Same-Sex Sexual Behavior in invertebrates is widespread and context-dependent: insights from a Systematic Synthesis

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, spanning arthropods, mollusks, annelids, echinoderms, nematodes, and flatworms. Reports were heavily biased toward insects, which represented nearly 90% of the dataset, reflecting research effort rather than true prevalence. Approximately three quarters of the cases were observed in natural conditions, while the remainder derived 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 last 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 explanation in some contexts, adaptive roles were supported by multiple cases, including enhanced mating practice, reproductive assurance, anti-predator strategies, and stress relief. Fitness consequences were rarely measured directly, 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. The synthesis of available evidence suggests that SSB is not an anomaly but a flexible component of reproductive strategies in invertebrates. Rather than being maladaptive, it emerges as context-dependent behavior influenced by ecological and social pressures, and in some cases may confer selective advantages.