ASSESSMENT OF ENERGY RESERVES BY DAMSELFLIES ENGAGED IN AERIAL CONTESTS FOR MATING TERRITORIES

Male Calopteryx maculata (Odonata) damselflies engage in prolonged, hi ghly escalated aerial contests for mating territories. A previous stud y (Marden and Waage 1990, Anim. Behav., 39, 954-959) found that winner s of contests had a higher fat content than losers in 88% of cases, wh ereas differences in physical variables related to size and flight abi lity were not related to the outcome of contests. Here that study is e xtended in order to determine whether contests proceed until one conte stant reaches a lower physical limit in fat reserve, or alternatively, whether contestants are able to assess each other's fat reserves and thereby settle contests before physical limits are reached. The physic al limitation hypothesis was not supported, as losers showed no reduct ion in variability of fat content relative to winners. The assessment hypothesis was supported by an increase in the accuracy of the 'fatter wins' rule with increasing contest duration. Winners were fatter in 1 7 of 25 (68%) short contests (duration <500 s), versus 20 of 21 (95%) long contests (>500 s), which suggests a gradual accumulation of infor mation during contests. The assessment hypothesis was supported furthe r by a negative relationship between contest duration and energetic as ymmetry between contestants in long contests. Duration of long contest s was also positively related to the total fat content of the two cont estants, which suggests that the ability of contestants to perceive re lative energetic status may vary depending on absolute levels of energ y reserves. A model that assumes an asymptotic increase in flight perf ormance with increasing fat content (i.e. Michaelis-Menten or 'saturat ion' kinetics) is proposed to explain the simultaneous effects of rela tive and absolute fat content on contest duration, and to examine poss ible mechanisms that damselflies use to assess each other's energy res erves.