Feather development

Feathers are products of the epidermis and keratinizing system. They are non-vascular and non-nervous. They have a tubular central shaft called the rachis; coming off either side of the rachis are the veins, which have a series of barbs with interlocking connections that are called barbules. The rachis and attached veins make up the spathe. The opposite of the rachis is the calamus, which anchors the feather to the body and is moved by attached dermal muscles. There are many different types of feathers that mostly follow this basic design with few variations based on evolutionary needs.[1]

Feathers are sometimes referred to as “elaborate reptile scales” just as birds are sometimes viewed as a subset of reptiles instead of their own category. Although this is a simplification, it originates from bird's homology with reptiles. Birds evolved from fast bipedal dinosaurs, but feathers evolved before them, and not for flying as what was originally thought. The theory of feathers evolving for flight unraveled in the 1970s when theropod dinosaurs (some common theropods were Tyrannosaurus rex and Velociraptors) were discovered to have feathers. By looking at melanosomes, which are structures whose shape and arrangement determines how light reflects off of the pigment, in dinosaur fossils, scientists have found that dinosaurs did in fact have bold plumage. Specifically, Jakob Vinter and his colleagues analyzed the melanosomes in Anchiornis huxleyi, which they were able to reconstruct into what the plumage would have looked like 150 million years ago when A. huxleyi lived.[2]

In 1861, a group of German quarry workers found the fossil of Archaeopteryx, which had feathers like modern birds, but also teeth in its mouth, a long, bony wing, and claws on its wings like a reptile. Archaeopteryx seems to be a transitional animal from reptile to bird. However, Archaeopteryx had already rather developed feathers, which didn't help to show how the feathers had evolved in the first place. In 1996, the missing fossil to help show the evolution of feathers was found. Sinosauropteryx was covered in thin, hollow filaments that covered its back and tail. Now, after finding many dinosaurs with primitive feathers, it is believed that feathers started out as these thin, hollow filaments, and eventually evolved into the fluffy coats of feathers seen on modern birds.[3]

There are a few theories for the reasons feathers evolved in the first place, if not for flight. The first is that they served a purpose as insulation. Theropods have been found covering their nests with their forelimbs, thought to be sheltering their young. Feathers known to be found on theropods would have played an important role for insulation. Another popular theory is that quick, bipedal dinosaurs used their feathers to run faster. It's possible that the feathers assisted the dinosaurs running up inclines, and this advantage of speed eventually lead to flight. The arboreal theory suggests that animals that lived in trees found it quicker and more energy efficient to leap from tree to tree instead of running down a tree, across the ground to another tree, and back up the new tree. This leaping eventually lead to gliding, and flight.[1]

Another newer theory is that feathers were selected for sexually.[2] Their bold plumage with its patterns, coloration, and even iridescent colors were to attract the opposite sex, which is very common in modern birds today. By uncovering new technology to look and analyze well-preserved melanosomes, scientists can reconstruct what the feathers on fossilized dinosaurs would have looked like, and they look rather flamboyant.

More current theories of the original purpose/advantage of flight involve more than one of these theories in combination with each other. Ken Dial, a flight researcher at the University of Montana-Missoula, shows how chicks often use feathers to gain traction as it runs from predators up inclines, but this flapping also helps the chick steady itself as it goes down again.[2] It is very possible that feathers had multiple useful functions first, before flight.

Feathers develop from the dermal papillae. Feathers begin to form from feather follicles, which are invaginations starting in the epidermis down to the dermis. It is in the dermis that the follicle and the pulp cavity begin to form the feather. The pulp cavity is the space that contains the feather follicle. The feather filament soon grows out of the follicle; this is due to cell proliferation, which is an increased number of cells as a result cell growth and division, at the follicle base. These new cells form two different tissues. There is the sheath, which is the main feather tissues and pulp caps. The sheath is the supportive layer that surrounds the feather, which falls off as it grows. Similarly, the pulp caps, which protect the dermal core, also fall off as the feather grows. The main feather tissues later unfurl, which causes the disposal of the sheath and the pulp caps as it assumes its functional shape. As the feather grows, its spathe, which is where the rachis and vanes attach, continues to form. When spathe is finished developing, the calamus begins to form within the base of the spathe. The calamus is the quill of the feather, which is the bottom portion that stays mainly within the pulp cavity. From there, the feather is fully developed and will remain as such until molting occurs, causing it to fall off. Feathers fall off during molting, which occurs at different times through the year depending on the type of bird. Birds can molt for seasonal, reproductive and many other reasons.

There are six different types of feathers, which are flight, contour, down, semiplumes, filoplumes, and bristle feathers.[4] [5] [6]