Sex Differences in Offensive and Defensive Investment in Crayfish Claws

Many animals possess weapons that have evolved to inflict damage to opponents during territorial encounters. Although the offensive capabilities of these weapons are important determinants of contest success, the defensive capabilities of the weapons are also important. However, current theory only accounts for investment into offensive performance, and not defensive endurance. To help better understand these dynamics, we investigated claw strength, cuticle thickness, and injury rates in male and female virile crayfish ( Faxonius virilis ). To measure the offensive capacity of a claw, we collected the max pinch of 3–5 pinches of each claw using a force transducer. To measure the defensive capacity of the claw, we dissected and measured the cuticle thickness of the carapace and seven sections of the claw: merus, dorsal manus, ventral manus, dorsal dactyl, ventral dactyl, dorsal pollex, and the ventral pollex. Injury rate was determined by visual inspection of the claw. Both male and female virile crayfish engage in aggression over access to both food and shelter. Thus, we expected each sex to invest similarly into the offensive and defensive abilities of their weapons. Interestingly, we found that despite males having proportionally larger claws, the average strength for a given size of the claw was similar between the sexes. Furthermore, we did not detect any sex differences in defensive ability as measured by cuticle thickness and injury rates. Overall, we demonstrated that both male and female virile crayfish invest similarly into the offensive and defensive ability of their claws. Support or Funding Information National Science Foundation (NSF) LEAP Scholars program