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How crocodiles get their scale patterns (Hint: It's unlike other animals)

STEVE INSKEEP, HOST:

We pursue one of the mysteries of the incredible variety of animals that you find in nature. NPR's Jonathan Lambert reports on a study in the journal Nature.

JONATHAN LAMBERT, BYLINE: Biologist Michel Milinkovitch's work on crocodiles began with befuddlement, specifically at the weirdness of their heads.

MICHEL MILINKOVITCH: I was 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.

LAMBERT: He was surprised because features like scales, feathers and hair form using basically a shared genetic toolkit that creates evenly spaced units across a body, with each unit turning into a feather, scale or strand of hair. But Milinkovitch, with the University of Geneva, couldn't square this regular process with the irregularity he saw on the croc's head.

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: Meaning a physical process instead of a genetic one. As mud dries, the top layer shrinks as it dries faster than lower layers, creating cracks. Or maybe the croc's patterning might form much like our brains, where the top layer grows faster than underlying layers, creating folds. To put these two options to the test, the team experimentally sped up the growth rate of the top layer of crocodile skin in developing embryos.

MILINKOVITCH: When we do that, we see that the crocodiles develop a brainy pattern on their skin.

LAMBERT: The team then built a computer model of skin growth based on their results. With slight tweaks to how fast or thick the skin grows, they were able to recreate the patterns of several crocodile species that looked pretty different. Traditionally, biologists would seek to explain those differences by genetic mutations, but those can take time to accumulate.

MILINKOVITCH: Here, it's so easy. You just modify a little bit of the mechanics.

LAMBERT: The classical narrative in evolutionary biology is that everything mostly boils down to genes. Genes are still hugely important, even for the crocodile's weird head. But this research shows that evolution can generate remarkable and diverse forms with just small changes, letting physics do the rest.

Jonathan Lambert, NPR News.

(SOUNDBITE OF MUSIC) Transcript provided by NPR, Copyright NPR.

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Jonathan Lambert is a correspondent for NPR's Science Desk, where he covers the wonders of the natural world and how policy decisions can affect them.