Males of the damselfly Calopteryx splendens xanthostoma (Charpentier) demonstrate territorial and non-territorial mate securing tactics. Non-territorial males obtain a territory in one of two ways: they either wait for a territory to become vacant, or they fight with and displace a territory holder. The estimated reproductive success of territorial males was a thousand times greater than that of non-territorial males, suggesting that males should fight hard to become, and remain, territorial. Our results show that the ability to win fights, and therefore be territorial, is dependent on energy reserves (fat). Energy reserves were correlated with the age of the individual. Young, pre-territorial, males had excess fat; territorial males that had gained their territory by fighting had intermediate fat reserves; older males that had been displaced from a territory had very low levels of fat. Previous studies of calopterygid territoriality have suggested that resident-intruder or other uncorrelated asymmetries are important in determining the outcome of a conflict. We provide an alternative explanation centring around Grafen's (1987) `desperado' effect and the energy constraint on a male's ability to obtain a territory.