PROJECT NUMBER: IOS 0952471
In order to understand the diversity of animal body forms found in nature, as well as the potential application of particular features to man-made devices, it is essential to first clarify the relationship between a given structure and the function(s) for which it has evolved. In studies of animal locomotion, this relationship has typically been examined in the context of a single, isolated function that is easy to elicit and measure, such as forward locomotion at a steady speed, without any clear indication of how important this function is for animals in the wild. A more complete understanding of the role of particular body structures can be gained by examining their effect on performance during complex, demanding locomotory behaviors that are known to affect an animal's survival or reproductive success. The current study will demonstrate this approach through integrated analyses of the biomechanics, behavior and performance of dragonflies hunting aerial prey. Studies will be conducted in a novel, outdoor artificial dragonfly habitat, in which natural predatory behaviors can be elicited reliably from known individuals, and filmed with a high-speed multi-camera system to reconstruct three-dimensional body motions. This will be the first project to explore the mechanical determinants of aerial predation, one of the most challenging and complex behaviors performed by flying animals. Through a series of manipulations, the work will provide a better understanding of the effects of body mass distribution, wing flexibility, and wing damage on flight performance. In addition, the project will train several undergraduate and high school students in basic experimental and field techniques, while providing data on prey capture rates and the effectiveness of wild dragonflies in controlling mosquito populations.
In order to understand the diversity of animal body forms found in nature, as well as the potential application of particular features to man-made devices, it is essential to first clarify the relationship between a given structure and the function(s) for which it has evolved. In studies of animal locomotion, this relationship has typically been examined in the context of a simple locomotory behavior that is easy to elicit and measure (e.g., forward locomotion at a steady speed), without any clear indication of how important this function is for animals in the wild (and thus how strongly it has been shaped by evolution). During this project, we adopted an alternative approach to investigating structure-function relationships in flying dragonflies, by examining flight biomechanics during a complex, demanding locomotory behavior that is known to affect both reproductive success and survival in the wild: aerial predation. To accomplish our goals, we developed a novel, outdoor artificial dragonfly habitat, in which natural predatory behaviors can be elicited reliably and filmed with a high-speed, multi-camera system to reconstruct three-dimensional trajectories of both the dragonfly and prey. In addition, we can quantify capture success rates of individual dragonflies by releasing prey repeatedly and scoring the proportion of successful captures, allowing us to examine the effects of a wide variety of factors (dragonfly species, sex, age, light levels, temperature, prey type, wing condition, etc.) on flight performance during this challenging locomotory behavior. We have now published and presented a series of studies on dragonfly predation - the first scientific studies to quantify the dynamic motions and physical requirements of aerial predation in any animal. Our results have been surprising in many ways. For example, in predatory contests with one species of dragonfly (the Spangled Skimmer) pursuing fruit flies, we found that although the pursuit takes place in open air, dragonflies approach their prey in such a way that the fruit flies rarely know the dragonfly is coming. Thus, fruit flies that routinely fly erratically and unpredictably are more likely to survive than those that detect the dragonfly and attempt to evade it - because the unpredictable flies sometimes escape from dragonflies by pure luck (by performing a random turn at exactly the right moment). In contrast to the Spangled Skimmer, other dragonflies found in the same habitat (e.g., the Blue Dasher) forego this careful approach strategy, instead accelerating rapidly and capturing prey more quickly – but nearly always alerting the prey to their approach, and thus needing to respond rapidly to the prey’s evasive maneuvers. The finding that closely related dragonfly species living in the same habitat adopt radically different predation strategies suggests that dragonflies may be more specialized for capturing particular types of prey than previously recognized. This highlights the importance of preserving the diversity of these important predators, as a single species of dragonfly would be unlikely to perform the same "service" (controlling populations of a range of smaller insects, many of which are pest species) as a naturally diverse assembly of dragonfly species. Through this project, we have also been able to collect some of the most complete and reliable measurements of exactly how successful dragonflies are at hunting aerial prey. Overall, we find that dragonflies are incredibly successful hunters: dragonflies typically capture more than 90% of the fruit flies they pursue, approximately 75% of mosquitoes, 60% of houseflies, and up to 40% of the large, biting deerflies – capturing most of these prey in less than half a second. We have found that several factors affect dragonfly capture success, including light (they perform better with more sunlight) and the condition of their wings (they perform worse as wings become tattered with age). Finally, although dragonfly species differ from one another in their pursuit strategies, a given species of dragonfly adopts the same exact strategy (including the same peak acceleration and approach behavior) when pursuing a wide range of prey, suggesting that quantitative analyses of dragonfly approach strategies may reveal effective, general pursuit strategies that would be robust to differing evasive behaviors. In addition to the scientific data gained, this project has provided a unique interdisciplinary training environment for 3 graduate students, 4 post-baccalaureate research assistants (3 of whom are now in PhD programs), and 6 undergraduates (many of whom are now in graduate programs, in fields ranging from wildlife conservation to aerospace engineering). A significant proportion of these students come from groups typically underrepresented in the sciences (e.g. women and underrepresented minorities). We routinely present the results of our research at international conferences and symposia, and conduct tours of our dragonfly research facility for K-12 students and other public groups. Finally, our work on dragonfly predation has been covered by several high-profile media outlets, including the New York Times, BBC, and Discovery Channel, reaching a wide audience and generating considerable public excitement.
