One of the most complex and highly intricate wonders of the flying world owes nothing to DaVinci’s studies on mechanical flight, the Wright brothers’ pioneering of aviation, or any other human-derived aeronautic technology. The most sophisticated piece of engineering used for flight has its origins in the Age of Dinosaurs and is one of the most common sights in our everyday lives: feathers.
Feathers as We Know Them
Modern-day feathers come in a seemingly infinite variety of sizes, shapes, and textures. Though their diversity is immense, each type is made of beta-keratin, a structural protein found in the skin of both reptiles and birds, and their branching structures have the same basic parts. The main shaft is composed of a hollow barbless base, known as the calamus or quill, and a central shaft, called the rachis. The rachis branches into main barbs and they branch even further into barbules.
The variety of feathers comes from small modifications to this basic branching structure to serve different functions. Feathers fall into a few general categories, which will be briefly describe here, and many more specific subcategories.
Bristles have a simple, stiff, and tapered rachis with few barbs. They are usually found on a bird’s head around their mouth, nostrils, and eyelids. Some experts think they are for protection much like eyelashes, others believe they serve a sensory function as evidenced by the nerve endings found at their base, and many support both theories.
Filoplumes are also simple and mostly bare of barbs except a tuft at the tip. They are found near contour feathers. Given their placement and the presence of unmyelinated nerve fibers, which are those that support peripheral sensory functions in their base, filoplumes act like whiskers by sensing the position of contour feathers.
Semiplume and down feathers are mostly hidden underneath outer feathers. Their loose branching structures appears fluffy and is highly effective for insulation.
Contour feathers include those that cover the surface of the bird. As their name suggests, these feathers follow the shape of the body, streamlining and weatherproofing it along the way like overlapping shingles. From the central shaft extends a series of slender barbs, each sprouting smaller barbules that are lined with tiny hooks. When these grasp on to the hooks of neighboring barbules, they create a structural network that is almost weightless yet remarkably strong. As the outer visage, these feathers also support decoration and camouflage.
Contour feathers also include the amazing evolutionary innovations mentioned in the introduction: flight feathers. Flight feathers are long, stiff, asymmetrically shaped, but symmetrically paired feathers on the wings or tail of a bird. They are built for durability, shaped for precision, and combined with musculature to produce the ultimate flying tool. The wing feathers, known as remiges, have uniform windproof surfaces, or vanes, on either side of the central shaft created by the interlocked hooks found on the barbules. These feathers are asymmetric with a shorter, less flexible leading edge that support stability and maneuverability. Similarly structured tail feathers, known as retrices, are arranged in a fan shape that allows for precision steering during flight.
While we can simulate some of these characteristics with our flying technologies, we have yet to create a machine that is as versatile, efficient, and effective as bird feathers in flight. Even more impressive, birds are not stuck with one set of feathers for their whole lives. Damaged or worn feathers can be replaced through the process of molting. During a periodic molt, old feathers are shed and new ones grow in their place keeping birds in top flying shape. You can’t say that about any of our manmade flying machines.
The Question of the Evolution of Feathers
The consensus among paleontologists is that birds, known taxonomically as the class Aves, are a group of maniraptoran theropod dinosaurs. Evidence found in the fossil record suggests that most major lineages of modern birds arose near the end of or right after the Cretaceous period (between 65-60 million years ago). Feathers now exclusively occur in avian dinosaurs (e.g., birds), but that was not always the case. With the discovery of the bird-like dinosaur Archaeopteryx in the 1860s and confirmed with further feathered dinosaur discoveries in the 1990s, feathers have been found on much earlier, non-avian species suggesting that their evolutionary beginnings stem at least as far back as the Jurassic.
Several theories have been explored and subsequently unraveled in recent years regarding the origin of birds and the evolution of feathers. Once the link between birds and reptiles was evidenced, some scientists theorized that birds did not evolve from dinosaurs. Instead, they are related by a distant common ancestor that has yet to be discovered. This theory, however, does not account for the striking similarities between the skeletons of birds and those of the highly feathered theropods.
Others theorized that maybe scales and feathers were both flat because feathers were an elongation of scales with frayed edges that eventually became the feathers we see today. They supposed that this growth over generations could have been prompted as an adaptation for flight. Maybe they helped these reptiles live in tree canopies by aiding gliding, which turned into the capability of flight. Such a “feathers-to-flight” theory would nicely tie up answers to all of the questions posed above and was fairly long-lived. With the discovery of hundreds of feathered, ground-running theropods, however, this theory proved to be discardable. So, too, dinosaurs far removed from theropods and even further removed from birds have been found with feathers that were not used for flight.
The feathers on the earliest non-avian dinosaurs did not look like the modern-day feathers described above. This fact has led to a new line of thinking about the transition from scales to feathers. From what we know from the fossil records, the earliest feathers, sometimes called protofeathers, were small, hollow filaments that appeared more like fuzz than feathers. Studying feathered specimens chronologically, the feathers slowly became more and more complex over time possibly because of an evolutionary impetus. The study of this feather development has prompted a new look into the genomic manipulation of placodes. Integumentary placodes are embryonic structures involved in the development of hair follicles, feathers, and teeth. Recent studies using modern genomic methods to identify feather-associated placodes have demonstrated the ability to turn scales into feathers. By turning key molecular circuits on and off at critical stages of scale development, researchers have been able to stimulate feather-like growths in alligator skin cells.
Though interesting, indeed, and something to keep an eye out for in new studies, none of this research is conclusive. Other studies suggest that convergent evolution might solve some of these riddles or more digging for fossils might be the best option. In any case, there is still much to learn about how the feathered dinosaur that you watch at your birdfeeder or hear outside your window evolved into what it is today.
Jane Thaler is a Gallery Experience Presenter in CMNH’s Life Long Learning Department. Museum staff, volunteers, and interns are encouraged to blog about their unique experiences and knowledge gained from working at the museum.