Survival selection imposed by predation on a physiological trait underlying escape speed

P>1. In contrast to other phenotypic traits, selection on physiological traits remains largely undocumented. We have evaluated survival selection imposed by predation by dragonflies on the activity of arginine kinase (Ak), a key enzyme delivering energy for escape performance in invertebrates. 2. To accomplish this, we conducted a semi-natural field enclosure experiment in which we manipulated predation by large dragonfly predators, and quantified escape swimming speed and Ak in the prey, the damselfly Enallagma vesperum. To avoid confounding selection on Ak with selection on other swimming speed-related variables, we also scored all morphological and behavioural traits thought to underlie swimming speed in these damselflies. 3. Dragonfly predators imposed considerable mortality and selected for faster swimming speed and higher activity levels of Ak. Furthermore, higher Ak levels contributed to higher swimming speeds, confirming the mechanistic role of Ak for escape performance. Although morphological (size of the caudal lamellae which generate trust) and behavioural (number of beats made by the abdomen during swimming and the start angle of the C-start) variables contribute to increasing swimming speed, we detected no selection on these variables. This may be due to functional redundancy. 4. Taken together, our results indicated selection on Ak and suggested that selection on physiological traits may be as strong as selection on morphology and life history traits.