Pay attention to the ladies: female aggressive behavior and weapon allometry provide clues for sexual selection in freshwater anomurans (Decapoda: Aeglidae)

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Contesting scarce resources can trigger the evolution of specialized morphological structures (i.e., animal weapons). While most research focus on male weapons, females might also bear weapons, although generally smaller and less conspicuous than male weapons. Social selection is evoked to explain female weaponry in which females fight for nonsexual resources such as food and shelter. Males might fight for similar resources but are expected to have proportionally larger weapons due to additional inputs from sexual selection. We tested whether males have proportionally larger weapons than females in two species of Aegla crabs. Interestingly, only males of one species had proportionally larger claws than females. Given that these larger weapons typically correlate to increased aggression, we expected males to fight more intensely than females. Thus, we compared intrasexual contests of males and females of the same species. Females fought similarly to males: latency, contest duration, and frequency of highly aggressive acts were similar between the sexes. Therefore, despite males having proportionally larger weapons in one species (as predicted by sexual selection), they fought similarly to females. Our results hint that fighting might not necessarily be the source of selection for sexual dimorphism we typically expect. Other sources, such as the frequency of fighting and predation pressure, might be selecting larger claws in males despite the similar fights, while fecundity costs might downsize female claws. We highlight that comparing female with male weapons and the associated fighting behavior shows that selection on weapons is not as straightforward as we might think. We show that studying the highly neglected female weapon allometry and usage allows us to infer on the selective process acting on animal weapons. We show here that males of one species have proportionally larger weapons, and, despite that, males fight similarly to females in every aspect analyzed. Therefore, fighting per se might not be the sole source of selection to explain sexual dimorphism. Other sources, such as frequency of fighting, and how expensive it is to produce gametes might be additional sources of selection.