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How Mother Nature engineers light and tough armours for her myriad species

What’s common between a rhino’s horn, pangolin’s scale, and porcupine’s quill? These natural weapons are made of the same protein found in human hair

beetles, ranjit lalThe tough light elytra (wing cases) of beetles are a composite of chitin with a substance called scalerotin. (Photo: Ranjit Lal)

It couldn’t be more ironical: the very effective, light armour they carry for their own homeland security (sometimes, used for offence) is swiftly leading to their own downfall — at our hands, and for reasons that ought to make us hang our heads in shame. The magnificent rhino, the quaint pangolin and the fierce porcupine are perfectly equipped to protect themselves: the rhino can upend a truck with a toss of its horn, the pangolin roll itself into a scaly ball which even lions cannot prise open, and the porcupine can put out the eye of the tiger with just one of its myriad quills. And these weapons are made of essentially the same thing: a structural protein called keratin — incidentally, it is also what our own hair is made of. And in our eternal wisdom, we believe that crushed rhino horn serves up as an aphrodisiac and pangolin scales have great medicinal value: as a result, both the animals are now teetering on the brink of extinction.

What should really intrigue us, however, is how Mother Nature’s engineering department has configured this protein to suit various needs and guises. The basics remain the same: the weapon is light (relatively speaking) and very strong. Keratins have been divided into two types: alpha keratin and beta keratin. The former is found in hair, wool and skin of animals, the latter in birds and reptiles. Keratins are a key structural material that can diversely make scales, hair, nails, claws, feathers, horns and hoofs and provide a protective, impervious layer to the skin.

I’ve never held a rhino horn so can’t vouch for how light or heavy it is (it would certainly be light relative to the rhino!), but feathers and porcupine quills are astonishingly light and structurally sound — they don’t seem to need handling with kid gloves! And porcupine quills can draw blood with consummate ease! Interestingly, spiders’ silk is also composed of keratin and is stronger than steel wire of similar dimensions.

For lightness and toughness there’s really nothing to beat keratin in the natural world — except for its only rival: chitin. This exists in the protective “exoskeleton” armour that most insects wear (thinking of it always makes me smile). In the old medieval days, gallant knights used to clad themselves in heavy metal armour (and chain mail) and clank around waving swords and spears as heavy as tree trunks. If they fell down, well, it was game over: they would stay down! Insects, in the meanwhile, with their segmented bodies trundle around at top speed, not in the least hampered by their tough composite exoskeletons. Chitin serves more for defensive purposes than as a weapon of offence, providing a protective hard shell, which is difficult to penetrate by a predator. The chitin-based hard “beak” of the octopus is an exception.

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In its pristine form, chitin is translucent, resistant and quite pliable and fairly tough. In insects (arthropods), it is often combined with a composite of other materials to form the tough exoskeleton. When combined with calcium carbonate, for example, as in the shells of crustaceans and mollusks, it becomes stiffer, tougher and less brittle than if the shell had been composed of calcium carbonate alone.

The soft squishy “skin” of a caterpillar is composed mainly of chitin, whereas the tough light elytra (wing cases) of beetles are a composite of chitin with a substance called scalerotin. In the wings of butterflies, chitin is arranged in a special manner that enables iridescence — and in the dragonfly’s wing, it is pliable enough to twist about its axis during flight enabling the insect to achieve maximum lift.

Chitin also forms the tough cell walls of fungi and enables fungi to heave itself out of the ground through piles of leaves and soil.

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While chitin is indigestible by all vertebrates, those that do eat insects usually have a symbiotic bacteria or protozoa in their guts that can break it down (I wonder if all the “survival” experts on TV who are constantly stuffing their faces with cockroaches and scorpions have these bacteria, though, apparently, chitin is used as a dietary supplement!). It is also used to make surgical thread, and there’s hope that we may make biodegradable plastic and paper from it, too.

If it’s keratin and chitin for mammals and arthropods respectively, it’s cellulose for plants, which provides the major structural component that gives rigidity to their cell walls. It is the most abundant naturally occurring organic compound, indigestible by all except for a selection of ruminants (who have symbiotic microorganisms to help them) and termites. For us, it simply helps us in being “regular”!

We’ve only recently begun exploring “composite” materials seriously — for the construction of planes, cars or anything that needs to be light and strong. Whether it is Kevlar or any other such composite for bulletproof jackets, or carbon-fiber polymers used for racing cars, these materials are showing us promise for the future. As usual, what’s mind-boggling is that Mother Nature in her workshop, got there so much before we did even though it must have taken her millions and millions of years of sheer perseverance and minute improvements, to achieve what she has now: rhino horn (compressed hair) can skewer the door of a Land Rover and plastic-looking porcupine quills can run you through and through. And, unlike plastic, these are all biodegradable.

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As for most of us: We’re content with deluding ourselves that pangolin scales will protect us from witchcraft, and look after our finances, and rhino horn (apart from tearing holes in Land Rovers) can put the swagger back into our walks and change us into Casanova!

First published on: 30-03-2022 at 12:26 IST
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