Escalated damselfly territorial contests are energetic wars of attrition

Thirteen pairs of neighbouring Calopteryx maculata (Odonata: Calopterygidae) males were manipulated such that members of each pair became residents in the same territory, thereby removing the normal resident-intruder asymmetry and permitting direct analysis of the physical and energetic factors affecting the outcomes of the prolonged, escalated contests that resulted. Energy reserves (fat remaining at the end of contests) were more often correlated with winning these contests than size or physical attributes related to flight ability. This pattern was also true for 11 natural contests in which persistent intruders displaced resisdents. Fat content varied with age, being lowest in immature (teneral) and older males, and highest in young males first appearing at the water. Our results indicate that escalated territorial contests in C. maculata favour males with the greatest energy reserves. High fat content in some males, especially young ones, may allow them to overcome the normal resident-intruder asymmetry and displace established territory resident. Since males rarely feed while at their territories and since territories are important for obtaining and protecting mates, energy reserves may be crucial to reproductive success and escalated fights may be especially costly.