Beyond reproduction: contextual expression of Same-Sex Sexual Behavior in invertebrates

Same-sex sexual behavior (SSB) in invertebrates encompasses a wide range of interactions, including courtship, mounting, copulation, and pair bonding between individuals of the same sex. Through a systematic review of 219 primary studies and 12 reviews, we found evidence of SSB in 203 species across arthropods, mollusks, annelids, echinoderms, nematodes, and flatworms. Although reports were heavily biased toward insects (≈ 90% of cases), but standardized reporting rates indicated this pattern reflects research effort rather than true biological prevalence. Approximately three-quarters of the observations came from natural conditions, while the remainder from laboratory studies. Male–male copulation and courtship accounted for the majority of reports, but female–female interactions and reciprocal behaviors were also recorded, the latter particularly in hermaphroditic taxa. The main explanatory frameworks identified were mistaken identity or indiscriminate mating, social and environmental influences, and adaptive functions. While non-adaptive mechanisms remain a plausible in some contexts, multiple cases support adaptive roles of SSB, including improved mating performance through practice, reproductive assurance, predator avoidance, and stress relief. Direct measurement of fitness consequences were rare, but when reported they revealed both costs, such as reduced lifespan or genital injury, and benefits, such as improved reproductive success under biased sex ratios. Overall, the synthesis of available evidence suggests that SSB is not an anomaly but a flexible component of reproductive strategies in invertebrates. Rather than being inherently maladaptive, it emerges as context-dependent behavior shaped by ecological and social pressures, and in some cases may confer selective advantages.