Re: Kickboxing Cassowary

[Michael Habib <mhabib5@jhmi.edu>]

Augusto Haro wrote:

> Eagles do not wrestle with prey larger than themselves, except for
> trained eagles that can grasp wolves. However, I doubt this being
> natural behavior. And theropod limb bones, while not pneumatic
> according to Britt (1991), have thin walls according to Sereno (1999).

Actually, wild eagles do kill prey much larger than themselves.  I have 
several references regarding the capture of pronghorns by eagles.  One 
study actually weighed the juvie pronghorns, and the average mass was 
15 kg.  An average golden eagle is about 4.5 to 5 kg.  Additionally, we 
can look at the loads sustained by other pneumatic taxa, such as 
peregrine falcons, which sustain massive decelerations pulling out of 
hunting stoops.

In any case, the thin-walled bones of eagles do not make them weak.  
Pneumatic bones are only weaker than thick-walled bones if the diameter 
is equal - but bird bones have expanded external dimensions.  Avian 
long bones are actually *stronger* in bending and torsion than their 
mammalian counterparts.  The internal bracing reduces the risk of 
failure by local buckling, which becomes more probable in thin-walled 
bones.  Failure by buckling is an important factor for a predator 
struggling with prey, and this appears to be why birds of prey have 
slightly thickened cortical bone relative to other terrestrial birds (I 
have a paper in the pipeline on this, actually - here's hoping the 
reviewers like it).  However, it only takes a rather small increase in 
cortical breadth to vastly decrease the local buckling (again, as in 
modern raptors and owls).  The walls of dromeosaurid long bones were 
not especially thin compared to many terrestrial birds, even though 
they might have been thin compared to those of mammals.  There isn't 
any evidence to suggest that they were especially fragile, in any case.

Cheers,

--Mike


Michael Habib, M.S.
PhD. Candidate
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280 0181
habib@jhmi.edu