In search of stress odours across species: Behavioural responses of rats to faeces from chickens and rats subjected to various types of stressful events

Abstract

Stressed animals have an increased risk of health and welfare problems, thus methods for easy and early stress detection are important for appropriate animal management. Using the ability of rats to distinguish between faeces odours from stressed and non-stressed conspecifics, we investigated whether rats could detect stress status in another species (the chicken), which would suggest a commonality in odorous stress signatures across species. We carried out four experiments to investigate the existence of stress-specific odours. In the first experiment using a T-maze, male Brown Norway (BN) rats (n=12) were found to sniff the faeces samples from stressed rats and chickens less relative to the samples from non-stressed individuals (P<0.05). In the second experiment, where odours were presented in an arena one at a time, male BN rats (n=16) sniffed faeces samples from stressed rats and chickens for longer than those from non-stressed controls (P<0.05). Within each test, the same responses to stress odours were seen independent of species of origin. This suggests that both in rats and chickens stress gives rise to specific volatile organic compounds (VOCs). In a third experiment, faeces from chickens, which had been stressed or non-stressed at hatching and subsequently exposed or not to acute stress at two weeks of age were tested on male BN rats (n=18). These rats were also tested with faeces from non-stressed and acutely stressed rats as well as herb odour (1-hexanol) used as control. Number of freezing episodes was higher when rats were exposed to any of the samples originating from stressed individuals compared to that observed with herb odour (P<0.05). Also, defensive burying was more likely to occur when rats were exposed to faeces from chickens stressed at hatching (P<0.05). Finally, a fourth trial analysed faecal samples from non-stressed and acutely stressed rats using gas chromatography coupled with mass spectrometry (GC–MS), and identified ten VOCs potentially involved in the distinctive smell detectable in faeces from acutely stressed rats. These findings confirm the existence of stress-specific odours in rats and indicate that, although not necessarily identical, a similar type of odour may be present in stressed poultry. In addition, this odour could be detected by rats in chicken faeces collected almost two weeks after the birds had been exposed to a stressful event. Our results suggest that patterns of VOCs may have the potential to be used as a tool for early, non-invasive screening of stress status in animals.