Contests between larval damselflies: empirical steps toward a better ESS model

Some hypotheses about facultatively aggressive interactions among damselfly larvae (Odonata: Zygoptera) have recently been generated from a model (Crowley 1984) based on the theory of evolutionarily stable strategies (ESS). The present study tests some of the assumptions and predictions of this ESS model with larvae of the damselfly Ischnura elegans van der Lind. in two sets of laboratory experiments. In contest experiments, interactions between two larvae on a thin dowel were observed in all possible combinations of the final three instars. The tendency to maintain control of the site and to evict the opponent was positively related to hunger, relative size (instar difference), and aggressiveness (labial striking), but negatively related to advancing toward a similar-sized opponent along the dowel. Contests were longer and involved more physical contact when larvae were more similar in size. No injuries or mortality resulted from the observed encounters. In distraction experiments, individual larvae were placed in a small feeding chamber with daphnid prey. Experimental larvae were surrounded by last-instar larvae visible through clear plastic walls; controls had no surrounding larvae. Time spent staring at others carried a feeding cost. This cost, the observed negative-exponential distributions of contest duration, and the apparent rarity of dangerous aggression, suggest that contests between similar-sized larvae could be considered wars of attrition. Other possible interpretations and some implications of these behaviour patterns for vulnerability to other predators are noted.