The mis-measure of animal contests: has direct assessment of rivals been overestimated?

Mutual assessment of differences in resource holding potential (RHP) is usually expected as an economical means of contest resolution; weaker rivals can retreat when they detect their inferiority and can thereby avoid futile, costly, persistence. Models of contest resolution in which retreat decisions are based on assessed RHP difference predict that contest duration should diminish as RHP difference between the rivals increases because differences are more readily detected. This prediction appears to have been fulfilled in contests of many different species, generating the impression that assessment of RHP differences is ubiquitous. But few studies have considered alternative models in which each rival simply persists in accord with its own RHP ('own RHP-dependent persistence'). In contests decided by own RHP-dependent persistence weaker rivals may retreat first because they are inherently less persistent. In these contests, duration depends primarily on the weaker (losing) rival's RHP rather than RHP difference between the rivals. However, the analyses most commonly employed to detect effects on RHP differences cannot discriminate between these alternatives. Many studies purporting to show a negative relationship between RHP differences and contest duration may thus actually reflect an incidental association between weaker rival (loser) RHP and RHP difference. I will discuss the importance and scope of this mis-measure of animal contests. I will also suggest some alternative analytical techniques capable of discriminating true effects of RHP difference.