attacks [22–26]. Physical adaptations for increasing foot strength (hence constriction ability) are more developed in owls which are small prey specialists [19]. In falcons and some owls, immobili- sation is aided by attempts to snap the spinal cord or crush the head with the predator’s beak. Falcons have evolved a stronger bite force and a specialized ‘‘tomial tooth’’ on the beak to aid in doing so [27,28]. Large prey are defined as being too big to be contained within the foot, and so cannot be constricted [20] (Supporting Information Videos S1 and S2). To prevent escape of large prey the raptor pins its victim to the ground using its bodyweight, then plucks away feathers or fur, exposing an area of flesh. For immobilisation, falcons will quickly attempt to snap the spinal cord to kill the prey, but accipitrids lack the physical specializations for this. Instead, accipitrids possess hypertrophied talons on D-I and D-II which are adaptations for maintaining grip on large struggling prey [20]. Accipitrids’ talons lock into their prey, keeping hold despite vigorous struggling, allowing the raptor to begin feeding. In such cases, death of the victim is hastened by massive bleeding from wounds sustained whilst being eaten alive. An understanding of how foot morphology affects predatory ability in extant birds of prey can inform interpretations of similar variation observed in extinct non-avian theropods. Previous comparisons of dinosaur hindlimb morphology have mainly concentrated on its contribution to locomotion ([29] and references therein). However, the hindlimb is more than just a component of the locomotor system, and in many theropod taxa the hindlimb exhibits features consistent with hooking and grasping functions. The purpose of this paper is to elucidate functional morphology of the deinonychosaurian pes by compar- ison to the findings of our sibling study [20], referred to hereafter as the Raptor Prey Restraint (RPR, or ‘‘ripper’’) model (Figure 1). When this approach is combined with consideration of phyloge- netic trends already recognized within theropoda (Figure 2), many of the peculiarities of deinonychosaurian anatomy can be interpreted as adaptations associated with specific predatory behaviours. We suggest the enlarged D-II claws of deinonycho- saurians were used to grapple prey in a fashion comparable to accipitrid birds of prey, and are part of a suite of features that indicate ecological separation within Deinonychosauria and Paraves. These findings open several new lines of research into the predatory abilities of extinct theropods, and the evolution of novel structures and behaviours leading to birds. This includes description of a new flapping behaviour that may have important implications for the origin of flight. Methods This paper is a qualitative application of the RPR model to non- avian theropods which is based on a quantitative assessment of predatory morphology and behaviour in birds of prey [20]. Measurements were taken of non-avian theropod specimens to test predictions of the RPR model and to emphasize trends in hindlimb proportions described elsewhere [15]. Through our work on birds of prey, it is clear that consideration of each digit is essential, as the way in which each varies is an indication of predatory specialty or locomotor habit. Previous analyses have considered only one digit (typically D-III [6-8,15]), and are much less able to assess variation in foot use. Indeed, proportions of accessory digits D-II and D-IV vary more strongly than D-III, depending on use. Principal morphological observations of Dromaeosauridae were made on MOR 747 (two complete and one partial pes of Deinonychus antirrhopus, Cloverly Formation, Aptian-Albian, Mon- tana; Museum of the Rockies, Bozeman, MT, USA). Examination The Predatory Ecology of Deinonychus Figure 1. RPR ‘‘ripper’’ behavioural model, illustrated by a small dromaeosaurid. (A) grasping foot holds on to prey. (B) hypertrophied D-II claw used as anchor to maintain grip on large prey. (C) predator’s bodyweight pins down victim. (D) beam-like tail aids balance. (E) low-carried metatarsus helps restrain victim. (F) ‘‘stability flapping’’ used to maintain position on top of prey (see Supporting Information Videos S1 and S2). (G) arms encircle prey (‘‘mantling’’), restricting escape route. (H) head reaches down between feet, tearing off strips of flesh (may explain unusual deinonychosaurian dental morphology). Victim is eaten alive or dies of organ failure. doi:10.1371/journal.pone.0028964.g001 of well-preserved troodontid pedal material (MOR 553S.TM068 and MOR 748.TM065: Troodon sp., Two Medicine Formation, Campanian, Montana) permitted further comparison within Deinonychosauria. Measurements were recorded from 52 non-avian theropod specimens (26 taxa, MNI = 45) and (where appropriate) added to the extant raptor dataset [20]. Where possible we measured metatarsus length, ungual size and curvature, and non-ungual phalanx length for all pedal digits. Some non-ungual measure- ments were taken directly from published descriptions. Claw attributes and measurements follow the nomenclature of Fowler et al. [20]. Statistical analysis (correspondence analysis, one-tailed t- tests assuming equal variances) was conducted on the combined dataset. Correspondence analyses (CA) were run in the R language and environment for statistical computing (version 2.11.1 for Mac OSX: www.R-project.org; [30]), to determine whether taxa group by pes morphology. Correspondence analyses were used because they are less susceptible than principal components analysis to outliers. Phalangeal and ungual measurements were converted into ratios to remove the effects of body size. In the first CA, two complete pedes of Deinonychus specimen MOR 747 were measured and added to the complete dataset of Fowler et al. ([20] Supporting Information; n = 42). The second CA focused on phalangeal proportions of non-avian dinosaurs, and excluded some incomplete specimens from the dataset (n = 30). Original data can be found in Supporting Information Table S1. PLoS ONE | www.plosone.org 2 December 2011 | Volume 6 | Issue 12 | e28964