Comparing Physiology, Behaviour and Hippocampal Gene Expression in Laying Hens Housing in Differing Experimental Housing Conditions, and that Exhibited Different Housing Preferences
Design of the housing environment and resources provided may influence the long-term welfare of commercial laying hens. Accessing nest boxes, perches and foraging materials appears to be a behavioural priority for hens, but whether the quality of each alternative is important to their overall experience is unclear. For this reason, hens were housed in an environment that was designed to be “Preferred” (P; n = 15) or “Non-Preferred” (NP; n = 15) for 26 weeks. P pens were large and had deep wood shavings as litter, while NP pens were smaller and had a wire floor. P pens contained more expansive nest boxes and perches than those in the NP pens, along with a peat/sand mix dust bath which was absent from the NP housing. P hens were also given daily positive reinforcers (e.g. food treats), while NP hens experienced negative reinforcers (e.g. water spray). At the end of the housing period, a range of physiological and behavioural measures were taken, and birds were given a series of two-way preference tests between their experimental housing (either P or NP) and a housing environment intermediate to both conditions. Tissue was collected from the HF, and the expression of mRNA transcripts for a range of candidate genes relating to adult hippocampal neurogenesis (AHN) and glucocorticoid regulation was measured using quantitative PCR. Preference for the experimental housing relative to the intermediate conditions was significantly higher for birds housed in P than in NP conditions. However, no other measure differentiated between hens housed in the two conditions. A minority of birds from each condition made the unexpected choice, and serum corticosterone was higher in these individuals than in hens that made the majority choice. In the caudal HF, expression of doublecortin (DCX) mRNA was also lower in hens that made the minority choice. The MR/GR mRNA ratio was lower in the rostral HF of NP housed hens that made the minority choice (in favour of the NP conditions), which might suggest that this subgroup of birds was the most stressed. Expression of proliferative marker PCNA was higher in hens that chose the intermediate conditions over either experimental housing condition. The validity of transcription as a proxy for AHN in chickens is currently uncertain, and consistency between qPCR assays was lower when cDNA came from different reverse transcriptions. However, the existing and novel measures of experience both suggest that variation between individuals, reflected by their preferences, may exceed differences arising from an inanimate housing environment designed to be relatively positive or negative for welfare.
Work was approved by the University of Bristol Animal Welfare and Ethical Review Body and conducted under U.K. Home Office Licences (PPL: 30/2779 and 30/3392). Animal use and care was in accordance with the Animals (Scientific Procedures) Act 1986, EU directive 2010/63/EU and the UK Home Office code of practice for the housing and care of animals bred, supplied or used for scientific purposes. Collection and post-mortem analysis of tissue was approved by the Animal Welfare and Ethical Review Body at Newcastle University (Project ID #549).