Bird Keratin Beaks
Location: Outside U.S.
Country: South Africa
Date: Fall 2012
Why the beaks of birds made of keratin not bones?
Since birds seem designed for flight, the interest would be to use materials that are durable and strong, yet lightweight. You would not want to built a plane out of the heaviest materials; instead you would use materials that provide the strength needed, but which are still not extremely heavy.
Even bird bones are very lightweight and contribute to success in taking off and flying. One might argue that perhaps heavier support systems appeared along the way in birds, but those trials were not successful and did not survive to pass that system to their progeny.
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Keratin weighs less than bone, yet it is hard enough to crack nuts, crush insects, or serve as a stiff tube to suck nectar out of deep throated flowers.
Bone growth requires blood flowing and bones do not generally grow outside of a body (to the best of my knowledge). A bone that protrudes from the body would not have enough blood flowing to the growth plates (called epiphyses) and thus would not grow.
Many ornithologists think that one evolutionary factor is that keratin is lighter than bone, so its easier for birds to fly. Selection and evolutionary development is complex, so that is probably not the only factor. If you look at the tremendous diversity of bird beak shape and function, you can speculate that keratin has been easier to mold by selection than bone would have been, for instance.
I don’t think that there is a clear “cause & effect” answer to this. For example, “Why aren’t finger and toe nails not made of bone?” “Why do we have cartilage between our joints, rather than just bones?” The general answer, but one that really isn’t “THE answer” is why does our body have a particular structural composition? Maybe somewhere eons ago some creatures did have different compositions, but those creatures did not compete favorably with creatures that had surviving compositions. That is the way evolution works.
In fact, bird beaks are made of both bones and an external sheath of keratin. The upper and lower jawbones have been lengthened and form the bony 'core' of most bird beaks. This core is then surrounded by keratin.
S. Unterman Ph.D.
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Update: November 2011