Lipid economy, flight activity and reproductive behaviour in the speckled wood butterfly : on the energetic cost of territory holding

Insect flight is a highly energy demanding type of locomotion. In butterflies, males may locate females by different behavioural tactics. The tactics correspond to different flight types that, in turn, are assumed to reflect different energetic costs. Costs need to be considered to fully understand the pay-offs of co-existing alternative tactics relative to the environmental context and the phenotypes of the individuals. We addressed the issue in the speckled wood Pararge aegeria, in which males either adopt a territorial wait-and-fight tactic (i.e. territorial perching) in a sunlit patch on the forest floor, or a fly-and-search tactic to locate females in a wider area of the forest (i.e. patrolling). Perching corresponds to high frequency of take-off flights and aerial combats with high levels of manoeuvrability and is assumed to be energetically more costly than longer, continuous flights at lower speed in patrollers. We tested the effect of different flight activity levels and of the behavioural tactics on lipid reserves and lipid use in males by laboratory and outdoor cage experiments. Low-activity males that had access to honey water were capable of synthesizing lipids; their lipid reserves increased with age. The effect disappeared in males that actively flew in the outdoor cages. Lipid reserves decreased significantly faster in territorial perching males compared to non-perching males, but resting metabolic rate did not differ between the alternative behavioural tactics. Territorial perching males had larger flight muscle ratio (i.e. thorax/body mass) than non-perching males. We discuss the evidence of the physiological costs of perching relative to the co-existence of perching and patrolling tactics.