Intraspecific variation in structures that display competitive ability: large animals invest relatively more

Abstract

The benefits of possessing (or enlarging) a display structure used in competitor assessment will vary between individuals of a species. For some individuals the cost of developing such a feature could outweigh any benefits accrued. Animals whose competitive ability is high can benefit from display, but there is little advantage in advertising low competitive ability. Thus, since large animals generally win fights it can be predicted that large animals should have relatively large display features. This prediction was tested using data on the moorhen, Gallinula chloropus. Evidence is presented that in this species frontal shields are used in competitor assessment. The size of an individual's frontal shield varied seasonally. Maximum shield size corresponded with peak aggressive behaviour. Shield size was positively correlated with body weight, which is the best predictor of the outcome of agonistic encounters in this species, and the slope of the regression was significantly greater than one. Thus shield size proved to be positively allometric as predicted. It remains to be tested whether or not positive allometry is a feature of all structures used to display competitive ability.