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Scientists normally explain the spectacular diversity of animal patterns by pointing to genetics. But a study in Nature suggests that, at least for crocodile heads, different forces might be at play. NPR's Jonathan Lambert explains.
JONATHAN LAMBERT, BYLINE: Biologist Michel Milinkovitch's work on crocodiles began with befuddlement - specifically at the weirdness of their heads.
MICHEL MILINKOVITCH: Just looking at the animal from up close, I was just so surprised by the pattern of very irregular scales on the head of the animal - very different on the left and on the right. And also, many of the edges are, like, incomplete. And this is so difficult to reconcile with all what we know about the development of feathers, hair and scales in other animals.
LAMBERT: Feathers, hair and scale on other animals all get built using a shared genetic toolkit that scientists understand pretty well. This toolkit involves genes that create evenly spaced units across a body - kind of like a polka-dot pattern - with each dot turning into a feather, scale or strand of hair. But Milinkovitch, with the University of Geneva, couldn't quite square this regular process with the irregularity he saw.
MILINKOVITCH: I realized that it looked very much like cracked mud. So it looks like it's a mechanical process which is generating this pattern.
LAMBERT: But Milinkovitch and his colleagues had no idea how that largely mechanical process - or, in other words, physical process - of skin growth could create these weird patterns. They had two main hypotheses. One was that they formed like mud, where the top layer dries and shrinks.
MILINKOVITCH: Because the layers below are not drying as fast, you have stress which is building up, and then you have cracks that are appearing.
LAMBERT: Alternatively, the irregular head skin might form like the folds of human brains, where the faster-growing top layer folds in on itself. To figure out whether crocodile heads are more like mud or more like brains, Milinkovitch injected a drug that speeds up growth into the top layer of skin of developing crocodiles.
MILINKOVITCH: When we do that, we see that the crocodiles develop a brainy pattern on their skin.
LAMBERT: That brainy pattern ruled out the mud idea. Milinkovitch's team then built a computer model of skin growth based on their results. They tried to re-create the irregular head patterns of a variety of crocodile species.
MILINKOVITCH: By modifying very slightly the parameters of the mechanical model, you know, you make the skin a little bit stiffer or you make the skin grow a little bit faster. You can get a Nile crocodile, an American alligator, a spectacled caiman.
LAMBERT: Those three species' heads look pretty different. And the fact that this diversity can stem from slight differences in how the skin grows, as opposed to requiring lots of genetic changes, is exciting, says Gareth Fraser, a biologist at the University of Florida.
GARETH FRASER: It really shows, you know, how versatile or how plastic these systems are, actually. And that's quite interesting, I think, because it's often nice to know that evolution is so vast, but it can also be so simple.
LAMBERT: The classical narrative in evolutionary biology is that everything mostly boils down to genes. Genes are still hugely important, even for crocodiles' weird head patterns. But this research shows that evolution can generate remarkable and diverse forms with just a few small changes, letting physics do the rest.
Jonathan Lambert, NPR News. Transcript provided by NPR, Copyright NPR.
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