Adult hippocampal neurogenesis as a biomarker of long-term experience in domestic laying hens (Gallus gallus domesticus)

Abstract

Consideration for the welfare of farmed animals has highlighted the goal of ensuring that they lead a “life worth living”, comprised of an acceptable balance between positive and negative experiences. Novel objective biomarkers of long-term or cumulative experience that are valence-specific may contribute to the assessment of this criterion. In humans, numbers of surviving hippocampal neurons produced through adult neurogenesis reflect self-reported mood. In rats and mice, adult hippocampal neurogenesis (AHN) is reliably suppressed by chronic stress, but stimulated by positive experiences. As AHN in the avian brain also appears to be downregulated by chronic stress and upregulated by cognitive enrichment, this project sought to quantify immature neurons in the brains of laying hens with known long-term experiences, to identify factors likely to have a notable influence on lifetime subjective welfare. Four studies measured levels of AHN in hens in relation to i) experimental housing conditions, ii) individual differences in use of outdoor areas, iii) severity of acquired fractures to the keel bone, and iv) alternative commercial housing systems (enriched-cage versus aviary) and physical body condition, on-farm. Over the course of the project, a methodological comparison was made between traditional morphological quantification of immature neurons by immunohistochemistry, and quantification of the transcription of genetic markers of AHN. Densities of immature differentiating neurons were suppressed by severe keel bone fractures and in association with poor feather coverage, low body mass and pale combs. The proliferative stage of AHN was also positively associated with the time that individual hens within a freerange system spent in the outdoor areas provided. The results suggest that interventions to reduce the prevalence of keel bone fractures and improve physical condition in commercial laying hens are likely to have significant benefits for the balance of cumulative chronic stress and positive affect experienced.