Exposure to lead (Pb) alters the outcomes of female-female competition for resources in the semaphore crab, Heloecius cordiformis

Sexually dimorphic species, such as the semaphore crab Heloecius cordiformis, provide useful models for understanding how environmental contaminants influence behaviour essential for fitness. While lead (Pb) exposure impairs male-male size-based competition, a component of intrasexual selection in this species, its effects on female competitive interactions remain untested. We experimentally examined whether Pb exposure alters female-female competition for burrow ownership and whether chela size predicts contest outcomes. Mature females were exposed for 96 h to 0, 10, or 100 µg/L Pb and then paired in size-matched and size-asymmetric contests for burrow access. Exposed crabs (10 & 100 µg/L) were less successful than control crabs in chela size-matched paired competitive interactions, spending less time in burrows and predominantly losing burrow ownership. While in size-asymmetric contests, female crabs showed no size-based advantage under any exposure condition, and outcomes were instead dominated by resource pre-emption (first entry into burrow) rather than physical contests. Field data across a contamination gradient showed weak, non-significant negative trends between sediment Pb and female chela length, further supporting the absence of selection pressure on chela size in females. Together, these results show that Pb may influence competitive interactions in female semaphore crabs, but its effect is unrelated to size, in contrast to males. This study provides evidence underscoring substantial sex-specific differences in behavioural competitive interactions and evolutionary responses to environmental stressors.