Interacting effects of size and prior injury in jumping spider conflicts

An animal's ability to win intrasexual conflicts is often determined by the combined influence of several different traits, such as size, experience, residency and prior injury. When each of two (or more) asymmetries favours a different rival, the outcome of a conflict will reflect each asymmetry's contribution to overall status. We investigated the interacting effects of size difference and prior loss of forelegs (a common injury in nature) on the ability of males to win intrasexual conflicts in Trite planiceps, a New Zealand jumping spider (Salticidae). When both rivals were intact, the probability of the larger rival winning increased with the size difference between rivals. When rivals were of matching size, injury was a strong predictor of outcome; intact spiders beat rivals missing one or both forelegs and spiders missing one foreleg beat rivals missing both forelegs. During conflicts between spiders that differed both in size and in injury state, these two variables had combined effects on outcome. The tendency for larger rivals to win increased with size difference, but the probability of the larger rival winning diminished if the larger rival of the pair was the more injured. We used inverse prediction from logistic regression models to quantify the size advantage that is sufficient to offset the disadvantage of prior injury.