On the nature of grief: uncovering the evolutionary roots of loss across the animal realm.

Understanding grief in non-human animals poses conceptual and empirical challenges. Unlike humans, whose grief is readily identified through affective, cognitive and behavioural manifestations, recognizing it in other species is complicated by differences in expression and by anthropomorphic bias. Research on grief has taken several approaches. Observational studies in the ethological tradition have documented grief-like responses (withdrawal, vocalizations and changes in social interaction) across diverse taxa. Separation studies have shown that disrupting attachment produces depressive-like behaviour both behaviourally and physiologically, underlining how early bonds shape later affect. Questionnaire studies have explored perceptions of grief in companion animals, revealing both cross-species continuities and potential interpretive biases. Meanwhile, evolutionary hypotheses for grief range from non-functional accounts, through functional accounts (either cognitive or socially bound), to integrative/mechanistic models. Across these lines of evidence, we suggest that grief is the response to the rupture of a social bond built by a deeply conserved attachment system and its homologous nonapeptide toolkit. We propose an inclusive definition of grief as a response to the disruption of significant social bonds, encompassing affective, cognitive and behavioural components across species where the body is either present or absent. This definition is intended to ground systematic, comparative study of grief across species. This article is part of the theme issue 'Mechanisms, development, phylogeny and functions of emotional expressions'.