Deinonychus - Paleobiology and Paleoecology - Predatory Behavior - Limb Function

Limb Function

Despite being the most distinctive feature of Deinonychus, the shape and curvature of the sickle claw varies between specimens. The type specimen described by Ostrom in 1969 has a strongly curved sickle claw, while a newer specimen described in 1976 had a claw with much weaker curvature, more similar in profile with the 'normal' claws on the remaining toes. Ostrom suggested that this difference in the size and shape of the sickle claws could be due to individual, sexual, or age-related variation, but admitted he could not be sure.

There is anatomical and trackway evidence that this talon was held up off the ground while the dinosaur walked on the third and fourth toes.

Ostrom suggested that Deinonychus could kick with the sickle claw to cut and slash at its prey. Some researchers even suggested that the talon was used to disembowel large ceratopsian dinosaurs. Other studies have suggested that the sickle claws were not used to slash but rather to deliver small stabs to the victim. Manning et al. (2005) ran tests on a robotic replica that precisely matched the anatomy of Deinonychus and Velociraptor, and used hydraulic rams to make the robot strike a pig carcass. In these tests, the talons made only shallow punctures and could not cut or slash. The authors suggested that the talons would have been more effective in climbing than in dealing killing blows.

Ostrom compared Deinonychus to the ostrich and cassowary. He noted that the bird species can inflict serious injury with the large claw on the second toe. The cassowary has claws up to 125 millimetres (4.9 in) long. Ostrom cited Gilliard (1958) in saying that they can sever an arm or disembowel a man. Kofron (1999 and 2003) studied 241 documented cassowary attacks and found that one human and two dogs had been killed, but no evidence that cassowaries can disembowel or dismember other animals. Cassowaries use their claws to defend themselves, to attack threatening animals, and in agonistic displays such as the Bowed Threat Display. The seriema also has an enlarged second toe claw, and uses it to tear apart small prey items for swallowing. In 2011, a study suggested that the sickle claw would likely had been used to pin down prey while biting it, rather than as a slashing weapon.

Biomechanical studies by Ken Carpenter in 2002 confirmed that the most likely function of the forelimbs in predation was grasping, as their great lengths would have permitted longer reach than for most other theropods. The rather large and elongated coracoid, indicating powerful muscles in the forelimbs, further strengthened this interpretation. Carpenter's biomechanical studies using bone casts also showed that Deinonychus could not fold its arms against its body like a bird ("avian folding"), contrary to what was inferred from the earlier 1985 descriptions by Jacques Gauthier and Gregory S. Paul in 1988.

Studies by Phil Senter in 2006 indicated that Deinonychus forelimbs could be used not only for grasping, but also for clutching objects towards the chest. If Deinonychus had feathered fingers and wings, the feathers would have limited the range of motion of the forelimbs to some degree. For example, when Deinonychus extended its arm forward, the 'palm' of the hand automatically rotated to an upward-facing position. This would have caused one wing to block the other if both forelimbs were extended at the same time, leading Senter to conclude that clutching objects to the chest would have only been accomplished with one arm at a time. The function of the fingers would also have been limited by feathers; for example, only the third digit of the hand could have been employed in activities such as probing crevices for small prey items, and only in a position perpendicular to the main wing. Alan Gishlick, in a 2001 study of Deinonychus forelimb mechanics, found that even if large wing feathers were present, the grasping ability of the hand would not have been significantly hindered; rather, grasping would have been accomplished perpendicular to the wing, and objects likely would have been held by both hands simultaneously in a "bear hug" fashion, findings which have been supported by the later forelimb studies by Carpenter and Senter.

Parsons and Parsons have shown that juvenile and sub-adult specimens of Deinonychus display some morphological differences with the adults. For instance, the arms of the younger specimens were proportionally longer than those of the adults, a possible indication of difference in behavior between young and adults. Another example of this could be the function of the pedal claws. Parsons and Parsons have suggested that the claw curvature (which Ostrom had already shown was different between specimens) maybe was greater for juvenile Deinonychus, as this could help it climb in trees, and that the claws became straighter as the animal became older and started to live solely on the ground. This was based on the hypothesis that some small dromaeosaurids used their pedal claws for climbing.