Increased risk and benefit simultaneously alter the outcome of territorial fights

Abstract

Competition for resource, including territories is seen in a broad range of taxa. There has been much research on factors that determine the intensity and outcome of competitions (e.g. resource holding potentials: RHP, and resource value: RV). No research, however, has shown how different factors can simultaneously alter the intensity and outcome of contests. We investigated the effects of RV on fighting behavior and success in inter-tidal, territorial male crabs. Under natural conditions the larger rival won contests irrespective of whether he was the resident or intruder (i.e. there was no residency effect). We then motivated intruders to fight hard by subjecting them to the threat of predation, and found that they were more likely to win contests. When we also motivated residents to fight hard by placing a female in his burrow (i.e. both rivals were simultaneously motivated to fight due to the perceived increase in resource value), we found that the contests escalated more and that resident males had a slightly increased chance of winning. This is the first report of two factors simultaneously affecting motivation and therefore altering the escalation and outcome of a contest.