The role of skill in animal contests: Analysis of a neglected RHP trait in fighting hermit crabs

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The resources that animals need to survive and reproduce are often limited in supply. Therefore, evolution has produced aggressive behaviours that allow individuals to take resources from others. The resulting fighting or contest behaviour has been the focus of intense study because the ability to win fights will have a strong effect on an individual's success in evolutionary terms. Most effort has focussed on the questions of why animals should fight and how they should make the decision to quit a fight and relinquish the contested resource. In contrast far less effort has been made to understand how animals fight, and what might differ between the aggressive behaviour of winners and losers. This is important for two reasons. First, our understanding of why animals fight is dependent on measuring their fighting ability. While it seems obvious that larger individuals should be stronger than smaller ones, in nature more fights take place when opponents are evenly matched. Therefore, it could be subtle differences in their aggressive behaviour that really drive differences in fighting prowess. Second, by investigating aggressive behaviour its-self we are directly investigating the traits for winning fights that we assume are produced by natural selection. In this project we will focus our investigations on a potentially important component of fighting ability that has been almost completely ignored to date: the possibility that winners and losers differ in skill. Skill describes the effectiveness of the movement patterns that animals use to form aggressive behaviours. We will use accurate filming and 3D animation technology to measure the movement patterns of fighting hermit crabs. These small crustaceans fight over the ownership of the empty snail shells that they rely on for protection. One crab, the defender spends most of the fight tightly withdrawn into its shell. The attacker, meanwhile, uses its shell to repeatedly strike the surface of the defender's shell. If the attacker fights well, this shell rapping behaviour will cause the defender to release its internal grip on its shell, allowing the attacker to evict it and take over the defender's newly vacated shell. We know that vigorous rapping (i.e. at a high rate) is important but should skill play a role as well? Here we will ask whether successful attackers that evict the defender fight more skilfully than those that fail to gain an eviction. First we will ask if their shell rapping is more efficient and accurate, by comparing the displacement distances and the positions of strikes between successful and unsuccessful attackers. We will also ask whether precision is important by comparing how concentrated the points of impact are for successful and unsuccessful attackers. Then we will examine how attackers decide when to switch between shell rapping and a less demanding tactic called shell rocking. In the second part of the grant we will look at the factors that could underpin variation in fighting skill. Do more skilful fighters, for instance, also show better performance of physically demanding or cognitive tasks? And how much of an individual's potential for fighting skilfully (technique) is actually expressed during a fight where its opponent should try to hinder skillful fighting? These studies will represent the first concerted effort to study the role of skill in animal contests. By providing a better picture of what makes a good fighter, we will gain new insights into the evolution of aggression. Furthermore, these studies will have potential broader impacts beyond biology. In sports science, skill is also composed of accuracy, precision, efficiency and appropriate tactics. The statistical approaches we will use for analysing the data on these parameters in hermit crabs could be adapted for use in programmes that seek to identify potential elite athletes or to reveal areas for improvement in individual training programmes.