Relating Adult Hippocampal Neurogenesis to Tonic Immobility and Longitudinal Ranging Behaviour in Commercial Laying Hens
Access to outdoor areas is provided as a means of enhancing welfare in commercial systems for laying hens, but proportions of time spent on the range by individual birds vary substantially. Adult hippocampal neurogenesis in mammals and birds is upregulated by positive experiences including environmental enrichment and exercise, while basal levels of proliferation have been associated with individual differences in reactive versus proactive coping style. We thus sought to explore whether individual differences in use of outdoor areas and in tonic immobility (TI) responses (indicative of fearfulness) were associated with hippocampal cell proliferation and neuronal differentiation. Radio frequency identification technology was used to track the ranging behaviour of 440 individual focal hens within a commercially relevant system over a 72-day period, after which tonic immobility durations were measured. Following hippocampal tissue collection from 58 focal hens, proliferation and neuronal differentiation were measured through quantitative PCR for proliferating cell nuclear antigen (PCNA) and doublecortin (DCX) mRNA respectively. Variation in ranging was not related to TI responses. Greater time spent in outdoor areas (the grassy range and stone yard) was associated with higher PCNA expression in the rostral subregion, while individual differences in TI duration positively correlated with proliferation over the whole HF. Basal proliferation in the chicken HF may thus be an indicator of reactivity, while levels in the rostral region may be stimulated by ranging experience. DCX expression in the caudal HF negatively co-varied with time on the range, but was not associated with TI duration. This suggests that ranging outside may also be associated with stress. Within laying hen flocks, individual differences in hippocampal plasticity thus relate independently to coping style and use of external areas.
Experimental use of the animals was approved by the Bern Kantonal Authority (BE-46/16) and the Animal Welfare and Ethical Review Body at Newcastle University (Project ID #549), and procedures complied with Swiss regulations regarding their treatment. Standard commercial protocols were followed, including ad libitum access to food and water.