Tradeoffs and Benefits of Extreme Animal Weapons in Snapping Shrimp (Alpheus spp.)

Abstract

Animal weapons are morphologies used in contests over limited resources like food, shelter, and mates. Individuals with larger weapons tend to win contests, so evolution can favor large weaponry. Yet, the specific functional benefits that weaponry provides are often unclear. Further, the mechanisms that prevent individuals from growing arbitrarily large weapons remain hotly debated. In this thesis, I quantified the functional and competitive benefits of large weapons in snapping shrimp, Alpheus spp. Then, I resolved two apparent paradoxes that arise from this finding. First, if large weapons are beneficial, then what prevents individuals from growing arbitrarily large weapons? Second, if weapons are costly, then how can they scale with positive allometry? Taken in sum, my findings demonstrate that the costs and benefits of weaponry interact to explain how weapon size varies with body size, sex, and season. In Chapter 2, I determined the competitive benefits of large weaponry. By performing behavioral experiments, I showed that weapons are used as armaments, not signals. Then, using high-speed videos with synchronous sound pressure measurements, I showed that as weapon size increases, the duration of the cavitation bubble and the pressure imposed by the snap also increase.In Chapter 3, I showed that snapping shrimp also benefit from large weaponry by improving pairing success. Using field observations, I showed that for male snapping shrimp, larger weapons were positively correlated with the probability of being paired and the relative body length of their pair mate. Females exhibited neither trend. That indicates a male-specific paring benefit of large weaponry. Furthermore, using the same dataset, I showed that females exhibit sex-specific tradeoffs between egg production and weapon size. These sex-specific costs and benefits explain why snapping shrimp males have larger proportional weapon sizes than females. It can also explain why that sex difference amplifies during the breeding season. Finally, in Chapter 4, I showed that snapping shrimp minimize their energetic maintenance costs to achieve positive allometry and weapon exaggeration. Energetic maintenance costs are the costs of maintaining homeostasis. Because tissues vary in their energetic maintenance costs, I used proportional tissue composition as a proxy. I discovered that as weapon size increased, the proportion of the claw comprised of energetically expensive soft tissue decreased. Thus, larger weapons had lower proportional energetic maintenance costs, which could facilitate positive allometry of weapon size. Additionally, exaggerated weapons, which I quantified using residuals from the weapon scaling relationship, had lower proportions of energetically expensive soft tissue compared to non-exaggerated counterparts. Overall, the dissertation shows that the costs and benefits of weaponry underlie predictable variation in weapon size in snapping shrimp. Canonically, this logic has been used to explain honest scaling relationships based on size, condition, or quality. I extended these classic theories to explain sex differences, seasonal oscillations, and exaggeration in animal weapons.