The Predatory Ecology of Deinonychus and the Origin of Flapping in Birds Denver W. Fowler1*, Elizabeth A. Freedman1, John B. Scannella1, Robert E. Kambic2 1 Museum of the Rockies and Department of Earth Sciences, Montana State University, Bozeman, Montana, United States of America, 2 Department of Ecology and Evolutionary Biology, Brown University, Providence, Rhode Island, United States of Ameria Abstract Most non-avian theropod dinosaurs are characterized by fearsome serrated teeth and sharp recurved claws. Interpretation of theropod predatory ecology is typically based on functional morphological analysis of these and other physical features. The notorious hypertrophied ‘killing claw’ on pedal digit (D) II of the maniraptoran theropod Deinonychus (Paraves: Dromaeosauridae) is hypothesized to have been a predatory adaptation for slashing or climbing, leading to the suggestion that Deinonychus and other dromaeosaurids were cursorial predators specialized for actively attacking and killing prey several times larger than themselves. However, this hypothesis is problematic as extant animals that possess similarly hypertrophied claws do not use them to slash or climb up prey. Here we offer an alternative interpretation: that the hypertrophied D-II claw of dromaeosaurids was functionally analogous to the enlarged talon also found on D-II of extant Accipitridae (hawks and eagles; one family of the birds commonly known as ‘‘raptors’’). Here, the talon is used to maintain grip on prey of subequal body size to the predator, while the victim is pinned down by the body weight of the raptor and dismembered by the beak. The foot of Deinonychus exhibits morphology consistent with a grasping function, supportive of the prey immobilisation behavior model. Opposite morphological trends within Deinonychosauria (Dromaeosauridae + Troodontidae) are indicative of ecological separation. Placed in context of avian evolution, the grasping foot of Deinonychus and other terrestrial predatory paravians is hypothesized to have been an exaptation for the grasping foot of arboreal perching birds. Here we also describe ‘‘stability flapping’’, a novel behaviour executed for positioning and stability during the initial stages of prey immobilisation, which may have been pivotal to the evolution of the flapping stroke. These findings overhaul our perception of predatory dinosaurs and highlight the role of exaptation in the evolution of novel structures and behaviours. Citation: Fowler DW, Freedman EA, Scannella JB, Kambic RE (2011) The Predatory Ecology of Deinonychus and the Origin of Flapping in Birds. PLoS ONE 6(12): e28964. doi:10.1371/journal.pone.0028964 Editor: Andrew Allen Farke, Raymond M. Alf Museum of Paleontology, United States of America Received February 28, 2011; Accepted November 17, 2011; Published December 14, 2011 Copyright: 2011 Fowler et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: Support for the Museum of the Rockies graduate student fund was provided by Damaris Waggoner and David Sands. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: df9465@yahoo.co.uk Introduction From its description by John Ostrom in 1969 [1], the Early Cretaceous carnivorous dinosaur Deinonychus antirrhopus (Thero- poda: Dromaeosauridae) became an icon of the ‘‘dinosaur renaissance’’. Depictions of Deinonychus as a fleet, intelligent predator, operating in packs to take down prey much larger than itself [2–4], captured the imagination of the public and researchers alike. Interest has grown yet further in recent years, following cladistic analyses that recovered Deinonychosauria [5] (Dromaeo- sauridae + Troodontidae) as the sister group to birds, prompting debate as to how flight might have evolved from a deinonycho- saurian-like ancestor. Despite this level of interest, much of what is typically assumed about the ecology of Deinonychus, and Deino- nychosauria in general, is based on speculation. Although the enlarged pedal D-II claw has generated much interest, surprisingly few analyses have compared dinosaur claw morphology to animals with known ecologies [6,7], mainly because of a paucity of research on claw morphology and function in general ([4,7,8]; although see [9,10]). In a novel experiment, Manning et al. [2] demonstrated that the hypertrophied D-II claw would not be effective for slashing and suggested instead that it was used by dromaeosaurids as a climbing crampon for gripping the hides of prey several times larger than themselves (see Supporting Information Text S1 for further review). We agree that the D-II claw is most effective as a hooked device, but modern analogues that have similarly hypertrophied D-II claws do not use them to climb up prey. In extant birds, variation in foot morphology is associated with variation in behaviour and factors such as speed, strength, agility, even diet [6,8,11–21]. Our recently published sibling study [20] investigated how foot morphology is related to predatory behavior in extant birds of prey. We showed for the first time that the Accipitridae (hawks and eagles) also possess a conspicuously hypertrophied talon on D-II and that this is utilized for prey immobilisation. It is important for extant predators to quickly subdue their victims, lest they escape or retaliate against their attacker. In extant raptors, prey immobilisation strategy is variable and mostly dependent on relative prey size [20] (‘‘immobilisation’’ is preferred to ‘‘killing’’ because accipitrids often do not wait until the death of their victims before feeding [22,23]). In all birds of prey, small prey (those that can be contained within the foot [20]) are immobilized by containment within the foot, assisted by constriction and beak PLoS ONE | www.plosone.org 1 December 2011 | Volume 6 | Issue 12 | e28964