Indicators on the inside: Physiology and equine quality of life

The recent move away from resource-based assessment of welfare exemplified by the Five Freedoms towards an animal-based framework such as the Five Domains Model, has highlighted the need to develop an Equine Quality of Life (EQoL) assessment framework based on evidence-based, observable behaviours that reliably reflect underlying emotional state. This Systematic Review aimed to identify potential physiological indicators of equine emotional states. A pair of online database searches #1 ‘(Emotion* ORaffective OR cognit*) AND (behav* OR welfare OR physiol*) AND (horse OR Equid* OR equine*)’ and #2 ‘(horse OR equid* OR equine*) AND (stress AND respons* AND physiol*)’ yielded 1600+publications. After application of predetermined selection criteria 124 were retained for analysis. Heart rate (HR), heart rate variability (HRV) and cortisol are the most commonly measured physiological indicators of equine emotion. The effects of negative and positive stimuli are not consistently reported across studies. Additionally, these measures are at risk of confounding by physical exertion. HR is considered a reliable indicator of equine emotional state due to changes being rapid and sensitive to the intensity of the experience. Increases are reported with arousal, pain, novelty, anticipation and exercise, whilst decreases occur during allogrooming and habituation. Although HRV is considered a more sensitive indicator of emotional state encompassing parasympathetic and sympathetic nervous system activity, a variety of methodological and interpretive questions about its reliability remain. A lack of association between behaviour and cortisol results is not uncommon. Cortisol increases due to acute stress, anticipation,stereotypy prevention and physical exertion are reported, whereas it decreases with chronic stress. The modifying effects of factors such as physiological habituation, coping style, past experience and individual variability remain unknown. Other suggested correlates of emotional state include eye temperature, ACTH challenge, haematological, immunological, neurological factors, respiratory rate and salivary alpha amylase but many of these lack validation in relation to association with emotional state.