The challenge of identifying positive emotional indicators in equids: A scoping review to evaluate if this approach is putting the cart before the horse (Equus caballus)

Driven by multiple converging factors, highlighting the negative impact of equestrian sports on equine welfare, recent scientific research has focussed on recognising positive states in horses (Equus caballus). In particular, this has yielded an increasing number of studies prioritising the identification of observable indicators of positive emotions. However, contrary to human emotion research, which exploits verbal self-reports, the study of non-human animal (hereafter referred to as “animal”) emotions must solely rely on indirect behavioural and physiological indicators. As such, the purpose of this review was to critically appraise the existing literature to evaluate viable markers of positive emotions in horses (Equus caballus), and contemplate ongoing challenges. The review identified several behaviours demonstrating validated associations with positive valence particularly in low arousal states in horses, including advanced upper lips, forward ear orientation, neck posture, vocalisations (snorts and whinnies), and affiliative contact. In contrast, other behavioural markers, such as spontaneous eye blink rate, appear to reflect a more complex cognitive process than emotion per se. Additionally, physiological measures such as heart rate, heart rate variability, and cortisol levels, which are frequently used as biomarkers of affective state, in fact capture autonomic nervous system activation, and likely indicate biological functions rather than positive emotions. Identification is further complicated by the inherently subjective nature of emotional experience, underpinned by complex and overlapping neural pathways. This is in part illustrated where methods employed by researchers to induce positive emotions, do not always produce the desired response. In line with relevant literature, this review advocates for a possible shift in focus, from assessing the outcomes of emotion (i.e., observable positive valence) to evaluating the experiences that facilitate optimal wellbeing and developing validated tools that can be applied in varied training and management contexts. This approach is informed by human psychological concepts such as flourishing, which is promoted through experiences that foster engagement, accomplishment, positive relationships and meaning. Notably, some of these dimensions, particularly engagement and accomplishment, may be observable and measurable in animals using the indicators proposed in the reviewed literature.