Tag: evolution

Among the myriad aspects of Nature that fill me with wonderment, there’s always something magical about the birds.  As eathbound humans, many among us have had that dream of flying above a landscape that we know so well, seeing everything from above, feeling utterly free and completely in control of our flight.  Then, we wake up to a reality that necessitates big lumbering sky-buses in order to achieve lift-off from the ground.

As I came across a lost feather a little while back, while I had a camera in hand with a macro lens attached, I thought to present a little bit about the wonder that is the feather.

Overview of Feathers

Feathers are epidermal growths that form the distinctive outer covering, or plumage, on birds and some non-avian theropod dinosaurs.

They are considered the most complex integumentary structures found in vertebrates, and indeed a premier example of a complex evolutionary novelty. They are among the characteristics that distinguish the extant Aves from other living groups. Feathers have also been noticed in those Theropoda which have been termed feathered dinosaurs.

Although feathers cover most parts of the body of birds, they arise only from certain well-defined tracts on the skin. They aid in flight, thermal insulation, and waterproofing. In addition, coloration helps in communication and protection.

Structure and Characteristics

Feathers are among the most complex integumentary appendages found in vertebrates and are formed in tiny follicles in the epidermis, or outer skin layer, that produce keratin proteins.

The β-keratins in feathers, beaks and claws are composed of protein strands hydrogen-bonded into β-pleated sheets, which are then further twisted and crosslinked by disulfide bridges into structures even tougher than the α-keratins of mammalian hair, horns and hoof.  The exact signals that induce the growth of feathers on the skin are not known, but it has been found that the transcription factor cDermo-1 induces the growth of feathers on skin and scales on the leg.

Lots of interesting technical detail, that doesn’t take away from the pure magic that is a soaring bird, as it glides through the sky without any effort.

Hope you enjoyed this post.

Technical Details of the Image

This image (and its detail) were captured with a Canon EOS 5D Mk III sporting an EF 100mm f/2.8 macro lens.  Exposure settings were 1/320 second at f/11 and 640 ISO.

Additional Information on the Evolution of Feathers

The functional view on the evolution of feathers has traditionally focused on insulation, flight and display. Discoveries of non-flying Late Cretaceous feathered dinosaurs in China, however, suggest that flight could not have been the original primary function as the feathers simply would not have been capable of providing any form of lift.  There have been suggestions that feathers may have had their original function in thermoregulation, waterproofing, or even as sinks for metabolic wastes such as sulphur.  Recent discoveries are claimed to support a thermoregulatory function, at least in smaller dinosaurs.  While feathers have been suggested as having evolved from reptilian scales, there are numerous objections to that idea, and more recent explanations have arisen from the paradigm of evolutionary developmental biology.  Theories of the scale-based origins of feathers suggest that the planar scale structure was modified for development into feathers by splitting to form the webbing; however, that developmental process involves a tubular structure arising from a follicle and the tube splitting longitudinally to form the webbing.  The number of feathers per unit area of skin is higher in smaller birds than in larger birds, and this trend points to their important role in thermal insulation, since smaller birds lose more heat due to the relatively larger surface area in proportion to their body weight. The miniaturization of birds also played a role in the evolution of powered flight.  The coloration of feathers is believed to have evolved primarily in response to sexual selection. In one fossil specimen of the Parave Anchiornis huxleyi, the features are so well preserved that the melanosome (pigment cells) structure can be observed. By comparing the shape of the fossil melanosomes to melanosomes from extant birds, the color and pattern of the feathers on Anchiornis could be determined.  Anchiornis was found to have black and white patterned feathers on the forelimbs and hindlimbs, with a reddish brown crest. This pattern is similar to the coloration of many extant bird species, which use plumage coloration for display and communication, including sexual selection and camouflage. It is likely that non-avian dinosaur species utilized plumage patterns for similar functions as modern birds before the origin of flight. In many cases, the physiological condition of the birds (especially males) is indicated by the quality of their feathers, and this is used (by the females) in mate choice.

Feathers and scales are made up of two distinct forms of keratin, and it was long thought that each type of keratin was exclusive to each skin structure (feathers and scales). However, a study published in 2006 confirmed the presence of feather keratin in the early stages of development of American alligator scales. This type of keratin, previously thought to be specific to feathers, is suppressed during embryological development of the alligator and so is not present in the scales of mature alligators. The presence of this homologous keratin in both birds and crocodilians indicates that it was inherited from a common ancestor. This may suggest that crocodilian scales, bird and dinosaur feathers, and pterosaur pycnofibres are all developmental expressions of the same primitive archosaur skin structures; suggesting that feathers and pycnofibers could be homologous.