Re: ALIMENTARY MY DEAR HOATZIN

[dbensen <dbensen@gotnet.net>]

>>The hoatzin's beauty is more than skin deep.  It appears to me that the
reduced phalanges of digit #3 are held alongside the longer digit #2
phalanges, buried underneath the skin, perhaps bound together by soft (i.e.
non-bony) tissue. The two functional manual digits remaining, #2 and #1, are
fully mobile grasping fingers capped with sharp unguals sheathed in
keratin.  These features persist for approximately 10 weeks in the
developing chick.  The phalangeal bones then fuse together into the
carpometacarpus to produce a conventional bird wing.  During the earlier
"climbing phase," the development of the chick's remiges (wing feathers) is
retarded in order that the fingers will be free from interference while
climbing.  In contrast, _Archaeopteryx_ and other claw-winged early birds
(for which feathers are known) exhibit a full set of remiges, leading to
questions concerning their climbing capability.  Interestingly, mature
hoatzins are said to use their wings in climbing, in spite of the loss of
their fingers!  I imagine that the keratinous claw sheaths drop off as the
wings develop.<<

Perhaps I can devise a scenario for the evolution of such a behavior.  Hoatzins
started out as  typical, finger-less birds that lived in the Amazon jungle.
Some of their chicks mutated and bore fingers on their wings just like baby
ducks and chickens.  These wings were beneficial to the chicks because they
allowed the chicks to climb with greater skill than they could otherwise
manage.  These chicks had a selective advantage and their greater skill at
climbing allowed the hoatzins to take advantage of leaves as a diet (Few birds
eat leaves because vegetable matter is difficult to digest and the animal that
eats them must have a large gut, which impedes flight).  Hoatzins are
re-evolving fingers.  Perhaps in a few million years we will have flightless
hoatizins that might descend back onto the ground . . .

Dan