Primates and proavians (was Re: [RE: A glider?])

["Tim Williams" <twilliams_alpha@hotmail.com>]

Ronald Orenstein wrote:

> I am not one for jumping to conclusions, but it would certainly be
> interesting o compare the structures on the forearms of this animal with
> the hair on the arms of South American sakis, which (if they do not
> actually glide) can make very extensive leaps apparently using the forearm
> hair for a bit of extra lift.  mind you, I wonder how this can work if the
> hairs are not stiffened?


This has also been described in another group of primates, the sifakas 
(_Propithecus_ spp.) from Madagascar.  Sifakas have a "brachial mat" of 
hair, which is useful in maneuvering the animal through the air during leaps 
(not true glides).  The person who described this behavior (and actually 
suggested it as an analog for a hypothetical tree-jumping proavian) was none 
other than Al Feduccia!

It was my impression that the hair along the arm did not so much provide 
lift, but drag.  As a drag-inducing device that improve maneuverability the 
"brachial mats" are more similar to the webbed hands and feet of flying 
frogs rather than the patagia of gliders, in which the flight surface 
generates lift.  The hair on the forearms of sifakas (and I guess sakis too, 
although I wasn't aware of this feature in sakis) improve the animal's 
control and accuracy during leaps essentially by using the brachial mats as 
a combined parachuting/steering device.

The "shaggy brush" on the arms of the new theropod (?_Sinornithosaurus_) is 
consistent with the "Pouncing Proavis" theory of Garner, Taylor and Thomas 
(1999).  The more elaborate feathers we see on the manus and forearms of 
_Caudipteryx_ represent the next stage of the development of drag-inducing 
devices.  At all stages, these brachial feathers were used by 
opportunistically arboreal maniraptorans for leaping onto prey.


Tim



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

Timothy J. Williams

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

Phone: 515 294 9233
Fax:   515 294 3163

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