Could acoustic production reflect thermal comfort in horses housed outdoors ?

The welfare of free-roaming, captive wild or domestic animals housed outdoors is often questioned. How subjects perceive climatic conditions is not always well detected as it is highly subjective, depending on species, age and experience. Hence, decisions on housing rely mostly on human anthropomorphic perception. In particular, indicators of comfort and positive emotions remain poorly documented. Acoustic signals, however, offer a promising window into animals’ internal states. In the present study, we hypothesized that acoustic signals could be used to identify positive or negative perceptions of environmental conditions in horses. Horses possess a diverse acoustic repertoire and the snort produced by nostril vibration, has been identified as a likely indicator of calm, positive emotions. Over 3 weeks in early spring, acoustic production and time budget were recorded for 20 adult horses living outdoors on pasture. Meteorological conditions and pasture composition were monitored simultaneously. Results showed that snorts were the most frequent acoustic signal and that their production varied considerably across observation sessions. Snort production was highly correlated with temperature: horses produced more snorts and showed more relaxed behaviours when the temperature ranged between 15-20°C. In contrast, lower temperatures around 5-8°C were associated with more “static” behaviours (reflecting energy conservation) and less snorts. Finally, snorts were more frequent immediately after horses entered a new pasture with higher food diversity than just before leaving, when food diversity had become more restricted. These findings suggest that acoustic production may help identify the subjective perception that animals have from their environment, allowing more individualized and adapted management practices.