RE: Naked heads (was Velociraptor Animation)

["Williams, Tim" <TiJaWi@agron.iastate.edu>]

David Marjanovic wrote:

> While many vultures (not all, *Gypaetus barbatus* is a notable
> exception) do have naked heads and necks like this, hyenas don't. So 
> this is equivocal.

_Gypaetus barbatus_ (the lammergeier) specializes in eating the marrow of
large bones - which they smash into manageable pieces by dropping onto rocks
from a great height.  They are also reported to do this for tortoises, to
get to the good stuff inside.  Yum yum.  In any case, lammergeiers don't
typically go diving headfirst into the torsoes of carcasses.

The correlation for scavenging birds having naked heads and necks is pretty
good: vultures from the Old World (Accipitridae) and New World (Vulturidae /
Cathartidae) show this feature, as does the carrion-eating marabou stork
(Ciconiidae).  Thus, bare heads evolved at least twice in the Aves (three
times if it evolved independently in the marabou and its kissing cousins,
the vulturids.)

As for causation...  The naked head and neck of scavenging birds has been
accorded a role in the bird's hygiene.  However, there may be an aerodynamic
reason as well; flight capabilities would not be helped by having gore,
blood and putrefying viscera clinging to the feathers of the face and neck.
Sticking a feathery head and neck deep into a corpse is like plunging a wire
brush down a filthy drainpipe.



Tim



------------------------------------------------------------ 

Timothy J. Williams 

USDA-ARS Researcher 
Agronomy Hall 
Iowa State University 
Ames IA 50014 

Phone: 515 294 9233 
Fax:   515 294 3163