Contaminants influence the outcomes of the male-male competition component of sexual selection under field conditions

Darwin's theory of precopulatory intrasexual selection states that individuals compete for access to mates for reproductive success. Size advantage is a critical factor in male-male competition across animal species, determining dominance, access to resources, the outcome of physical contests, and reproductive opportunity. As a result, this body size becomes exaggerated over evolutionary time. Despite extensive research on the physiological effects of metals, there has been less focus on how these contaminants impact behaviours critical for reproduction and sexual selection, particularly in Semaphore crabs. Crabs were collected from a control location, Swansea, and three contaminated locations (Powells Creek, Badu Mangrove and Bicentennial Park) and were paired in size-matched and size-asymmetric male-male competition contests for access to a burrow, a critical resource for reproduction. The results showed that individuals from locations with elevated contamination were less successful in competitive interactions, spending less time in the burrow than individuals from the control location. Additionally, while larger males from the control location consistently outcompeted smaller males in dyadic interactions, this size advantage was significantly reduced at contaminated locations, where smaller males more often won burrow ownership. Furthermore, morphological measurements revealed elevated concentrations of Pb, Dioxins, and PCBs at contaminated locations were correlated negatively with male body size and chela size, especially in mature purple males. The results demonstrated that selection pressure on large size may be relaxed in males under contaminant stress. This research also provides field evidence supporting our previous laboratory findings, which highlight the influence of exposure to lead contamination on the outcome of male-male competition, a component of intrasexual selection in Semaphore crabs.