. Carefully supervised pedigree breeding
Behaviour
Baseline values (Table 2) differed significantly between the breeds for “alert behaviour”, “feather ruffle” and “crowing” which were more frequent in WL, while “relaxed behaviour” and “preen” were more frequent in RFJ. WL tended to “ground peck” more than RJF. Females showed higher baseline of “relaxed behaviour” and males displayed higher baseline levels of “feather ruffle”. A significant sex × breed effect was seen in “drinking” and “wing flap”, which both were performed the most by RJF males
Discussion
Both Red Junglefowl (RJF) and the domesticated White Leghorn (WL) layers showed an immediate response to the restraint stress, as evident from behaviour as well as endocrine changes. The reactions of the ancestral breed were more acute, with a pronounced behavioural reaction and a significant increase in corticosterone levels, whilst the recovery was slower and the overall responses sustained for a longer period in the domesticated birds. This shows that the stress response and recovery process
Acknowledgements
The project was supported by grants from the Swedish Research Council (VR) 621-2011-4731, the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS) 221-2011-1088, and ERC (project Genewell 322206), and within the auspice of the Swedish Centre of Excellence in Animal Welfare. For excellent technical assistance we thank Lejla Bektic. We thank ARUP Institute for Clinical and Experimental Pathology for supporting this project.
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Equal contribution.