[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

AT LAST!! Wing Assisted Incline Running (long)



Greetings,

In the latest Science, the following:
Dial, K.D. 2003.  Wing-Assisted Incline Running and the evolution of flight.
Science 299: 402-404.

As I have hinted since SVP 2001, this is a MAJOR work on the origins of bird
flight, the paleobiology of eumaniraptorans, and in modern organismal
biology in general.

The primary study: the discovery that at least some galliform birds (chukars
(_Alectoris chukar_) in the main study; Japanese and bobwhite quails and
ringnecked pheasants in the supporting material) engage in a behavior
hitherto ignored by humans.  From a young age, and on through adulthood,
these ground birds run up the sides of trees and other objects (sometimes at
greater than 90 degrees!) while using wingbeats to generate forces toward
the substrate, and thus maintain traction even when running at inclines
steeper than vertical.

One one level this is very cool in that it demonstrates there is still much
about living animals (even in common species) that we don't know about.  I
will again add one of my refrains here: for people still in college (or
before), we desparately need more zoologists out there studying living
animals!!  You can still work on paleontological issues (especially  big
ones, like locomotion, respiration, senses, etc.) while working on critters
that you can poke and prod and get to do things...

What is probably of greater interest to folks on the list is that Dial has
gone further than simply document this behavior (and detailing the forces
involved).  Instead, his experimental design involved running birds
throughout their early ontogeny on both rough and smooth surfaces under
several different wing conditions.  The first condition is with the wings
unaltered; the second was with the feathers present but trimmed; the third
was with the wing feathers plucked.

Those with plucked feathers had limited incline running ability, with little
ontogenetic change in the maximum angle they could "climb".  Those with
trimmed feathers showed a considerable increase in the angle of climb, which
increased additionally as the birds got older.  Finally, the fully feathered
birds could reach greater angles still.

This demonstrates that there is a real benefit to a small but present
enlargement of the wing feathers.  This suggests, further, that in an animal
engaged in running locomotion and with some feather-like integument (but no
true wing feathers) that there would be an advantage to those variants in
the population which had slightly longer arm feathers than the rest of the
population.  If arm feather length was a genetically-controlled trait, then
one has the conditions under which good old fashioned Darwinian selection
could favor the increase in arm feather length in successive populations.

(Incidentally, WAIR behavior is associated with predator escape in living
birds.  Thus the selective forces invovled are extremely prevalent in
nature, and a scenario where WAIR represents the the adaptive scenario under
which wing feathers were elongated in the early stage of avian flight does
not have to invoke otherwise unknown or rare behaviors (insect net
trappings; acceleration of ground running by flapping; shading to catch
fish; etc.)).

Some of my own thoughts follow:
The initial form of the Ostrom version of the cursorial scenario of bird
flight was this: the hindlimbs of birds remains a perfectly good running
organ, rather than being incorporated into the flight mechanism (as in bats
and pterosaurs).  In that sense, WAIR represents a "transitional behavior"
of sorts between a strictly cursorial model of bird flight origins and a
strictly arboreal model of bird flight origins.  Thus WAIR is a cursorial
behavior to allow access to arboreal conditions; or, as I said to a few
folks at SVP 2001 after seeing the presentation, its "from the ground up the
trees".

To make the arboreal crowd happy, the origins of true avian aerial flight
(i.e., flapping without the feet contacting any hard surface... :-) would be
from creatures able to access the trees.

Given that the primary predators of theropods would be OTHER theropods, it
is truly a Theropod Origin of Flight... :-)  (Okay, okay, lizards and
terrestrial crocs and the like would have been dangerous to small-bodied
theropods, too).

Furthermore, WAIR behavior by itself (that is, Winged Assisted Incline
Running without the ability to fly) would have a selective benefit to
theropods: that is, predator avoidance is useful for most critters,
regardless of whether they could fly later.  Once you have a taxon capable
of this behavior, you could then have one (or more) branch elaborating it to
become true fliers.  Additionally, this behavior could be present throughout
ontogeny for small forms, but would be additionally be reasonable for the
juveniles of larger taxa as well.  (Analogy: _Varanus komodonesis_, which is
arboreal as juveniles but terrestrial as adults).

So when did WAIR begin? If we take a phylogenetic scenario with
compsognathids, tyrannosauroids, and ornithomimosaurs as basal coelurosaurs;
oviraptorosaurs, therizinosauroids, and alvarezsaurids as basal
maniraptorans; and dromaeosaurids, troodontids, and avialians as
eumaniraptorans; my initial prediction would be that WAIR was present
basally in Eumanirapora, and arguably at the base of Maniraptora, but was
not present in the ancestors of compys, tyrants, and ostric dinos.  The
reasoning:
* Basal dromaeosaurids (e.g., _Microraptor_), basal troodontids (e.g.,
_Sinovenator_), and basal avialians (e.g., _Archaeopteryx_, _Rahonavis_) are
all small-bodied animals with more laterally-oriented glenoids than
primitive coelurosaurs.  Where known, they have elongated arm feathers.
Where found, they have broad sterna (although in some they do not appear to
be well-developed).  This suite of features would be consistent with some
form of WAIR behavior.  Whether baby _Deinonychus_ and _Troodon_ could do
this would depend on the degree of lateralization of their glenoids, among
others: we do not yet have specimens to resolve this.

* As for more basal maniraptorans: the broad arm feathers are present in at
least some (e.g., _Caudipteryx_), and at least some have more
laterally-oriented glenoids (_Heyuannia_), and most seem to have
well-developed sterna.  However, polarization of the the orientation of the
shoulder girdle in the basal forms for the oviraptorosaur-therizinosaur
clade or for alvarezsaurids does not currently support a lateralized form;
however, it will be worth it to go back and reexamine some of these taxa.

* The orientation of the shoulder girdles, the small size or lack of
ossification of the sterna, and the small size of the arm feathers (where
known) for basal coelurosaurs suggest that these would not have been able to
perform WAIR.

Finally, it may be that some of the traits that have been used to suggest
that oviraptorosaurs and/or deinonychosaurs were secondarily flightless are
instead related to a primary WAIR ability.  To be fair, thought, it would
not be unexpected that if one were to get some maniraptorans up into the
trees with this flapping-assisted running locomotion that you might get
multiple independant origins of aerial flight.

Must head home now.

Enjoy,

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796