Feathers by Thor Hanson (2011)

Feathers: The Evolution of a Natural Miracle explores the details of feathers and why they are so amazing. The book is filled with interesting facts about all the different kinds of feathers (Example: most birds have between 1 and 25 thousand, but only a few dozen flight feathers), but it is mostly focused on exploring the incredibly lightweight, watertight, insulated, beautifully-colored, multi-functional natural wonder that even evolutionary scientists call a “miracle.”

Interesting Info on Feather Design

“Feathers are unbelievable,” Feduccia said, and his voice took on a tone of wonder I would hear again and again… “They have all of these incredible aerodynamic features – lightweight, with graded flexibility; they’re perfect airfoils; they can work together in slotted wings with high lift at low speeds.

Feathers cluster “in well-defined tracts,” which “offers two advantages: It distributes plumage across the entire body while allowing skin between the tracts to remain relatively bare” for “regulating body temperature.” They “may also play a role in how feathers move, helping to concentrate the relevant muscles in discrete lines… Each follicle is surrounded by strong muscles and nerves that give birds surprising agility with individual feathers. They can fluff them for warmth, lift them for preening or display, and even make fine adjustments during flight to maximize aerodynamic efficiency… Coordinating such movements is quite an engineering feat. It would be like a person straightening their part with a thought, twitching individual ear hairs, or accurately judging wind speed from the play of a breeze across their eyebrows.”

“Engineers call feathers the most insulating material ever discovered.” Tiny birds, while operating at a body temperature several degrees higher than ours, can maintain a “difference between the outdoor air temperature… as large as an astonishing 140 degrees Fahrenheit.” The complex layers of barbs and barbules can can efficiently trap a large amount of air molecules “as a barrier.” “With their intricate air-trapping microstructure, down feathers are the most naturally insulative material on earth, and birds have the ability to fluff them up manually, essentially adjusting their R-value at will.” The lightness of this material allows birds to fly. Birds and even other animals will scavenge stray feathers to insulate nests and burrows.

Different kinds of feathers are created by “varying the location and timing of keratin production at the follicle collar… To accomplish these feats, the follicle’s cells must act in perfect concert, a symphony of starts and stops that is controlled by a particular gene” (the Sonic Hedgehog hox gene). Human industry has yet to create a synthetic material that matches the insulation power with the same lightness and durability: “Feathers grow that way naturally, but manufacturing such finely branched filaments is extremely difficult.” (These downy feathers, however, are not waterproof, and require a covering of watertight contour feathers, and extra parental care for downy young until they grow that outer layer. This presents an evolutionary challenge, as described below.)

Feathers keep birds from freezing by being so insulative, but it’s just as amazing that they don’t make birds overheat. “When a bird takes flight, it suddenly finds itself producing seven, ten or even twenty times the body heat it had while perched.” Since they already operate “within a few degrees of the point at which proteins in living cells break down faster than the body can replace them,” temperature regulation is crucial, and involves adjusting feather positions and increasing blood flow to bare portions (apteria). Additionally, a bird’s “complex system of nine or more air sacs to supplement their lungs,” which “increases the efficiency” to allow flying, also “dramatically expands the surface area available for internal evaporation,” releasing extra heat through the mouth by panting.

On the “amazing” “flexibility” of feathers for real-time flight adjustments: A falcon “dove after a lure… accelerating up to 157 miles per hour before neatly catching it and pulling up,” experiencing a calculated gravitational force of “twenty-seven Gs“! (“Fighter pilots risk losing consciousness at anything over nine.”) Other examples of “airflow management” include reducing drag to increase flight efficiency. “Vultures, eagles, and other soaring birds use small adjustments of their spread wing-tip ‘fingers’ to manipulate air currents or change speed and orientation, and all birds utilize feather movements to instinctively alter the turbulence patterns around their wings. Slots can be opened or closed to direct air… covert feathers can be raised or lowered like tiny flags.” (No wonder aircraft engineers study birds to find ways to increase gas mileage!)

“Owl feathers feature barb extensions” that not only increase efficiency but also “muffl[e] the sound of their approach” – except for the Scops Fishing Owl, which hunts prey underwater and doesn’t need the stealth factor!

The watertightness of outer/contour feathers is not fully understood but seems to involve a high number of “touch points,” and “air pockets” between them, that repel water molecules. “Considering their light weight, flexibility, and thinness, feathers offer one of nature’s most versatile and efficient waterproofing membranes.” There are also beautiful adaptations: Diving cormorants have a slight structure modification that allows their “outer feathers” to get soaked, which adjusts their buoyancy as they dive for fish, “while still keeping their skin and down feathers sealed inside a watertight blanket.” At the other end of the spectrum in the dry desert, the sandgrouse has a different feather structure that absorbs so much water that birds have been observed “methodically soaking their chests” in pools to allow “thirsty chicks… to eagerly drink at Papa’s breast, sucking water straight from his feathers.”

While many birds “snap” their wings in “percussive notes” for mating rituals, the club-winged manakin takes it to another level with the “odd shape” of its feather wings: “This rapid vibration brought the wings together repeatedly, striking the enlarged clublike secondaries together in a way that forced the bent one to saw back and forth across a row of tiny ridges on the adjacent shaft… Each wing was indeed acting as a tiny violin, with the bent feather tip serving as the pick or bow, the ridges as strings, and the swollen, hollow feather shafts as the resonating chamber, amplifying and sustaining the tone.” (This was not understood until the relatively recent “breakthrough” of “high-speed video.”)

Birds regularly replace their feathers through molting, which is needed to maintain function after wear-and-tear and also to try to help manage the ubiquitous issue of bird lice. Sometimes molting changes colors that correlate with the mating season.

Interesting Theories on Feather Evolution

Hanson describes the old scales-to-feather hypothesis that never had any evidence and the new Stage I to V theory that seems to at least have some evidence for it from evolutionary development. Hanson describes Archaeopertyx as well as the recently uncovered feathered dinosaur fossils, but he notes the “temporal paradox,” highlighted by minority BAND scientists (Birds Are Not Dinosaurs), that the earlier stages are in all the later-dated fossils, while Archaeopteryx’s much older feathers are the asymmetrical flight feathers, thought to be the last stage to evolve. Hanson seems to suggest that the discovery of the even-earlier Anchiornis resolved this paradox, because the bird had some lower-stage feathers, but since it also had the flight feathers, we still seem to have a curious sudden appearance of those. (There is also evidence that Archaeopteryx molted, suggesting the function has been around about as long as feathers themselves.)

On the evolution of theropod dinosaurs into birds, Hanson describes some evidence for the current consensus but also notes the dissenting views of Alan Feduccia, a self-described “old-school Darwinian” who thinks birds came from a different ancestor. On the Stage theory of feather evolution, he “questions the usefulness of Prum’s downlike Stage II feathers,” which “lose most of its insulative value when wet… Young ostriches caught out in the rain often die of exposure, even in the African heat. In Prum’s model, however, contour feathers evolved after downy plumes.” Feduccia also thinks the “host of similar traits” between birds and theropods “came about” through “convergent evolution,” which points to the curious flexibility of one person’s homology to be another’s convergent evolution.

A simpler example of convergent evolution: Carrion birds lack feathers on their heads, which seems to keep them from getting blood and guts stuck to their heads as they plunge them into their carcass meals. “For carrion birds, the loss of feathers is such a good idea that it has evolved at least twice, in different places, in totally different groups of species… The New World and Old World vultures are not related; their likeness evolved from the practicalities of their grisly diet.”

(In an unrelated example, the book notes the “more than two dozen independent and unrelated times membranes [flaps of skin] evolved for vertebrate gliding and flight” in non-birds.)

The current consensus for the evolution of bird flight involves Wing Assisted Incline Running by climbing steep slopes or trees, a hypothesis that is a sort of hybrid between the ground-up and tree-down hypotheses, which both had inconvenient difficulties.

On the development of flight with cooling mechanisms: “Innovation in nature often occurs at stress points, places where competing adaptive pressures create an evolutionary dilemma… powered flight and specialized cooling mechanisms developed in tandem.” If “dinosaurs were warm-blooded creatures, then the basics of avian cooling must have already been in place in theropods… the result is a complex system of feather manipulation, controlled blood flow, and evaporative cooling that allows most birds to dispel far more heat than they produce, even while flying on a warm day.”

Challenges to Evolution and Creation

I think the “sudden appearance” / “temporal paradox” of feathers in the fossil record, and the questionable usefulness of the increasing stages point to difficulties for theories of unguided, gradual development. A few examples of convergent evolution add a curious inconvenience. And many of the amazing features seem incredibly complex.

At the same time, I think these features also present challenges to young-earth creationism and its “perfect paradise paradigm.” While complex mating rituals would certainly fit the original commands to “be fruitful and multiply,” the clear adaptations for predator/prey relationships are more curious.

Did owls have barb extensions before the Fall if they didn’t hunt small mammals? Did vultures have bald heads? Did diving birds have adaptations to survive dozens of G forces and adjust their feather buoyancy if they didn’t eat underwater fish? And if the pre-Fall climate was globally lush, as some have conjectured, did the desert sandgrouse have its uniquely absorbent structure?

I suppose the observation that many feathers involve multiple functions would support an idea that these features could have existed with different, but still beautifully designed, functions (although multi-functionality also makes it easier to imagine gradual evolution of complex features).

However, provided the theologies are equally valid, such designs seem to me more naturally indicative of an old-earth creation or theistic evolution type of view, with animals surviving in harsh environments and predator/prey relationships with beautifully designed features that allow a variety of creatures to survive in a “very good” but not yet “perfect” world.

Regardless of how they got here, feathers are marvelous, and thanks to Thor Hanson’s book, I can appreciate their wonder just a little bit more.

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