Abstract
Understanding the determinants of fighting ability (or resource-holding potential, RHP) is key to elucidating the evolution of aggressive behaviour, as current tests of contest theory rely on realistic proxies for overall RHP. Traditionally, RHP is considered equivalent to body size but it is increasingly clear that a wider range of morphological and physiological traits contribute to fighting ability. In situations analogous to contests, such as courtship displays in animals and competitive sport in humans, the role of skill has long been appreciated but this component has been neglected in analyses of animal fights. Here, we investigated two spatial components of skill, accuracy and precision, during shell fights in hermit crabs, where an attacker repeatedly strikes (raps) its shell against that of a defender. By analysing the points of impact of these strikes, we found that attackers that rapped with coarse-scale accuracy were more likely to win the fight, indicating that the ability to target a ‘sweet spot’ on the defender's shell is an important determinant of contest success. Furthermore, we found that this element of skill correlated with temporal performance (vigour). Taken together these results show that spatial skill is an RHP component. Moreover, in contrast to the traditional assumption that fighting ability is equivalent to body size, RHP is actually underpinned by a suite of interlinked traits including performance capacities, morphology and skill.
Highlights
Understanding the determinants of fighting ability is key to elucidating the evolution of aggressive behaviour, as current tests of contest theory rely on realistic proxies for overall RHP
By analysing the points of impact of these strikes, we found that attackers that rapped with coarse-scale accuracy were more likely to win the fight, indicating that the ability to target a ‘sweet spot’ on the defender's shell is an important determinant of contest success
There was a significant effect of the proportion of points of impact (POIs) in zone 1 (c21, 87 1⁄4 5.94, P 1⁄4 0.015) but not zone 2 (c21, 87 1⁄4 0.45, P 1⁄4 0.50) on fight outcome, indicating that attackers that successfully elicited an eviction landed a higher proportion of raps in zone 1 than attackers that failed to elicit an eviction (Fig. 3)
Summary
Understanding the determinants of fighting ability (or resource-holding potential, RHP) is key to elucidating the evolution of aggressive behaviour, as current tests of contest theory rely on realistic proxies for overall RHP. By analysing the points of impact of these strikes, we found that attackers that rapped with coarse-scale accuracy were more likely to win the fight, indicating that the ability to target a ‘sweet spot’ on the defender's shell is an important determinant of contest success. As there are no data to suggest where a ‘sweet spot’ might be located, we analysed both the coarse- and fine-scale spatial distributions of the points of impact (POIs) of repeated raps in an effort to ascertain whether accuracy and precision are important at a local or general level: for example, is it enough to hit a certain part of the shell (e.g. body whorl, apex, lip) or must the attacker hit more localized targets to secure victory?
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