Social behaviour has traditionally been studied through the lens of conspecific interactions, yet animals routinely interact with heterospecifics in ways that shape survival, resource acquisition, and information use. Mixed-species groups (MSGs) are among the most widespread expressions of such heterospecific sociality, occurring across diverse vertebrate lineages, yet their evolutionary origins, the ecological conditions that favour their formation, and the traits determining who associates with whom remain poorly understood. This thesis addresses these questions across three interconnected scales, asking why heterospecific sociality evolves, when mixed-species groups form, and which species interact to form such associations.
The first chapter examines the evolutionary distribution of sociality across vertebrates using a global comparative framework spanning birds, mammals, and fishes. Integrating species-level behavioural and ecological data with phylogenetic methods, I test the relationship between conspecific and heterospecific sociality, asking whether mixed-species grouping is a distinct evolutionary phenomenon or an extension of the pressures favouring sociality more broadly. Heterospecific sociality proves phylogenetically structured rather than randomly distributed and rarely evolves directly from solitary ancestors. Instead, it arises most often in lineages that have already evolved conspecific group living, indicating that mixed-species associations build on foundations established through earlier transitions to sociality. Heterospecific grouping thus emerges not as an isolated ecological phenomenon, but as a recurring outcome of the same selective pressures that favour conspecific sociality, extended across species boundaries to exploit complementarities unavailable within single-species groups.
Chapters 2 and 3 shift scale to ask which species associate, examining the mechanisms structuring MSGs within species-rich coral reef fish communities across two Indian Ocean island groups. Testing whether associations are shaped by phenotypic traits, I find that species do not associate randomly from the pool of ecologically similar co-occurring species. Body size and shape show non-random patterns of similarity, consistent with shared movement and ecological constraints structuring group membership. Extending this to visual traits, associated species are often more visually similar than expected by chance, particularly in body patterning, a pattern consistent across island systems. This reveals a previously underappreciated role for visual phenotypes in group cohesion and species recognition, establishing partner choice, not just ecological opportunity, as a driver of group composition.
The final chapter asks when and why MSGs form, examining ecological drivers of shoaling among herbivorous reef fishes in the Andaman Archipelago. Testing three interacting mechanisms, predation risk, interference competition from territorial damselfishes, and algal resource availability, I find predation influences group size but is not consistently the strongest predictor of grouping dynamics. Resource accessibility and competition with damselfishes emerge as equally or more influential, suggesting MSGs help individuals overcome competitive barriers and access defended resources, not merely reduce predation risk.
Together, these findings show that mixed-species sociality cannot be understood at a single biological scale but reflects a hierarchical process. Evolutionary history determines which species possess the capacity for heterospecific association; ecological conditions determine when that capacity becomes advantageous; and phenotypic compatibility determines who ultimately associates. These mechanisms are complementary components of one scale-dependent process, reconciling long-standing debates over the primary driver of mixed-species grouping. By linking macroevolutionary patterns with ecological mechanisms and species-level associations, this thesis extends the study of social evolution beyond species boundaries, arguing that heterospecific sociality is a structured, recurrent component of animal social systems deserving a central place in contemporary theories of social evolution.