Reproductive costs and benefits underlie sex differences in weapon investment in the snapping shrimp
Jason P. Dinh, S.N. Patek. Biology Department, Duke University, Durham, NC, United States

Sexual selection drives the evolution of weapons. We previously showed that large individuals bear disproportionately large weapons because they endure low energetic maintenance costs and attenuated morphological tradeoffs. Here, we identified sex differences in weapon costs that underlie seasonal and sex differences in weapon size in the snapping shrimp, Alpheus heterochaelis. We found that large weaponry disproportionately benefits males and burdens females. For males, as weapon size increased, the likelihood of being paired increased, and the relative size of female pair-mates increased. No trend was seen in females. By contrast, for females, as weapon size increased, egg production decreased. Thus, males boost reproduction by pairing with larger females, whereas females reduce reproduction by decreasing investment in eggs. Our findings are consistent with sex differences and seasonal oscillations in weapon investment: males have larger weapons compared to females, and in the breeding season, when reproduction is at a premium, males increase weapon size but females decrease weapon size. We conclude that differential costs drive sex and seasonal differences in weapon investment.