Much of our understanding of the control and dynamics of animal movement derives from controlled laboratory experiments. Laboratory experiments allow investigators to focus on simple movement patterns and to elicit similar behaviours repeatedly, facilitating biomechanical analyses and reducing the impact of individual variability. The repeatability and control provided by laboratory experiments is thus essential for teasing out fundamental principles of animal locomotion. Many of these fundamental principles involve the relationship between locomotory structures (e.g. appendages, muscles, joints) and their functions (e.g. power, control, efficiency); yet, when examined outside of their ecological context, the relevance of structure–function relationships and their significance to wild animals can be difficult to assess.
Linking laboratory analyses of simplified locomotory tasks to studies of natural, complex behaviours can help reveal the ecological relevance and significance of structure–function relationships. This approach has been well developed in studies of terrestrial sprinting and escape behaviour, particularly in lizards (e.g. Irschick et al. 2005
), but has rarely been used in studies of animal flight (for an example, see Srygley & Kingsolver 2000
). To demonstrate the utility of this approach for animal flight studies, we examined the effects of wing damage on dragonfly flight performance, in both a laboratory drop–escape response and the more natural context of aerial predation.
Many flying animals suffer episodic (e.g. moulting birds) or cumulative (e.g. insects) loss of wing area, and although the aerodynamic predictions regarding the cost of reduced wing area are straightforward, previous studies investigating the consequences of area loss have yielded conflicting results. In bumble-bees (Bombus terrestris
), artificial wing wear (approx. 18% area loss) increases mortality (Cartar 1992
). Laboratory experiments show that bees with clipped wings have a higher wingbeat frequency but no increase in metabolic costs, leading to the suggestion that reduced manoeuvreability makes bees with wing damage more susceptible to predators (Hedenström et al. 2001
). Increased susceptibility to predators has also been proposed as a consequence of wing area loss in butterflies, as individuals with their hindwings removed (approx. 50% area loss) fly in equally curvy paths but at a lower overall speed (Jantzen & Eisner 2008
). Yet, an earlier study found that butterflies with clipped wings (15–20% area loss) display no difference in flight activity, dispersal rates or recapture probability in the wild (Kingsolver 1999
Thus, the studies performed to date do not demonstrate convincingly that wing area loss carries a functional cost that has a negative impact on fitness, as they lack either a functional explanation (in the case of bumble-bees) or a demonstrated fitness cost (in the case of butterflies). It is important to note that in these studies, fitness has been evaluated only in the context of short-term survival (i.e. evasion of predators over the course of several days), although for many species the need to escape from predators may be a relatively infrequent event as compared with other flight behaviours.
We examined the effects of hindwing area loss—a form of wing damage commonly observed in wild dragonflies—on flight biomechanics during a simple laboratory flight test and during aerial predation. Aerial predation is the only means by which odonates (dragonflies and damselflies) acquire resources, and is a critical component of fitness. The substantial mass gain that results from hunting contributes to flight muscle mass and mating success in males (Marden 1989
), as well as to abdominal mass and fecundity in females (Anholt 1991
). However, aerial predation is also a risky business: individuals that forage more and gain more weight have a higher mortality rate, presumably owing to the inherent risk of being preyed upon themselves while pursuing a meal (Anholt 1991
). Thus, aerial predation is a critical flight behaviour that may have multiple fitness consequences.