feathers so as to make such an action difficult or clumsy [1,31,69]. Further, paravian manual unguals are enlarged and strong expression of flexor tubercles suggests that the claws were capable of exerting considerable force, yet the limited range of motion of the forelimb [31] seems strongly adapted for flapping, rather than the flexibility required for prey manipulation (or indeed, climbing [61]). Comparison to extant raptors provides a combined functional hypothesis for the forelimbs and unguals that has not been previously considered. During immobilisation, it is common to observe extant raptors encircling their prey with their wings, a posture known as ‘‘mantling’’ [70]. This is observed across all extant raptor families and is thought to either assist in preventing prey escape, or conceals the victim from other predators, lest they attempt piracy. Under the RPR model, if the same strategy was employed by paravians subduing prey, then the large manual unguals may have been used to pull escaping prey back under the feet of the predator in a raking action. This reconstruction lowers the hands to be used near the feet, consistent with the orientation of the palms while in this posture. Exceptionally large prey immobilisation strategy Fossil associations of Deinonychus and the ornithischian Tenonto- saurus (which is of larger body size than Deinonychus) have led some workers to hypothesize a predatory relationship between the two, including the possibility of pack-hunting in Deinonychus [3]. Coordinated pack-hunting was considered unlikely by Roach and Brinkman [71], although mobbing was thought possible. It is rare to see extant predators taking prey that are significantly larger than themselves; however, Roach and Brinkman note that golden eagles (Aquila chrysaetos) have been observed to take down small deer and sheep [72–74]. Published accounts of this rare (indeed, disputed) behaviour are anecdotal, but the process by which golden eagles kill large prey is of interest. In most accounts, eagles form a tight fist with their feet, and stoop their prey, striking it at speed [75]. Clearly, this behaviour is not possible for non-volant deinonychosaurians. However, rarer accounts ([76] and references therein) record eagles ‘‘prey-riding’’; embedding their talons deep into the backs of their much larger prey and holding on as the victim’s vigorous retaliations serve only to widen the wounds. Prey-riding can be considered as an extension of the typical accipitrid predatory strategy for dealing with large prey, except that here the victim is too large to be pinned down by the raptor’s bodyweight. In order to prevent escape the raptor merely holds on with its hypertro- phied talons. Some anecdotal sources suggest that piercing of internal organs by talons hastens the death of the victim (also previously suggested for smaller prey) [68]. Experiment and observation has shown this to be unlikely ([20] and references therein). Instead, the victim is probably immobilized by weakening through exhaustion and/or loss of blood. Prey-riding behaviour was recently filmed for solitary golden eagles attacking reindeer calves [77], and has also been suggested to have been employed by the extinct Haast’s eagle, Harpagornis moorei: the largest species of raptor known to have existed ([78]; see Supporting Information Text S1). However, prey-riding by eagles is very rarely observed and should not be considered typical. Also, since enlarged D-I and D-II talons are characteristic of all accipitrids [20], most of which have not been observed prey-riding, then this behaviour is probably not a significant selection factor affecting ungual size and morphology. Prey-riding in eagles is a similar behaviour to the ‘‘climbing crampon’’ hypothesis of Manning et al. [2,62] whereupon the enlarged D-II claw of deinonychosaurians is suggested to have The Predatory Ecology of Deinonychus evolved to maintain purchase on exceptionally large prey. However, as in extant accipitrids, the hypothesis that the hypertrophied D-II claw evolved specifically for this behaviour is unlikely. We do not exclude the possibility that Deinonychus and other dromaeosaurids may have successfully attacked prey much larger than themselves, but their anatomy suggests that, as with all known tetrapod predators, they mostly preyed upon animals smaller than themselves. Hallucal reversal and evolution of the grasping foot It has been proposed that the reversal of D-I, the hallux, evolved to grip branches for perching, and as such is an important component of some models for the origin of flight [7,52,79]. Although there has been some debate, the hallux of Archaeopteryx is now thought to have been medially directed rather than fully reversed ([80]; see Supporting Information Text S1). A fully reversed hallux was reported for the basal bird Jeholornis [81,82], although this is also disputed as all Jeholornis specimens are compressed in a fashion similar to Archaeopteryx such that apparent hallux reversal may be a preservational artifact. As such, the first appearance of a fully reversed hallux is uncertain, and may not be strictly definable since translocation was probably gradual. Middleton [83] documented the variable position of the hallux in extant birds and concluded by asking which functional changes and selection pressures led to the evolution of a reversed hallux. In contrast to perching-only hypotheses, the RPR model proposes that a grasping foot first evolved for predatory purposes in terrestrial paravians. Selection pressure for increased grasping ability (benefiting predatory success) favored gradual translocation of the hallux to a progressively more reversed position where it could oppose the other digits providing more even grip. This demonstrates a viable selection pathway whereby the necessary grasping ability and hallux reversal required for perching could be exapted from a predatory function in a wholly terrestrial predator, without invoking a hypothetical pre-flight arboreal or scansorial stage for non-avian theropods. Grasping for predation or an arboreal habit? In their description of the small basal dromaeosaurid Microraptor zhaoensis (Lower Cretaceous, China), Xu et al. [52] describe a number of pedal characters which they refer to as ‘‘consistent with an arboreal habit’’: pedal digit I (hallux) is relatively distal in position; pedal unguals show higher curvature than other non- avian theropods; and distal non-ungual phalanges are elongated. Similarly, Feduccia et al. [79] remark that ‘‘[hallucal] reversal is an unequivocal arboreal adaptation for grasping branches’’. A distally positioned or reversed D-I (hallux) and elongated penultimate phalanges are both features that enhance grasping ability. High ungual curvature enhances the hooking ability of the ungual. These features would all be of considerable use to an arboreal animal, but as shown here and elsewhere ([20] and references therein), they are also proven predatory adaptations. Although these two interpretations are not mutually exclusive, it is a challenge to be able to differentiate them morphologically and elucidate whether one function has been exapted from the other ([61]; a similar problem has been encountered in carnivorous mammals; [9]). Some features of paravians seem to support the predatory model over the arboreal model, at least as a primary or initial function. The same adaptations for grasping (along with other predatory adaptations) are seen in both small and large bodied deinonychosaurians, including taxa too large to have been arboreal. The subarctometatarsalian condition of the metatarsus, exhibited by basal Deinonychosauria (including Microraptor) and PLoS ONE | www.plosone.org 9 December 2011 | Volume 6 | Issue 12 | e28964