Crabs are known for their aggressive mating competitions, where males fight for dominance and access to female mates. However, a new study has found that exposure to the toxic metal lead (Pb) can disrupt these competitive interactions, putting some males at a disadvantage. The research, conducted on the Semaphore crab, Heloecius cordiformis, suggests that contamination of coastal habitats by lead can lead to changes in the strength and direction of sexual selection in crab populations. This has important implications for understanding how environmental pollutants can impact the evolution of species. Sexual selection and intrasexual competition are key topics in evolutionary biology.
How Lead Exposure Affects Crab Mating Competitions
Male-male competition is a crucial component of precopulatory intrasexual selection in many species, including crabs. In this process, males compete for access to resources important for reproduction, such as territories, burrows, or females. Successful males pass on their advantageous traits to offspring, leading to the exaggeration of these traits over evolutionary time.
The researchers exposed male Semaphore crabs to two different concentrations of lead (Pb) – an environmentally relevant dose of 10 μg/L and a behaviorally relevant dose of 100 μg/L – and then observed their competitive interactions in laboratory experiments. They found that crabs exposed to the higher 100 μg/L concentration of lead were less successful in their competitive interactions, spending less time in burrows and more often losing ownership of the burrow to unexposed crabs.
Lead Exposure Negates the Size Advantage
Further experiments revealed that the established size advantage of larger-clawed males in competitive interactions was abolished by lead exposure. In the absence of lead, larger-clawed males were more successful in competing for burrow ownership. However, this size advantage was lost when crabs were exposed to both the 10 μg/L and 100 μg/L concentrations of lead.
The researchers suggest that the loss of the size advantage under lead exposure could be due to the larger crabs accumulating more of the metal in their bodies, potentially impairing their competitive abilities. This contaminant-induced relaxation of selection on male size and claw size was also evident in the researchers’ field surveys, which found that male crabs in more metal-contaminated locations had smaller carapace widths and claw lengths compared to those in less contaminated areas.
Implications for Evolutionary Processes
The findings of this study have important implications for understanding how environmental pollutants can impact the evolutionary processes of sexual selection and intrasexual competition. The contaminant-induced loss of the size advantage in male crabs could result in a relaxation of selection pressure on body size and claw size in contaminated locations. This could lead to a shift in the trait composition of crab populations over time, with smaller males becoming more common.
Such impacts on the sexual selection process could also have broader consequences for the resilience and adaptation of populations in the face of environmental change. The researchers suggest that the effects of pollutants like lead on sexual selection warrant further investigation across a range of species to better understand the eco-evolutionary consequences of anthropogenic impacts on natural systems.
Author credit: This article is based on research by Rosemary Patrick, Kim Colyvas, Andrea S. Griffin, Michael Stat, Wayne A. O’Connor, Mohammad Mahmudur Rahman, Mallavarapu Megharaj, Geoff R. MacFarlane.
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