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Re: Birds' Warm-Bloodedness Probed

Ron Baalke's recent submission of an AP article about Anusuya
Chinsamy's work triggered a serendipitous chain of events which lead
me to the fifth annual paleobiology symposium here at Penn's geology
department on Friday, March 18th.  At that symposium I picked up quite
a bit of information which will likely be of interest to members of
this list.  Since we've recently had discussions about the origins of
birds here, I'll begin with a discussion of Anusuya's presentation.
Since Anusuya is here at Penn, I have the good fortune of being able
to ask her to comment on my comments.  Hopefully that means you can
trust anything I say about her work, but any errors below are mine all
mine!!!  :-)

Anusuya is a post doc in Peter Dodson's lab, where she has been
sectioning the bones of ancient birds and dinosaurs in order to infer
how the animals grew and therefore what sort of physiology they had.
I have not yet seen Anusuya's work in print, so all of this is coming
straight from the bird's beak, as it were.  Apparently a recent NY
Times article claimed (or implied) that Anusuya is casting doubt on
the theropodian origin of birds.  That is most assuredly not the case.
In fact, I think that she has made the connection more firm (as you'll
see below), and she most definitely feels that cladistic
analyses--like that performed by her co-author on a recent _Nature_
paper, Louis Chiappe--firmly link birds to theropods.

The phylogeny that you should refer to for the rest of my comments
is (if you'll pardon the crudity and attempt to take no information
aside from geneology from this partial diagram):

                                           (modern birds)
                                 ichthyornis  /
                        hesperornis \        /
                                 \   \      /
                         patagopteryx /
                                  \  /
                  enantiornithines \/
                                \  /
                        sinornis \/
                              \  /
                 archaeopteryx \/
                            \  /
         dromaeosauridae     /
        troodontidae \      /
                   \  \    /
                    \  \  /

Now a little bit about bones; it's not critical that you understand
this paragraph, but it will help you to appreciate the context of the
work if you put in the effort.  Vertebrate bones are a composite
material whose structural integrity derives mainly from a calcium
hydroxyapatite mineral matrix embedded in a proteinaceous (mainly
collagen) resin.  Interspersed through the bone are living cells
called osteocytes, which maintain the bone while it's alive (e.g.  by
moving calcium around).  When the bone is growing or remodelling, it
is laid down by cells called osteoblasts, which direct the formation
and deposition of the proteins and hydroxyapatite crystals.  When bone
grows slowly, the osteoblasts lay down the proteins in orderly
rows--the protein molecules are linear chain polymers which, for the
purposes of this discussion, you can imagine as threads.  When bone
grows quickly, however, the osteoblasts churn out the proteins and
then have to move on quickly; the threads are thus put down at all
orientations.  In the slow growing case, the parallel fibers are
visible in thin sections of the bone.  In the latter case, the fibers
are jumbled like the knit of a fabric, and the bone is referred to as
"woven".  In ectothermic vertebrates, bone growth is episodic; periods
of rapid bone growth are interspersed with periods of slow deposition.
This feature is observable in thin sections of adult bones which
appear to have rings much like the rings of trees.  Mammal and bird
bones do not show this pattern, however.  Since these animals grow
quickly, their limb bones look woven as described above.

Anusuya has sectioned the bones of modern birds such as secretary
birds, ostriches and cassowaries.  In no case were any growth rings
apparent in the bones of these animals.  This is in contradistinction
to the bones of theropod dinosaurs such as syntarsus, Coelophysis and
Troodon, which have prominent growth rings (last thursday, Anusuya
received a Tyrannosaur's (!) fibula which she will begin sectioning
shortly).  Anusuya has also sectioned the bones of hesperornis, which
like modern birds shows no sign of growth rings.  Hesperornis was a
late Cretaceous animal with a world wide distribution.  In some
respects it may have been similar to modern penguins and loons.  It
was a flightless animal that used its feet to propel itself through

Contrariwise, the bones of Patagopteryx and Enantiornithes, animals
that lived 80 and 70 million years ago respectively each had easily
visible growth rings in their bones.  These growth rings provide
fairly direct evidence that the physiology of these animals was
different from the physiology of modern birds and more similar to that
of their inferred theropod ancestors.  As I said in the beginning of
this message, I think that this evidence from bone histology
strengthens the case for the dinosaurian origin of birds.  I say that
because it places intermediate forms in the proper biostratigraphical
context.  Since theropods don't show the same sort of bone histology
as modern birds, it's nice to have evidence that animals that are
clearly ancestral or closely related to the ancestral stock of modern
birds have bone histology similar to the modern birds' putative
theropodian forebears.

Anusuya's interpretation that her bone slices indicate heterothermy
(variable body temperature) in an animal--specifically
Enantiornithes--that most consider to have been an active flapper
leads to a fairly obvious problem; how could a cold-blooded animal
fly?  Anusuya compares the proto-bird's physiology to that of modern
crocodilians and lizards, which are clearly capable of short bursts of
high metabolic activity.  Although I admit of some skepticism for this
view, I would go one step further and suggest that modern hymenopteran
and lepidopteran insects provide possible living analogues to the
proposed semi-warm-blooded birds.  There are moths and bees that live
in fairly high northern latitudes.  These animals often cannot fly
when their thoracic muscles are at ambient temperatures.  To
compensate, they use their muscles to warm up their bodies prior to
takeoff.  Once they've initiated flight, their continued muscular
exertion keeps their bodies warm enough to sustain it.

It also may help to keep in mind that our typical view of a
warm-blooded/cold-blooded dichotomy is not supported even by studies
of contemporary vertebrates.  Some species of tuna and marlin maintain
their eyes and/or brains at temperatures above that of the surrounding
water.  They do this with specially developed muscles and vascular
systems that pre-warm blood going to these areas of their bodies.  In
this sense they are partially homeothermic.  Other animals (e.g. great
white sharks) have similarly intermediate physiologies.  It shouldn't
be too surprising to find that ancient animals had as much
physiological diversity as do modern animals, and the position (a la
Bakker) that all dinosaurs were the same in this respect not only to
each other but also to us is likely to be a gross oversimplification.

Skipping back to birds, it also shouldn't be too surprising that the
evolution of flight would bring about major changes in physiology.
Indeed, the bones of Enantiornithes has osteocytes unlike those known
from any modern animal.  In Anusuya's slides these osteocytes looked
fuzzy; they had tiny processes radiating out in all directions.  Since
these bones were not terribly vascularized (another indicator of a
slow metabolism), it could be that these osteocytes represent
adaptations for the metabolic demands of flight--that they performed
some sort of transport function analogous to that of the blood vessels
in the bones of modern birds.  This lead me to wonder about
specializations in the bones of pterosaurs, so I asked Anusuya if she
knew anything about them.  Unfortunately it seems that not much is
known about the histology of pterosaur bones.  They appear to be thin
walled and highly vascularized like those of modern birds, however.

Since this message is already quite long, I'm going to stop here.  I
intend to post some discussion of at least one more talk, though.
Harrass me about oxygen isotope indicators of homeothermy if I don't
get to it by the end of the week.

Mickey Rowe     (rowe@lepomis.psych.upenn.edu)