Quadratic resource value assessment during mantis shrimp (Stomatopoda) contests
Resource value assessment, in which competitors adjust behaviours according to the perceived value of a contested resource, is well described in animal contests. Such assessment is usually assumed to be categorical or linear; for example, males fight more aggressively when females are present than absent, or as female fecundity increases. Here, to our knowledge for the first time, we show quadratic resource value assessment, in which resource value is highest at a certain level and decreases in either direction. The mantis shrimp Neogonodactylus bredini occupies coral rubble burrows in a size-assortative manner: individuals of a certain body size inhabit burrows of a certain size. Using mock burrows of various sizes, we tested whether mantis shrimp (1) chose burrows predicted to be the best fit for their body size and (2) were more aggressive during, endured higher costs during and were more likely to win contests over burrows predicted to be best fit. Individuals chose burrows larger than their predicted best fit burrows. In contests, intruders without burrows were more likely to evict burrow residents when the burrow was slightly smaller than the intruder's predicted best fit size. Intruder success decreased as relative burrow size increased or decreased from this value. Intruders won by delivering more strikes and by being aggressive first. In contrast to intruders, burrow residents showed little evidence of resource value assessment. A literature review revealed that quadratic resource value assessment may play a role in contests over resources from territories to parasite hosts. Therefore, our results impact theoretical models of contest behaviour and may lend insight to how contests affect resource distributions.