derived further in troodontids (fully arctometatarsalian), is an adaptation that affords cursorial benefits: it is difficult to envisage a scenario in which it would be selected for in an arboreal animal. Further, grasping adaptations of the feet are maintained in the otherwise more cursorial troodontids, whose reduced forelimbs (also seen in the basal dromaeosaurid Tianyuraptor; [84]) would render them poorly adapted as climbers. It is possible that deinonychosaurians exapted their predatory grasping foot from arboreal ancestors. However, since the ancestors of Paraves were large-bodied terrestrial carnivores, this hypothesis requires basal Paraves to evolve an arboreal habit and adaptations, which are then subsequently lost in Deinonychosauria (potentially also becoming secondarily flightless; [85]). While possible, the multiple behavioural and morphological shifts render this hypothesis less parsimonious than if Paraves were terrestrial carnivores like their ancestors, and the grasping foot evolved initially for predatory purposes being exapted later for perching in Avialae. ‘‘Stability flapping’’ and the ‘‘flapping first’’ hypothesis The origin of the flapping stroke is an important independent step in the origin of flight [86,87]. Here we describe ‘‘stability flapping’’, a behavioral component of the RPR model, and propose that it could have been employed by paravians during predatory activity. During the struggle following large-sized prey capture by accipitrids, the hypertrophied D-II talons are locked into the prey, preventing the feet from assisting stabilization of the raptor [20]. To counter this, vigorous stability flapping is typically executed by the raptor in order to first get on top of its prey, then to constantly maintain this position, allowing it to use its full bodyweight to pin its victim to the ground [20] (Supporting Information Videos S1 and S2). If only small corrections are required, the wings are extended for balance with only occasional light flapping, and vertical movement of the tail [20]. Stability flapping supports a ‘‘flapping first’’ model where flapping and associated aerial capability, including generation of lift, can be evolved independently of a flight function. Large feathered wings were present in basal Paraves and Deinonycho- sauria such as Archaeopteryx, Microraptor, and Sinornithosaurus [41,52,88], and the presence of feathered forelimbs in larger species is demonstrated by preserved quill knobs in Rahonavis [89], and Velociraptor [90]. However, there has been much debate as to the aerial capabilities of these taxa and their importance in the evolution of powered flight. What use is half a wing? Even a relatively small aerofoil and weak flapping capability could be employed for stability flapping, affording a greater chance of predatory success. The low aspect ratio wings seen in Archaeopteryx [76] and basal Deinonychosauria [91] are similar in shape to those of extant accipitrines (Figure 11), woodland raptors that capture prey by surprise ambush and frequently utilize stability flapping The Predatory Ecology of Deinonychus (Supporting Information Videos S1 and S2; [20]). Short, broad wings confer great maneuverability at a cost of overall speed or soaring ability, and would have been well-suited for stability flapping. Forelimb movement in Deinonychus was ‘‘comparable to the form of the avian flight stroke’’ [92], and similar in other basal Paraves and Deinonychosauria ([31,93]; although Gatesy and Baier [87] questioned the precise similarity between the forelimb movement of Deinonychus and that of extant pigeons). Even if Deinonychosauria were not capable of a full avian-like flapping ability, they may have been able to perform a rudimentary flight stroke during stability flapping. Similarly, long feathered tails are conspicuous in accipiters and aid in maneuverability and balance during stability flapping. Basal Paraves and Deinonychosauria possessed long bony tails which are shown to have been well feathered (e.g. [52,88]), and would have assisted balance during predation [1] and stability flapping. Stability flapping is less physically demanding than flight, and represents a previously unrecognized intermediate aerial ability. Padian and de Ricqles [94] define four requirements of ‘‘flight’’ and suggest that all are fulfilled in Archaeopteryx: (1) an airworthy wing, (2) a flight stroke capable of generating a vortex wake that will propel the animal forward, (3) a metabolic level capable of sustaining flight for substantial intervals, and (4) the neuromuscu- lar coordination that permits effective navigation in a three- dimensional world. Vigorous stability flapping involves most of these requirements, but each can be functional in a less developed state than is necessary for flight. Hence, stepwise acquisition and development of Padian and de Ricqles’ flight requirements might have been facilitated by gradual evolution of less energetic behaviours leading to stability flapping. The presence of feathered forelimbs is well documented even in taxa basal to Paraves (e.g. the oviraptorosaurians Caudipteryx and Protarchaeopteryx, and therizino- saurosauroid Beipiaosaurus; [5,51,95]). Few would suggest that these are airworthy wings, but they may have provided some aid to balance (even outside of a predatory role). It is conceivable that vigorous stability flapping evolved from the simple outstretching of forelimbs for balance, developing through an intermediate stage consisting of short flaps and tail movement. Both of these behaviours are often employed by extant birds of prey for small positional corrections [20]. The step from stability flapping to powered flight requires significant generation of forward thrust. Although directional thrust is employed for positional changes during stability flapping, it is not yet clear how this might be adapted into a method of propulsion. Stability flapping (and other flapping behaviours [96,97]) would have been most effective at small body sizes and might have been a factor driving selection for miniaturization in Coelurosauria [94,98]. The ‘‘flapping first’’ model provides a viable selection pathway whereby decrease in body size through Coelurosauria is Figure 11. Wing proportions of birds. (A) Archaeopteryx. (B) Variation of wing aspect ratio in extant birds, from left (low) to right (high): goshawk (Accipiter gentilis), golden eagle (Aquila chrysaetos), northern royal albatross (Diomedea sanfordi). The short broad wings of Archaeopteryx are similar to the goshawk, where they afford great maneuverability. Image in (A) altered from Longrich [101]. doi:10.1371/journal.pone.0028964.g011 PLoS ONE | www.plosone.org 10 December 2011 | Volume 6 | Issue 12 | e28964