Developmental density shapes adult mate guarding strategies in an invertebrate

Post-copulatory mate guarding is a widespread reproductive strategy that reduces sperm competition but can generate sexual conflict when male and female optima diverge. While mate guarding is known to respond plastically to immediate social conditions, the extent to which early-life social environments of both sexes shape adult guarding behaviour remains poorly understood. We experimentally tested how developmental density of males and females influences post-copulatory mate guarding and associated behaviours in the nuptial gift–giving cricket Gryllodes sigillatus. Using a full factorial design, we reared males and females under high or low same-sex densities and crossed them in standardized no-choice mating trials. We quantified mate-guarding duration, the intensity of guarding, female feeding time on the nuptial gift and ampulla retention time. Contrary to our prediction, males reared at low density guarded females for significantly longer than males from high density. Females reared at low density were also guarded for longer, with largely additive effects of the two treatments. Further analyses revealed that longer mate guarding prolonged female feeding and ampulla retention. These results demonstrate that developmental social environment can have lasting, sex-specific effects on post-copulatory behaviour and that mate guarding is functionally linked to mechanisms that ensure sperm transfer. Our findings highlight how early social cues can bias adult reproductive tactics and, consequently, the outcomes of sexual selection in polygamous mating systems.