The feather is an extraordinary biological invention and the key to the success of modern birds. It has to be light and flexible to give birds fine control over their airborne movements, but tough and strong enough to withstand the massive forces generated by high-speed flight. It achieves this through a complicated internal structure that we are only just beginning to fully understand, with the aid of unlikely research assistants - fungi.
At a microscopic level, feathers are made of a protein called beta-keratin. The same protein also forms the beaks and claws of birds, and the scales and shells of reptiles. It's close (but less rigid) relative, alpha-keratin, makes up the nails, claws and hairs of mammals. Zoom out, and we see that feathers have a central shaft called the rachis with two vanes on either side. Each vane is composed of barbs that branch off the rachis. Even thinner barbules branch off from the barbs, and are held together by small hooks that give the feather its shape.
What's much less clear is how the keratin fibres and filaments are organised into the rachis, barbs and barbules. To work that out, scientists would typically slice the rachis in cross-sections and look at it under an electron microscope. But feathers don't give up their secrets so easily. Their fibres are stuck together with a chemical glue that makes them virtually impossible to separate. Imagine gluing a bundle of matches together and cutting them cross-ways. You could see the fibres that make up the component matches, but if they were glued together tightly enough, you wouldn't be able to tell where one match started and another began. So it is with feathers and their keratin.
Theagarten Lingham-Soliar from the University of Kwazulu-Natal solved the problem by recruiting fungi as research assistants. He used four species, which like to grow on keratin, to digest the complex molecules that glue individual filaments together. The process was very slow. Even after a year, the feathers seemed in pretty good shape and it was only after 18 months that they had broken down enough to be studied under the microscope.
The wait was worth it. For the first time, the microscope revealed how feathers are organised. For a start, they contain the thickest keratin fibres ever recorded, with a 6-micrometre diameter that's ten times greater than their next thickest rivals. The long fibres make the rachis strong and stiff, and each is made up of even smaller 'megafibrils' and 'fibrils'. Most of the keratin fibres are aligned along the shaft of the rachis but some are wrapped around them. These wraparound fibres prevent their long-ways comrades from buckling. Without them, the slender rachis might turn into a bulging barrel.
The keratin fibres were lined with small lumps or nodes that stick up from the main axis at regular intervals. Each node is capped with hooks or a ring, and those of neighbouring fibres are staggered. They act like the bricks of a wall, linking together to prevent cracks from spreading and resisting forces that would fracture the rachis. They also help to anchor the fibres into the glue that binds them together, much like steel rebars used in the construction of high-rise buildings.
But the most surprising feature of the fibres is that they're almost identical to the structures of barbules, and very similar to the downy plumage of baby chicks. This has implications for the evolution of feathers. You might think that the central feature - the rachis - came first, followed by the structures that branch off it. But the fact that the rachis itself is made of barbule-like fibres suggests that the barbules came first. Only later did they unite to form a solid rachis.
Reference: Proc Roy Soc B doi:10.1098/rspb.2009.1980
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