Assessment of energy reserves by damselflies engaged in aerial contests for mating territories

Male Calopteryx maculata (Odonata) damselflies engage in prolonged, highly escalated aerial contests for mating territories. A previous study (Marden & Waage 1990, Anim. Behav., 39, 954-959) found that winners of contests had a higher fat content than losers in 88% of cases, whereas differences in physical variables related to size and flight ability were not related to the outcome of contests. Here that study is extended in order to determine whether contests proceed until one contestant 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 physical limitation hypothesis was not supported, as losers showed no reduction 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 17 of 25 (68%) short contests (duration <500 s), versus 20 of 21 (95%) long contests (>500 s), which suggests a gradual accumulation of information during contests. The assessment hypothesis was supported further by a negative relationship between contest duration and energetic asymmetry between contestants in long contests. Duration of long contests was also positively related to the total fat content of the two contestants, which suggests that the ability of contestants to perceive relative energetic status may vary depending on absolute levels of energy reserves. A model that assumes an asymptotic increase in flight performance with increasing fat content (i.e. Michaelis-Menten or 'saturation' kinetics) is proposed to explain the simultaneous effects of relative and absolute fat content on contest duration, and to examine possible mechanisms that damselflies use to assess each other's energy reserves.