Same-Sex Sexual Behavior in Invertebrates: A Systematic Synthesis of Prevalence, Study Type, and Interpretation.

Same-sex sexual behavior in invertebrates includes interactions such as courtship, mounting, copulation, tandem running, and pair formation between individuals of the same sex. We conducted a systematic synthesis of 222 primary studies and 12 reviews and found published evidence of SSB in approximately 207 species spanning arthropods, mollusks, annelids, echinoderms, nematodes, and flatworms. Although most reports involved insects (approximately 89%), standardized reporting rates suggest this bias reflects research effort rather than true biological prevalence. Observations came from field studies, although several taxa, particularly Diptera and Hymenoptera, were disproportionately represented by laboratory work. Copulation-like interactions were the most frequently reported behavior, followed by courtship and mounting. Where authors proposed explanatory frameworks, mistaken identity or indiscriminate mating was the most invoked interpretation, although adaptive functions and socially mediated explanations were also recurrent. Direct measurements of fitness consequences were rare and yielded mixed outcomes, suggesting that the consequences of SSB are context dependent rather than uniformly beneficial or maladaptive. Overall, the available evidence supports the view that SSB in invertebrates is neither anomalous nor reducible to a single mechanism. Instead, it appears as a heterogeneous component of the behavioral repertoire whose expression is shaped by mating system, study type, and taxonomic history.