Environmental downgrading to battery cages induces depression-like states in hens

Depression is one of the most common and impairing mental health disorders in humans. Captive animals may also experience depression-like states, particularly under restrictive housing conditions and in individuals with higher vulnerability to stressors (e.g. more fearful). This study investigated whether transferring laying hens (n = 60) from a cage-free environment to conventional battery cages led animals to display two core symptoms of clinical depression: anhedonia and low mood during the first two weeks post-transfer. Personality was included as a potential moderator of affective responses. Additionally, expression of tumor necrosis factor alpha (TNF-α), an immunological marker often elevated in clinical depression in humans and animal models, was quantified. Anhedonia was assessed using a palatable food consumption test (sweetcorn) conducted for 5 days before and 5 days after transfer. Low mood was assessed through an attention bias test. Personality was evaluated using four behavioral assays (open field, novel object, stationary human, and tonic immobility tests). A composite Affective Index was also calculated by integrating the behavioral measures of anhedonia and low mood into a single outcome variable. TNF-α expression was measured at baseline (pre-transfer) and 80 days post-transfer. Battery-caged hens exhibited behavioral indicators consistent with depression-like states during the first two weeks after transfer, including reduced sweetcorn intake and increased attention to threat, particularly among more fearful individuals. The Affective Index corroborated that battery-housed hens experienced more negative affective states overall, with fearful individuals being especially vulnerable. Contrary to predictions, TNF-α expression decreased in caged hens 80 days post-transfer, possibly reflecting immunosuppression due to chronic stress. These findings highlight that conventional rearing systems used in egg production can trigger depression-like states in these animals. These results support the need to critically review housing conditions in commercial production systems and to promote management strategies that minimize suffering, especially in vulnerable individuals.