Density dependence and fighting in species with indeterminate growth: a test in a fiddler crab

Many forms of selection are density dependent. It is often assumed that all individuals of a given sex compete for the same resources, but in many species different types of individuals (e.g. morphs or body sizes) compete for different resources. Selection on competitive traits should be examined relative to the density of the relevant competitors (and contested resources) rather than total population density. Crucially, a predictable decline in effective competitor density with size might affect selection on fighting behaviour in species with indeterminate growth. We investigated whether male fighting behaviour over burrows in the fiddler crab Uca annulipes is consistent with size-dependent burrow usage that affects the density of relevant competitors and contested resources (burrow availability is limited). We show that larger males occupied larger burrows and occurred at lower densities, so they must travel further to locate new, suitably sized burrows. This should favour larger males investing more in each fight that they initiate. Indeed, larger males fought for significantly longer than smaller males, which increased their likelihood of winning a burrow. The observed increase in fight duration is not readily explained by ontogenetic changes in fighting costs (i.e. ‘giving up’ thresholds). It is worth testing whether increased fight duration with size/age occurs in the innumerable other species with indeterminate growth that compete for refugia (e.g. hermit crabs, reef fish) because a decline in density with body size is inevitable owing to cumulative mortality, and a physical constraint on the minimum-sized refugia that can be entered is commonplace.