The Effects of Proportional Processing and Multiple Opponents on Contest Assessment in Male Green Swordtail Fish (Xiphophorus hellerii).

When animals compete over essential and limited resources, how they gather information about fighting ability is a crucial factor influencing their decision-making. Most research in animal contests asks how decisions are made when facing a single competitor; however, in many cases, individuals face multiple potential opponents and may incorporate information on this social environment. In addition, recent research suggests that animals perceive contest-relevant stimuli like body size in a proportional, not absolute, manner; this proportional processing has rarely, if ever, been incorporated into studies of contest assessment. Green swordtail fish (Xiphophorus hellerii) live in social aggregations, in which males may defend females from multiple potential opponents. Here, we asked how focal male green swordtails defended live females when presented with two simulated males that differed by known sizes. We found that focal males spent less time near the larger, more salient, of the two competitors as the mean size of both simulated competitors increased. That is, focal males mainly used information on the social environment to make competitive decisions, as opposed to information about own or relative fighting ability as commonly assumed in most contest theory. We also found that males who spent less time with the largest competitor shifted their attention to the defended female, devoting more time near this resource. Our findings suggest that, when there are multiple potential competitors, common models of decision-making in contests may be less applicable than previously assumed. Further, given the common use of proportional processing across animals, we suggest that future work on contests incorporates this type of perception.