Claws on deinonychosaurs

[Richard and Jo Cowen <cowen@blueoakfarm.com>]

This is a comment on an online paper published recently in Biology  
Letters of the Royal Society of London. I think the abstract is  
freely available, but the main paper requires a subscription.

Manning, P. L., et al. 2005. Dinosaur killer claws or climbing  
crampons? Biology Letters Online Publishing, October 11, 2005

Abstract (freely available online, I believe): Dromaeosaurid theropod  
dinosaurs have a strongly recurved, hypertrophied and hyperextensible  
ungual claw on pedal digit II. This feature is usually suggested to  
have functioned as a device for disembowelling herbivorous dinosaurs  
during predation. However, modelling of dromaeosaurid hindlimb  
function using a robotic model and comparison of pedal ungual  
morphology with extant analogue taxa both indicate that this  
distinctive claw did not function as a slashing weapon, but may have  
acted as an aid to prey capture.

News story: http://news.bbc.co.uk/1/hi/sci/tech/4332272.stm
BBC News OnLine

Here are some points from the full paper:

First of all, you have to know that this project was done in  
connection with a TV show for the BBC (The Truth About Killer  
Dinosaurs). Second, a 5-author paper that had multiple reviewers and  
editors and proof readers shouldn't be quoting "speeds" in meters per  
second squared: those units denote acceleration. It makes you wonder.

They designed a hydraulic limb based on the structure of Velociraptor  
and Deinonychus. The leg bones were mimicked by steel rods,  
controlled by hydraulics; and the hyperextension of the big claw was  
effected by a control cable. The claw movements were reconstructed  
mostly from the beautifully preserved foot of Deinonychus. The model  
was based on a body mass of 40 kg for the animal: roughly that of a  
Dobermann.

The big claw, the only reason for building the apparatus, was made  
from an aluminum core with a sheath of Kevlar and carbon fiber set in  
epoxy resin, which was sharpened appropriately. Then they banged the  
claw into a pig carcass mounted on a frame at speeds of 2 m "per  
sec2" and 11 m/sec2. Assuming these are real speeds misquoted, they  
represent roughly 4 mph and 20 mph.

The claw did not penetrate far into the pig, so the authors conclude  
it didn't cause slashing wounds. Therefore, they suggest, the claws  
were used like crampons to climb up the flanks of their prey until  
they could bite them!!!

I really have problems with this, though I'm not in a position to  
solve those problems.

We don't know what the "pig carcass" really was. In the photos in the  
paper it looks like a clean, cold, piece of pork from the local  
village butcher (it has one of those violet stamps on it). It's  
mounted as a solid lump on a metal frame: no "give" at all as there  
would be in a living animal. No wonder the claw didn't penetrate  
much. Let's go back to the abstract: if the claw is "usually  
suggested to have functioned as a device for disembowelling", and the  
device just banged the claw into a cold cleaned rigidly-mounted slab  
of meat, then they didn't test the disembowelling hypothesis at all.  
What you need is a claw applied appropriately to the guts of a warm  
animal. More on this below.

We don't know just how the claw impacted the meat. If it hit the meat  
more or less directly, of course it would try to penetrate, even if  
it didn't get very far. If it hit the meat in a mode in which the  
claw was moving to close just as it hit the meat, then the claw  
motion would have been dominantly slashing on impact, and would have  
been much more likely to tear the flesh in a cutting action. The  
authors don't say, but since they are so keen to tell us the "speed"  
of the claw, that means to me that the claw's motion wasn't  
dominantly slashing. The picture gives the same impression, though  
it's difficult to tell just how the claw was moving on impact. So  
again, the hypothesis was not tested.

Apart from anything else, I don't like the crampon hypothesis.  
There's nothing potentially lethal to bite even if you succeed in  
getting up on to the flanks of a big animal.

The other thing is that this whole "test" and then the subsequent  
inferences are predicated on the supposition that the raptors were  
attacking prey much larger than they were: a roughly 90 pound  
predator (a largish dog) attacking 500 to 1000 pounds of pig. This  
vision, I suppose, derives from brainwashing by the Bakker images.

My image is that it's much more likely that these raptors were taking  
on prey roughly their own size, or perhaps smaller. They'd knock them  
down then disembowel them with the foot claw, before or after tearing  
out their throats with their teeth. So any experiment would be better  
performed on a complete, warm, wriggling Bambi, with its belly skin  
stretched by muscular exertion rather than being relaxed in death.  
I'm not advocating this, of course, just making a logical point.

But wait: there are two sets of people who get close to that  
activity. One consists of surgeons, whether they work on humans or  
animals; and the other consists of hunters as they gralloch their  
prey. I'm none of the above, so I'd welcome feedback.

There are differences in technique, however, that have to be part of  
the assessment. All the above are concerned to cause minimal damage  
to the creature, the first because they're trying to save its life,  
and the second because they want a clean gralloch, without puncturing  
the rumen, for example:
http://wireviz.tripod.com/id3.html
with pictures and instructions!

But if you're a predator, the more damage you cause, the better, so  
perhaps a raptor-type claw is a good thing. At least one commercial  
deer-cleaning knife has a "gut-hook" on it:
"http://www.deer-uk.com/Skinner.gif

Whatever deinonychosaurs were doing, it was unique. But some  
interpretations seem to me to be better than others.