Um conto sobre a agressividade: valor de recurso, tomada de decisão e adaptações morfológicas

Abstract

Agonistic behavior is ubiquituous in nature. Hence, understanding how and why it occurs is fundamental for unravelling its influence in the evolution of behavior. The goals of this thesis are: to test the influence of predictable environmental cycles on fighting behavior, how animals decide when to quit a fight and the morphological adaptations for efficient weaponry. Two variables are typically used to determine an individual‟s motivation to fight: resource value and fighting costs. Although they are well studied, there is no information on the effects of predictable environmental cycles on these variables. Hence, in the first chapter we tested the influence of tidal cycles on the motivation and contests of the sea anemone Actinia equina. We simulated a tidal cycle in the laboratory in which we exposed a group of anemones to it but not others. We elicited fights between these groups, also measuring their startle responses to assess territory value. Our results suggest that territories with water flow are more valuable because individuals exposed to flow had: shorter startle responses, higher winning probability and longer contests. However, these individuals are also less likely to escalate their aggression levels, suggesting that fighting costs are also higher. Following that rationale, understanding how individuals make the decision to withdraw from a contest is important to understand the evolution of such behavior. Two theoretical models explain that decision: one in which the individual uses information only about himself; and the other predicts that individuals can assess each other‟s fighting ability. To test this in the second chapter, first we tested which morphological and performance traits are important for an individual to win a contest. Afterwards, we tested which model explains better the decision to withdraw in fights between males of Aegla longirostri. Our results indicate that body size and claw height are important to settle contests. Regarding the decision to withdraw, crabs seem to use a mix of both models because contest variables corroborate with the quantitative predictions of both assessment models. Crabs‟ claws are regarded as weapons in these fights, and as such, are used to subjugate opponents. Given this role, having an efficient weapon can increase the individual‟s fitness. According to theory, this efficiency has a cost: the higher the weapon efficiency, less variable will be its shape when compared to an structure that is used both to fight and to signal. In the third chapter, we investigated the shape and performance of the claw of three Aegla species that use their claws differently: one species uses only for fighting (A. longirostri), other for fighting and signaling (Aegla abtao) and the last one only for feeding (Aegla denticulata). Our data corroborate this theory: fighting claws are biomechanically efficient but with a conserved shape, whereas the fighting and signaling claw is also biomechanically efficient, but its‟ shape is more variable. Overall, we demonstrated that predictable environmental cycles are important for contest behavior, that theoretical models of assessment need to be reworked and the biomechanic efficiency constrains weapon shape.