Acoustic markers of negative arousal in lambs: evidence from behavioural and eye thermal profiles

Abstract

Vocalizations are known indicators of emotional arousal in animals, but validation using simultaneously collected physiological and behavioural measures remains limited to a few species. This study investigated sheep vocal expression of negative emotional arousal using stress-related behaviours and eye temperature as non-invasive arousal indicators. To this aim, twenty lambs underwent a short-term isolation test with two phases aimed at eliciting different levels of arousal in response to separation from conspecifics: partial isolation, where lambs maintained visual, acoustic and tactile contact with conspecifics through a fence, and full isolation (complete separation). During full isolation, lambs expressed higher bodily activation--spending more time running, jumping, and changing state behaviours--and produced more open-mouthed bleats (321 vs 27) than in partial isolation, validating higher arousal. Eye temperature also increased from partial to full isolation (however only in small lambs and not large ones). Calls emitted in full isolation were characterised by higher frequencies, were less tonal (more chaos) and had shorter durations. When combining behavioural and physiological assessment of arousal and testing their impact on the spectro-temporal structure of vocalizations, we found that bodily activation, but not eye peak temperature, impacted on the frequency distribution and tonality of the calls. Call duration increased with eye temperature, but only in lambs expressing high bodily activation, while the mean of the second formant increased with eye temperature in smaller but not larger lambs. Overall, lamb vocalizations indicate arousal and are correlated with bodily activation, while co-variations with physiological measures depended on behaviour and individual traits. HighlightsO_LIAnimal calls indicate emotional arousal, but few species have validated measures C_LIO_LILamb calls became higher in frequency, less tonal and shorter with negative arousal C_LIO_LICalls indicated emotional arousal and were correlated with bodily activation C_LIO_LIChanges in calls with eye temperature depended on behaviour and individual traits C_LI