Evolutionary Microstructure of Bird Feathers

1
Evolutionary Microstructure of Bird Feathers
Archaeopteryx
Sarah Mouri

Fossil from the Solenhoefen Limestone (Jurassic)
of Germany. http//www.ucmp.berkeley.edu/diapsids/
birds/.
Figure 1
The complex structure of avian feathers,
including all information from fossilized
feathers such as those present in Archaeopteryx,
suggests strongly that feathers evolved only once
in the history of Vetebrata (Bock). This
conclusion is based on the complicated morphology
present in all feathers, as well as their
particular and intricate mode of embryological
development. Had feathers present in fossil and
recent forms originated more than once in the
evolution of tetrapods, one would not expect the
entire suite of complex morphological attributes
to have evolved in the two or more independent
lineages of birds possessing feathers (Bock).
Despite Archaeopteryx having had similar feather
structure to modern birds, this does not
necessarily suggest that it was an accomplished
flier. Archaeopteryx had several predecessors
with the beginning structures of feathers.
Explicitly stated theories for the origin of
feathers had originated toward the close of the
19th century with Pycraft (1894, 1896, 1910)
arguing that feathers arose in connection with
flight. Since then the flight theory, in which
feathers are thought to have been developed for
flight adaptation, and the insulation theory, in
which feather development is believed to have
served to insulate body heat and incubate eggs
and adapt its structure for flight after the
adaptation for wings, have become the most widely
accepted. The three species chosen for the
study, 1. Japanese Quail 2.Ruby throated
Hummingbird and 3. Indian Peacock, are
ecologically similar because they have adapted to
similar environments, away from extremes, such as
desert or arctic habitats. Quail and peafowl are
limited fliers, as are most of the species in
their order, Galliformes. Hummingbirds are highly
specialized fliers, categorized in the family
Apodiformes, along with the several species of
swifts. Despite their difference in flight
capabilities, peafowl and hummingbirds have
developed iridescence generally believed to
function in mate attraction (Kinoshita, 2002).
Abstract Feather macrostructures are
deceptively simplistic and the Scanning Electron
Microscope allows a detailed observation of the
greater complexity of feather structures. The
microstructure of feathers contains a wealth of
anatomical information including coloration
characteristics, flight capabilities, and
habitat. The experiment was designed to explore
the feather microstructure of three separate
species of birds 1. Japanese Quail Coturnix
japonica 2.Ruby throated Hummingbird Archilochus
colubris and 3. Indian Peacock Pavo cristatus.
Quail and peacock belong to the same order,
Galliformes however, iridescence was not present
in Japanese Quail. Results were used to spark an
evolutionary discussion of modern feather
microstructures, comparing the three species
general structures and methods of achieving
iridescence or lack there of. The quail served as
a control variable microstructures of quail
feathers remained similar to those of
Archeopteryx lithographica (Figure 1) a feathered
dinosaur notorious for its representation of
early bird development (Figure 2). Hummingbirds
and peacocks have both achieved iridescence, but
humming birds made use of trapped air bubbles,
while peacocks had uniquely shaped blocks, which
constructed the barbules of their feathers.
Evidence supported that iridescence
characteristics in hummingbird and peacock
feathers were more likely a convergent/analogous
trait than a homologous trait. A further
exploration, concentrating specifically on the
flight feathers of the peacock and hummingbird,
would enhance the studys overall support of
convergent evolution between the two species.
Figure 2
Evolution of Birds and Flight
National Geographic Society
Methods and Materials Preparation of avian
feathers for the SEM has not been widely
practiced. Attempts in this study were made to
clean the specimens with warm water and a mild
soap, which resulted in so much residue upon
drying, it was nearly impossible to see the
microstructures. Sonification altered the
molecular properties of the feather fibers, which
disabled the hooks and barbs zipper effect.
The specimens contained too many artifacts to be
left untreated, but dusting with pressured air
removed enough of the particles that the
microstructures were clear images. All
iridescent feather samples were taken for their
color distinction, and quail down feathers were
chosen to illustrate the complex microstructure
of the seemingly most simplistic feather. The
samples were sputter coated with gold-platinum,
doubling the standard time to 120 seconds for
coating. After 60 seconds an abundance of
charging remained on the images. Micrographs were
taken using an AMRAY 1810 SEM.
Image 3Quail Down
Basal Nodes of Downy Barbule
Image 8 Peacock Green
1.
Internode
National Geographic Society
Node
Coturnix japonica
3.
National Geographic Society
or rachis
Pavo cristatus
Figure 3 Feather Macrostructure www.nurseminerva.
co.uk/adapt/feathers.htm
Image 9 Peacock Blue
Image 1
2.
National Geographic Society
Image 7 Peacock Green to Blue
(eyespot)
Archilochus colubris
Introduction The origin of feathers and the use
of feathers for flight have initiated several
different theories in evolutionary biology. While
considering the origin of feathers and the origin
of birds are linked it is not difficult to see
why early evolutionists believed the feathers
evolved for the use of flight. However,
evolutionists could not justify why a trait for
flight would adapt so much earlier before the
action of actual flight was executed. Since 1996,
the discovery of coelurosaurian dinosaurs (the
closest known taxon to birds) with remnants of
several kinds if integumentary structures,
including feathers, has revolutionized our
thinking about the origin of feathers (Chen et
al.,1998 Ji et al., 1998 Padian et al., 2001).
When most people think of feathers, they
envision only typical contour feathers covering
the body of birds. But feathers come in a wide
variety of forms (Lucas and Stettenheim, 1972
Stettenheim, 2000). In addition to contour
feathers, remiges (wing feathers) and rectices
(tail feathers), there are down feathers of
several types, semiplumes, filoplumes, several
categories of bristles and large specialized and
often bizarre courtship plumes. A contour feather
has a complex morphology consisting of a central
rachis to which barbs are attached on two margins
to form a vane (Figure 3). The barbs possess two
rows of barbules of different types, the proximal
and distal barbules, whose morphology permits
them to lock together thereby maintaining the
integrity of the vane (Bock, 2000). The two rows
of barbules are more commonly known as hooks and
barbs, which are responsible for the zipper
effect locking neighboring barbs together.
Preening behavior in birds functions to zip up
their feathers therefore maintaining the
structure of their feathers. Without preening,
water and cold would be able to penetrate down to
the skin the feathers used for flight would no
longer be able to push against the air without
dragging, and several other problems that may be
detrimental to the bird could occur. The
diversity of functions and biological roles of
avian feathers is also great. Most people think
of feathers serving only for insulation of body
heat and for flight, which are the two largest
contending theories of evolution. However,
feathers are also involved in protective
coloration, signaling of all types (aggression,
species recognition, flashing signals) between
conspecific individuals as well as between
interspecific individuals. Feathers function as
courtship displays of all types, streamlining of
the body (Buhler, 1990), cleaning of the plumage
rather than the body itself, and sound production
(Bock, 2000). The structure of all feather
categories is directly related to the function of
each type. (Continued Above)
-Rachis
Results and Conclusions 1. Quail Down (Image
1, 2, and 3) Micrographs of quail feathers
illustrated the typical microstructures known to
all modern birds (Figure 3). As seen in Image 1,
an overhead view of down is not much different
than the billowy mass of fluffy fibers seen with
the naked eye. However, as the sample is
enhanced (Image 2 to Image 3) the complex
microstructures become clearer. Distinct nodes
and anti nodes are visible and the basal cell
and pennulum can be distinguished. 2.
Hummingbird Red and Green Contour Feathers
(Images 4, 5, and 6) What may appear to be
artifacts are in fact air bubbles on the surface
of the barbules. The air bubbles trap and
reflect light, which causes the iridescence to
occur. The color visible to humans is dependent
upon the size of the bubble and the amount of air
it holds (McKernan, 1998). Red feathers
contained many more air bubbles than green
(compare Image 1 and 3 Green to Image 2 Red),
and as the brilliance of iridescence becomes
duller, the number of bubbles decreases and the
spacing between them increases. 3.
Peacock Eye-spot Feather (Images 7, 8, and
9) Image 7 illustrates the unusual structure of
peacock display feathers in general. Compared to
the hummingbird (Image 4) and quail feathers, the
general microstructures were not present. Hooks
and barbs are not present, which makes sense when
looking at the display feathers as a whole. A
peacocks display feathers are not zipped
together like the tail feathers of hummingbirds
and quail. The display feathers are not the real
tail feathers, and have no function for the
hooks and barbs structures. The distinct blocks
seen in Images 8 and 9 are how the peacock has
accomplished iridescence. Rather than air
bubbles, the individual blocks constructing the
barbules of every feather are saddle shaped.
The periodic structure of the submicron melanin
granules in every block is considered responsible
for the optical interference (Kinoshita, 2002)
Image 2 Quail Down
-Barb
-Hooks and Barbules
-Barb
- Air bubbles /
Image 4 Hummingburd Green
contour (Adult male)
-Rachis
Air bubbles
Image 5 Hummingbird from red throat
(Adult male)
Image 6 Hummingbird
Green contour (Adult male)