Exploring emotional expression in invertebrates: mechanisms, functions and phylogeny.

The idea that invertebrates may exhibit basic forms of emotion, and possibly sentience, is gaining traction as evidence of their advanced cognitive abilities continues to mount. While high-level cognition does not imply emotional states, invertebrates offer a unique window into whether such states can arise in small-brained animals. With their relatively simple yet highly structured nervous systems, they serve as tractable models for dissecting the mechanisms underlying emotional expression. To probe these processes, researchers have developed behavioural paradigms exposing different species to contexts likely to elicit emotional responses. From a phylogenetic perspective, studying emotional expression in invertebrates can reveal whether basic emotional systems are conserved across species. This review synthesizes current knowledge of non-verbal emotional expression in social insects, focusing on mechanisms and functional roles. We apply a multi-component approach to examine key building blocks shared across emotions and taxa, offering insights into their evolutionary origins. We also identify critical gaps in knowledge and propose directions for future research. By examining the proximate and ultimate causes of emotion, this framework enhances our understanding of the biological basis and evolutionary significance of emotions, promoting interdisciplinary research that bridges neuroscience, ethology and evolutionary biology. This article is part of the theme issue 'Mechanisms, development, phylogeny and functions of emotional expressions'.