Which is harder: chitin or keratin? Which is heavier? Could an animal hypothetically have an exoskeleton made of keratin? If yes, would that animal need to molt its exoskeleton as animals with chitinous exoskeletons do?

Chitin and keratin both vary somewhat in their mechanical properties; in particular, some crustaceans mineralise their chitinous exoskeletons to make them harder and stiffer. In general, however, chitin and keratin both have densities on the order of 1200 kg per cubic metre, compared to 1000 kg per cubic metre for water. Keratin is somewhat stiffer than chitin. I don't have any comparative data handy for hardness, or strength for that matter - can anyone help?

In a sense, exoskeletons made of keratin do exist. After all, lizards and snakes are covered in keratinous scutes, and these must indeed be periodically shed. I suspect that a hypothetical arthropod-like animal with a continuous exoskeleton of keratinous plates would need to moult in essentially the same way as real arthropods, since it would face the same structural problem of being unable to add new material to the plates of its exoskeleton. A keratinous structure like a fingernail, a hair, or a horn can grow by the addition of new keratin at the base, but a zone of growth underlying each exoskeletal plate would only allow the plates to thicken, not expand laterally. The plates would presumably have to replaced in a moulting process to accommodate growth.

I thought that chitin has a crystaline structure so that once it hardens it is fixed. This allows it to form complex shapes under the existing exoskeleton, which is then inflated with haem to grow to size and then it "goes off" and hardens. I thought the problem with keratin is that it is fibrous with little cross-linking, so it is prone to splitting - much more so than chitin. An animal with a keratinous exoskeleton could probably avoid moulting due to constraints on secretion of the keratin, but it may need to shed old keratin that gets too worn to function properly.

If the exoskeleton were composed of keratin it could be continuously secreted from glands on the surface of the body, expanding to keep pace with the growth of soft parts. Tortoises have a keratinous surface on their carapace that continues to grow with them without the need to shed. Pangolins also have keratin plates that grow individually, but which form a protective structure that grows with the animal. I doubt that keratin would provide an ideal supportive structure though, due to the limitations in its strength in certain planes.

Last edited by Paolo Viscardi (15th Apr 2008 10:27:59)

I didn't realise that the fibrous structure of keratin made it especially prone to splitting - that's interesting. As far as I know, however, the keratin plates on a turtle's shell don't exactly grow continuously. Instead, they're periodically replaced from underneath by larger plates, with the old plate either being shed or remaining in place on top of the old plate. Perhaps a system like this could be made to work for an arthropod-style exoskeleton made of keratin (if I ever become a mad genetic engineer designing artificial life forms in some underground laboratory, I'll give it a try), but it would really just amount to replacing the plates of the exoskeleton one at a time instead of all at once.

Well, think of nails/claws and hair - they grow by continuous excretion and their length (in a natural environment) is determined by wear in nails/claws. Rhino horn is much the same - requiring abrasion to wear off old bits of the horn to prevent it growing out of control. Shedding frequency largely determines growth in hair, although the hair also becomes worn as the tips age (think split ends).

A secretion mechanism like that in our fingernails would easily provide a mechanism by which a growing exoskeleton could be established and maintained. By excreting at the same rate as the body grows (should be easy to achieve) there is no need to moult - unless serious damage is sustained.

Yes, but nails, horns and strands of hair all grow outwards from a permanent zone of growth positioned at the base of the structure. (Well, some of my own hair follicles are proving to be more permanent than others, but that's beside the point.) For an exoskeleton that's made of large adjoining plates, the problem would be finding room for the growth zones without disrupting the arrangement of plates. One solution might be to have the plates overlap, so that the growth zone of each plate was along an edge that was positioned under, and protected by, an overlying plate. This would restrict the plates to an imbricating arrangement, but yes, it's another possibility.

Pangolin scales are made up of keratin. And they overlap, right? But then, that's not an exoskeleton...

I was just thinking that Corwin had re-invented scales :-)