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.