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Mayr: On the origin of feathers



This Letter to Science from the end of last year might have eluded Ben
Creisler's dragnet.  The Letter is a rebuttal of the hypothesis of
Koschowitz et al. (2014) that pennaceous/planar feathers in theropods
originally evolved for display.

Mayr G. (2014) On the origin of feathers.  Science Dec
19;346(6216):1466. doi: 10.1126/science.346.6216.1466-b. No abstract
available.

First paragraph: "Feathers are one of the most characteristic
evolutionary novelties of birds, and until recently a broad consensus
existed that typical vaned feathers (pennaceous feathers) evolved in
an aerodynamic context to streamline the body and form lift-generating
surfaces. Based on the occurrence of essentially modern-type
pennaceous feathers in a number of flightless non-avian theropod
(“beast-footed”) dinosaurs, Foth et al. recently proposed that these
feathers originated in a context other than flight (1). In their
Perspective “Beyond the rainbow” (24 October, p. 416), M.-C.
Koschowitz et al. argue that vaned feathers evolved for signaling with
structural coloration. This hypothesis is, however, highly implausible
from an evolutionary point of view."

Overall, Mayr's logic is sound.  He consequently invokes WAIR
(“wing-assisted incline running”) as the alternative hypothesis for
why theropods initially evolved pennaceous feathers.  WAIR at least
provides an aerodynamic context for the evolution of pennaceous
feathers.  However, WAIR doesn't account for the presence of
pennaceous feathers along the hindlimb and tail in pennaceous
theropods.  AFAIK, neither hindwings or a pennaceous tail have any
roles in WAIR.  Also, in many pennaceous theropods, including basal
avialans (= 'proto-birds') it's not clear if the pectoral muscles and
skeleton were capable of executing the flight stroke required for WAIR
in these taxa. Some studies have indicated that the orientation of the
shoulder joint of proto-birds prevented the humerus from being raised
above the back, meaning a complete flight stroke was not possible (but
this is disputed by others).  (Juvenile birds that use WAIR have
incipient wings, but a fully evolved flight stroke like their
neornithean parents.  After all, in modern birds WAIR has over a 100
million years of aerodynamic refinement behind it.)

Mayr recognizes that proto-birds and related theropods had hindlimbs
adapted for running.  Neither the fore- or hindlimbs were adapted for
climbing or living in trees.  (I only mention this because an
aerodynamic function for the first pennaceous feathers is often used
to support an arboreal origin of avian flight... thus it goes that the
ancestors of birds must have been arboreal.  I don't subscribe to this
view.  Interestingly, wind tunnel tests on _Microraptor_ by Palmer
(2014) indicated that the aerodynamic surfaces offered no clear
advantages for gliding in an arboreal environment.)

In their original paper, Koschowitz et al. (2014) use the term
'pennaceous' to mean 'planar' - not necessarily bipinnate (i.e.,
having the barbs interlocked by barbules within the feather vane, to
maintain integrity).  (Note that barbules are typically not preserved
in fossils, so their presence can only be inferred indirectly in these
specimens.)  It is possible that planar feathers initially evolved for
display, and were subsequently co-opted (exapted) for an aerodynamic
function via barbules to produce a bipinnate vane.

The aerodynamic function of pennaceous feathers might not have been
WAIR or even arboreal gliding.  As mentioned on this list, pennaceous
feathers could have been used in elaborate display rituals that
involved aerodynamic behavior.  Or pennaceous feathers could have
improved maneuverability, prior to the development of a power stroke -
at least in the line leading to birds (Evangelista et al., 2014).
Improved maneuverability could be useful for weaving rapidly over
uneven terrain, or for during attacks on prey (launched from above or
level with the prey), or balancing on or alighting from larger prey.

The 'aerodynamic' functions of pennaceous feathers may have differed
among different proto-bird taxa, with independent experiments in
pre-flight behavior.  _Archaeopteryx_ might have used its 'wings' for
one behavior, _Sapeornis_ for another, and _Jeholornis_ for yet
another behavior.  For example, _Sapeornis_ appears to have excellent
perching adaptations, but other proto-birds tend not to (if any at
all).  So it might not be easy to delineate which particular behavior
led directly to flight in birds.  As an analogy, in today's world,
different flightless and near-flightless birds may use their wings for
various purposes (e.g., kagu, lyrebirds, scrubbirds, tapaculos,
kakapo, etc).



References

Koschowitz et al. (2014) Beyond the rainbow.  Science 346: 416-418.

Evangelista et al (2014) Shifts in stability and control effectiveness
during evolution of

Paraves support aerial maneuvering hypotheses for flight origins.
PeerJ 2:e632 arXiv:1401.3209 [q-bio.PE]

Mayr, G. (2014) On the origin of feathers. Science 346: 1466.

Palmer, C. (2014) The aerodynamics of gliding flight and its
application to the arboreal flight of the Chinese feathered dinosaur
_Microraptor_.  Biol. J. Linn. Soc. 113: 828–835.