Re: Origin of feathers

["John V Jackson" <jjackson@interalpha.co.uk>]

-----Original Message--- From: Dinogeorge Date: 16 April 1998 08:21


>In a message dated 98-04-15 06:34:25 EDT, jwoolf@erinet.com writes:
>
><< Or decreased mass will work too.  While gravitational acceleration
doesn't
>vary
> with mass, energy of impact does.  All other factors being equal, a 1kg
>animal
> takes less damage from a fall than a 10kg animal will. >>


It's not the weight, it's the size.  Load to be resisted proportional to
cube of length, strength of bone proportional to square of length - so
falling more dangerous for bigger animals.

Dinogeorge replied:

>Exactly so. Mass decrease in dino-birds was accomplished by (1) small size
of
>individuals, and (2) lightening of the skeleton through hollowing of the
>bones.


Well all right, aerial animals are better off smaller and that usually means
lighter so it looks like a bit of a minor quibble.  One thing that used to
puzzle me is why lightness is of no advantage to other animals?  Why don't
mammals have hollow bones?  (Well they do of course, but not to the extent
of birds.)  I believe it's to do with the trade off between strength under
normal loading, and resistance to breakage in unusual impacts.  Fliers are
able to exploit more efficient use of bone for normal purposes, because they
don't experience the need to resist impacts to the side of the limb bones
for example, very often - because a fall usually means nothing to them - and
the further the fall, the less of a problem.

(And of course air sacs in other parts of the body have nothing at all to do
with making birds lighter.  But then, no-one has yet said that they did!)

JJ



-----Original Message--- From: Curtis B. Olson Date: 16 April 1998 08:27

>Do adaptations evolve to deal with
>regular behaviour, or random events?

What's wrong with random events?  Surely we have untold numbers of
adaptations resulting from the accidents that have befallen our ancestors.