Evolutionary Origin of Feathers
WELCOME AND INTRODUCTION TO THE
SYMPOSIUM. Paul F.A.Maderson* and
Dominique G.Homberger. Brooklyn College,
NY, and L.S.U., Baton Rouge.
[email protected].
The problem of the evolutionary origin of
feathers has attracted attention for nearly 140
years. As with many other neomorphic features,
the HOW (what was the form of the antecedent
structures, and how was it changed?) and the
WHY(y/hzt were the selective factors involved?)
of feather evolution are inherently insoluble. The
practice of scenario construction in evolutionary
biological discourse has its critics. However, we
believe that debate concerning neomorphs, with
judicious consideration of their possible roles in
ancestral clades, has value in that it provides a
focus for careful examination of relevant data and
thus spot-lights critical areas for future research.
We welcome workers with disparate technical
expertise and differing theoretical backgrounds. In
a format new to SICB symposia, we have elected
to have many speakers and we ask that questions
be submitted on the available printed forms.
These questions will make up the agenda for a
formal discussion period at the end of the third
session. This will be recorded, edited and
published as a symposium report in conjunction
with the expanded platform talks.
EVOLUTION OF AVIAN FEATHERS Walter J.
Bock, Columbia Univ, New York, NY
[email protected].
Feathers are distinctive features of birds and
have clearly evolved from reptilian scales.
Contrary to recent claims, there is no evidence
that feathers existed in reptilian dinosaurs.
Analysis of their origin is a historical-narrative
explanation based strictly on comparisons of the
morphology, functions and biological roles of
scales and feathers as there is no fossil record of
intermediate structures. Feathers possess a
number of functions, including water-proofing,
concealing coloration, species specific and
courtship markings, thermal insulation and flight,
of which the last two are the most important for
the analysis of their evolutionary origin. Currently
two major opposing H-N E exist for the origin of
feathers based on these major functions and their
accompanying biological roles. No evidence is
presently available which serves to conclusively
disprove either. The best approach is to consider
each within a broader explanation of the
sequential evolution of a number of other avian
functions and biological roles such as thermal
regulation, nesting, resting, general locomotion
always considering the organisms at all
intermediate stages in this evolution as viable
wholes. Such an analysis provides more support
for the thermal regulation theory for the origin of
feathers with flight secondary, but does not prove
or disprove either theory.
ABSTRACTS
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INTEGUMENTARY MORPHOLOGY OF
MODERN BIRDS - AN OVERVIEW. Peter
Stettenheim. Lebanon, NH.
peter. stettenheim@yalley. net
Avian integument is basically similar to that of
reptiles and mammals but it is elastic, often
transparent, and thinner than in the other classes.
Over most of the body, the epidermis produces
feathers in ordered rows forming tracts, separated
by more or less featherless areas, the apteria. The
integument is specially differentiated into horny
coverings on the beak and claws, and the three
types of scale on the legs and feet. Many birds
have various integumentary outgrowths on the
head and neck, such as combs, caruncles, and
wattles. The entire skin is a sebaceous secretory
organ, and many birds also have secretory glands
at the base of the tail, in the outer ear, and around
the anal openings. Feathers are set in follicles
furnished with blood vessels, smooth muscles,
nerves, and sensory nerve-endings. Arising the ..
-synthesizing general epidermis, a feather grows
basically as an elongating cone from the bottom
of its follicle and its parts differentiate and
undergo B-keratinization as they move upwards.
The basic structure of all feathers consists of a
shaft, primary and secondary branches, and their
projections. This plan, implemented with the
physical properties of feather keratin, has yielded
enormous variety in the morphology and functions
of feathers in modern birds. Down feathers are
not simple precursors of contour feathers but are
specialized for thermal insulation.
A PHYLOGENETIC PERSPECTIVE ON THE
LINEAGES IN WHICH FEATHERS
ORIGINATED. Stuart Sumida, Cal.St.Univ., San
Bernadino. [email protected].
A number of hypotheses have been suggested for
the origin of birds and thus feathers. Although
distributions of functional complexes have been
frequently used to test phylogenetic hypotheses,
analysis of feather origins remains hampered by
the incomplete fossil record of the [relatively] soft
tissues. Functional speculation regarding feather
origins has focussed on four possible alternatives:
(1) flight; (2) insulation; (3) display; (4) some
combination of these. Scaled skin provides tensile
strength in vertebrates that use significant body
wall tension for stability or transmission of
locomotor forces. If feathers originally developed
for flight, associated body stiffening may have
become more important than that of skin. Dinosaur finds in China demonstrate that feathers
originated in taxa mat still retained several
primitive reptilian features. These fossil data
indicate: (1) feathers, and thus birds, originated
among theropod dinosaurs; (2) the earliest
feathers appear not to have been fully developed
flight feathers; so that (3) their initial function
was probably for insulation and/or display. As the
earliest function of feathers may not have been for
locomotion, it could be speculated that the
transitional animals represented by the Chinese
fossils retained skin of the tensile properties of
reptiles and combined it with the novel
characteristics of feathered insulation or display.
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Evolutionary Origin of Feathers
ARCHOSAUR-AVIAN RELATIONSHIPS:
REVIEW AND COMMENT. Peter Dodson.
Univ. of Penn., Philadelphia.
[email protected].
Birds have long been recognized as archosaurs
of Mesozoic origin. Diverse archosaurs populated
Mesozoic communities world-wide. For nearly
140 years Archaeopteryx (U. Jur) has stood almost
unchallenged as the ancestor of modern birds.
Faute de mieux, its study has long been taken as
the study of bird origins, but the issue demands
re-examination in light of new fossils and the
application of new techniques of phylogenetic
analysis. The past decade has witnessed explosive
growth in Cretaceous fossils with new finds from
Argentina, Spain, Madagascar and especially
China, where a seemingly unbroken transition
from small non-volant theropod dinosaurs, to
proto-birds, to primitive birds, to modern birds is
compressed into a brief stratigraphic interval.
Cladistic analyses seemingly confirm bird origins
from theropods and put to rest poorly supported
rival hypotheses involving Triassic dinosaurs,
"thecodonts" and crocodilians. Could currently
accepted orthodoxy be regarded as an artifact of
phylogenetic systematics? Does the method ignore
too much critical dam to be reliable? The wealth
of new material invites a re-examination of the
phylogenetic position of Archaeopteryx, which
may now be superfluous in the narrative. It is now
an opportune time to re-examine both the fossil
evidence for the origin of birds as well as the
methods that lead us to these conclusions.
THE DEVELOPMENT OF THE SAUROPSID
INTEGUMENT : REVIEW AND COMMENT.
Paul F.A.Maderson*, Lorenzo Alibardi, and Sam
Tarsitano. Brooklyn College, NY, Univ. of
Bologna, and S.W.Texas St. Univ.
[email protected]
Given the central role which "embryonic data"
have had in theories of feather evolution, the
pertinent data base has always been sparse.
Indeed, if "developmental data" are defined as
including adult patterns of cell and tissue
differentiation, such were ignored for the first 100
years of such theories. Typological constructs of
"a reptile scale" and "an avian scale" are invalid
and misleading. New data on the diversity of
morphogenetic and cytodifferentiative processes in
developing, renewing and regenerating skin,
within and between extant sauropsid clades, are
informative with respect to adult function.
Surprising gaps remain in our knowledge, even at
the level of descriptive developmental anatomy.
We spotlight issues in urgent need of study.
Considered in empirically approachable contexts
of skin development, form and function in living
amniotes, inherently unresolvable questions
concerning feather origins may at least be
clarified.
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SKIN AND FEATHERS - INTERPRETING
THE PALE0NT0LOGICAL EVIDENCE IN
DINOSAURS AND BIRDS. Derek E.G.Briggs*
and Paul G.Davis. Univ.of Bristol, and Univ.of
Portsmouth, [email protected].
Critical data for the origin of birds, and for the
evolution of flight, revolves around the evidence
for integumentary structures preserved in fossils.
Data on rates of decay of different tissues, and the
processes that lead to their preservation, allow
interpretations of the fossil evidence to be
constrained. This leads, in turn, to more rigorous
analysis of the context, timing and significance of
the origin of feathers.
THE KERATINS OF SCALES AND
FEATHERS Roger H. Sawyer*, Travis Glenn and
Jeffery French. Univ. of S. Carolina, Columbia.
[email protected].
The cornified epidermal appendages of birds
consist primarily of highly insoluble, cysteine
rich, 3nm fibrous proteins of relatively low
molecular weight which give a beta-type X-ray
diffraction pattern. In the chicken these unique
proteins, known as beta-keratins, are the products
of a large multi-gene family including the claw,
scale, feather and feather-like keratin gene
clusters. Although these genes have been
sequenced for the chicken, no information is
available for other taxa. The recent discovery of
feathered dinosaurs suggests an ancient origin for
these genes. Thus, we have undertaken the
characterization of the beta-keratins in extant
archosaurs to elucidate their diversity, age, and
origins.
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SOCIETY FOR INTEGRATIVE AND COMPARATIVE BIOLOGY
Evolutionary Origin of Feathers
AVIAN EPIDERMAL LIPIDS: WITH
COMPARISONS TO THOSE OF REPTILES.
Gopinathan (Gopi) K.Menon* and Jaishri Menon.
Calif. Acad.of Sciences, SF, and William Paterson
Univ., NJ. [email protected]
The multiple roles of avian epidermal lipids
await full understanding and appreciation. Avian
epidermis is a unique sebokeratinocyte so that the
entire epidermis functions as a lipid secreting
gland. The lipogenic potential and ability to
modulate the type of secretory lipids sustain
thermoregulation at high ambient temperature via
evaporative cooling, and to conserve body water
under xeric stress - "facultative water proofing"
of the integument. Studies elucidating the latter
mechanism clearly show that transepidermal water
permeability is a passive process, and mat the
stratum comeum lipids alone provide the
permeability barrier. Some comparisons with the
lesser understood reptilian water barrier will be
made in this context. The highly exaggerated
lipogenesis in the basal epidermal cells that
accompany permanent feather loss and formation
of the neo-apterium in ciconiiform birds, and
replacement of definitive feathers by bristles in
the gruiformes, may have implications in
modulating feather morphogenesis via modified
carotenoid/retinoid gradients in the skin.
Unbridled speculations on these aspects will be
made to provoke another look at feathers as a
model for altered morphogenesis during posthatching development.
THE ROLE OF THE PLUMAGE IN HEAT
TRANSFER PROCESSES IN BIRDS. Blair
O.Wolf* and Glenn E.Walsberg. Univ.of Ariz.,
Tucson, and Ariz.St.Univ., Tempe.
[email protected]
The structure and coloration of avian plumages
have evolved to meet several physiological and
ecological functions. While we focus here on
thermoregulatory function, plumages may also be
importantly modified by selection pressures for
social signalling, crypsis, water repellency, and
aero- or hydro-dynamics. Heat transfer through
avian plumage involves several complex processes
including radiation, convection, conduction, and
also latent heat flux associated with water
evaporation from the skin surface. Radiative heat
gain, from direct solar radiation, is a potentially
overwhelming source of heat (10 to 20 times the
basal metabolism of a small bird) and is a critical
determinant of an animal's heat-balance in nature.
Properties of the animal (color, plumage form)
and the physical environment (changes affecting
convection, e.g., wind speed) can have large
effects on radiative heat gain. Wind speed also
influences both rates of heat loss from the animal
and radiative heat gain. Evaporative water loss
(EWL) is an essential means of heat transfer for
birds when body temperature is exceeded by
ambient. Cutaneous evaporative loss (CWL) in
some species can account for more than 50% of
total EWL. Variation in the thermal environment,
as well as plumage structure, can importantly
modify rates of CWL in heat stressed birds.
ABSTRACTS
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FUNCTIONAL AND EVOLUTIONARY
MICROANATOMY OF AVIAN SKIN.
Dominique G. Homberger. Louisiana St.Univ.,
Baton Rouge, [email protected]
In birds, the skin differs greatly among various
regions of the body, e.g., feather tracts, apteria,
scaly skin of the legs, rhamphotheca, etc. The 3D arrangement of its components (smooth and
striated muscles, elastic and collagenous
membranes and rods, fat tissue, sensory
receptors, etc.) reflects specific local
biomechanical demands. So do the specific
locations where the skin is secured to the body
and where fat bodies and airsacs underlie the
skin. Thus, the avian skin is a highly complex
organ system, whose functional and evolutionary
significance can be understood only by
considering its feather-bearing properties as well
as its relationships to the rest of the avian body. A
complex microanatomy of the skin is likely to
have evolved prior to the evolution of feathers. A
comparison of avian skin with crocodilian skin as
a model for featherless, yet regionally diversified
skin may provide the data needed to reconstruct
the historical sequence of structural changes that
have occurred during the evolution of feathered
skin from a featherless precursor.
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SELECTIVE FACTORS IN THE ORIGIN OF
FEATHERS AND FUR. John Ruben. Oregon
St.Univ., Corvallis. [email protected]
Conventional wisdom has long held that feathers
and fur evolved in response to selection for
enhanced thermoregulatory capacity in the
ancestors of birds and mammals. However,
incipient respiratory turbinates (indicative of
elevated metabolic rates) first occurred in largebody, Permian Era mammalian ancestors. These
data suggest that at least the initial stages of
mammalian metabolic rate evolved in response to
selection for increased capacity for sustained
activity rather than for thermoregulatory
purposes. Similarly, the probable ectothermic
status of the earliest fully-feathered bird,
Archaeopteryx, probably signals that the initial
appearance of feathers had more to do with the
evolution of flight than for the maintenance of
avian endothermy. Consequently, heat-retention
parameters of pelage and plumage (i.e., fine,
dense underfur, downy feathers) probably
appeared long after the intial stages in the
evolution of elevated metabolic rates in the
ancestors of mammals and birds.
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Evolutionary Origin of Feathers
MECHANISTIC MODELS DESCRIBING
PLUMAGE AND PELAGE HEAT AND MASS
TRANSFER. Warren P.Porter*, Srinivas
Budaraju, and Warren E.Stewart. Univ.of
Wisconsin, Madison.
[email protected]
Feather and fur are porous media in an
engineering sense. We will illustrate how our
mammal model can be modified to predict
metabolic rates of birds across their body size
range in different climates. The mammal model
can now predict the classic mouse-to-elephant
curve of Benedict using our porous media model
coupled to body heat and mass transfer. We
illustrate for mammals and birds how to tie the
energetics - heat and mass transfer model to gut
capacity and foraging costs that vary with climate,
body size and insulation to determine maximum
discretionary mass and energy. We can then
calculate optimal body size that maximizes
reproduction using global climate data at half
degree grid resolution for the earth. The results
are consistent with Bergmann's rule. In summary,
we illustrate how individual energetics can be
coupled to population and community level
phenomena via key variables in behavior,
physiology and morphology.
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FACTORS INVOLVED IN EVOLVING A
PROTOFEATHER. Alan H.Brush, Univ.of
Conn., Storrs. [email protected]
Feathers likely evolved from conical tubercles
not platelike structures. The morphology of the
earliest/most primitive feather is unknown.
Minimally, forming such requires a capacity to
synthesize feather (^)-keratin (providing the
molecular phenotype), a follicular mechanism for
assembly and interacting epigenetic factors. A
variety of morphologic phenotypes followed
rapidly once the minimal structural element
existed (? recognisable as a single barb), derivable
from a single tubercle structure of appropriate
size. Possibly recognizable externally as a bristle,
it need never have existed as a morphological
unit. Rather, if individual placodes gave rise to
multiple barb ridges these could fuse distally to
produce the equivalent of natal down. An ability
to form the simplest barb permits derivation of all
extant feather morphs. Branching is controlled by
follicle symmetry: feather shape and relative size
of parts are regulated by barb length, a function
of growth period. A feather Phylogram invoking
these conditions gave a morphology identifiable as
natal down by simple fusion of proto-barbs. In
response to various selective forces, this simplest
structure could be modified easily and rapidly, via
changes in the regulation of the fusion and growth
periods acting on pre-existing, basic machinery,
as has been shown for other systems.
SOCIETY FOR INTEGRATIVE AND COMPARATIVE BIOLOGY