Assessment Games and Competitive Behaviour of the Mangrove Tree-Dwelling Crab, Selatium Brockii (De Man, 1887) (Decapoda, Grapsidae)

Abstract

Game theoretical models have been developed to understand the decisions of individuals to continue or stop fighting. These can be split into self assessment models (cumulative assessment model (CAM), war of attrition without assessment (WOA-WA), energetic WOA), where contestants only have information on their own fighting persistence, and mutual assessment models (sequential assessment game: SAM), where contestants exchange information on each other's competitive ability. Here, we assess if self or mutual assessment occurs in fights between male mangrove tree-dwelling crabs, Selatium brockii (De Man, 1887). Contests were staged in the presence of a female between focal crabs belonging to either of two different absolute size classes against a smaller, size-matched, and larger opponent. In line with mutual assessment models and CAM, contest duration increased when contestants were more matched in competitive ability (i.e., size) but was not significantly different between focal crab sizes. However, this pattern differed between focal crab size classes when competitors were smaller, with contests lasting longer for the larger size class. Different behaviours with different levels of aggression were employed during contests, suggesting that mutual assessment occurs. However, unlike current model predictions, the different behaviours were not used in increasing order of aggressiveness. Instead, the sequence of behaviours used during a contest both increased and decreased in aggressiveness. These observations suggest that contests are a form of unbalanced partial assessment, corroborating other recent empirical findings, and challenging theoreticians to formalize such contest dynamics.