Cell Proliferation in the Adult Chicken Hippocampus Correlates With Individual Differences in Time Spent in Outdoor Areas and Tonic Immobility

Abstract

Access to outdoor areas is provided as a means of enhancing welfare in commercial systems for laying hens (Gallus gallus domesticus), but substantial individual differences exist in their proportional use. Baseline cell proliferation levels of Adult Hippocampal Neurogenesis (AHN) have been associated with individual differences in reactive vs. proactive coping style, and in both mammals and birds, AHN is upregulated by positive experiences including environmental enrichment and exercise. We thus sought to explore whether individual differences in use of outdoor areas and in tonic immobility responses (indicative of fearfulness) were associated with hippocampal cell proliferation and neuronal differentiation. Radio frequency identification technology was used to track the ranging behavior of 440 individual focal hens within a commercially-relevant system over a 72-days 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 mRNA, respectively. Individual differences in tonic immobility duration positively correlated with PCNA expression over the whole hippocampal formation, while greater time spent in outdoor areas (the grassy range and stone yard) was associated with higher proliferation in the rostral subregion. Basal proliferation in the chicken hippocampal formation may thus relate to reactivity, while levels in the rostral region may be stimulated by ranging experience. Doublecortin expression in the caudal hippocampus negatively co-varied with time on the grassy range, but was not associated with tonic immobility duration. This suggests that ranging outside may be associated with stress. Within laying hen flocks, individual differences in hippocampal plasticity thus relate to coping style and use of external areas.

Keywords: Gallus gallus domesticus; adult neurogenesis; animal welfare; avian brain; free-range laying hens; hippocampal formation; individual differences.

Figure 1

Mean proportions of available tracked time (%) spent in each of the four areas by hens in the four experimental pens.

Figure 2

Relationships between the proportions of available time spent in outdoor areas by individual hens and corrected gene expression in the rostral and caudal subregions of the hippocampal formation. (A) PCNA expression in relation to the total percentage of time spent in outdoor areas (i.e., the stone yard + grassy range) by focal hens. (B) Doublecortin (DCX) expression in relation to the percentage of time that focal hens spent on the grassy range. Gene expression values are unstandardized residuals following correction for LBR expression, rostrocaudal subregion, sample, and pen, in linear mixed models.

Figure 3

Relationship between durations of tonic immobility (seconds) for individual hens and residual expression of (A) PCNA and (B) doublecortin (DCX) in rostral and caudal subregions of the hippocampal formation, after correcting for LBR expression, rostrocaudal subregion, sample, and pen in linear mixed models.