Re: ALIMENTARY MY DEAR HOATZIN

["Ralph W. Miller III" <gbabcock@best.com>]

Betty Cunningham wrote:

> The hoatzin is not unique with the juvenile having a finger on it's
> wing.   ---SNIP---
>
> In all cases of this finger that I have heard of it is not retained as
> an adult. I would be interested to know if the finger falls off or is
> re-absorbed, however.  Does it have the characteristics of a skin thing
> or the characteristics of a gross anatomy thing.

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.

Page 313 of Bakker's _The Dinosaur Heresies_ compares the manus of a hoatzin
adult, a hoatzin hatchling, _Archaeopteryx_, _Deinonychus_, and a primitive
reptile.  You can see similar illustrations on-line at
<www.dinosauria.com/jdp/buckna/three.htm> (on page 10 of 14 pages).
Bakker's take is that the hoatzin chick's hand represents a reexpression of
suppressed genes which hearken back to its maniraptoran ancestry, i.e. an
evolutionary reversal.  Given that extant bird embryos exhibit unfused
phalanges, it may also be seen as a case of retarded development of the
avian wing (as Matthew Troutman has suggested).  Too bad no modern birds
exhibit the reptilian tails or teeth of their forebears (although the
merganser's notched beak constitutes a reasonable imitation of teeth).

On his post dated March 14, 1998, Matthew Troutman notes that small wing
claws are present on hawks, ducks, ostriches, rheas, rails, and pelicans,
and may serve a display function, among other things.  However, this is not
to say that these birds have "fingers."

-- Ralph W. Miller III       gbabcock@best.com