Scientists Unlock the Secrets of Crocodile Skin and Its Irregular, Mystifying Patterns

We commonly refer to scaly, itchy skin rashes as “crocodile skin.” Scientists, however, have just proven that real crocodile skin—specifically the parts on the reptile’s head, snout and jaw—is much more special than previously thought.

But unlike the hair, feathers and scales of most other vertebrates, crocodile head scales uniquely develop as a result of tissue mechanics, not genetics, researchers suggest. Their findings were published in the journal Nature on Wednesday.

The study was inspired by an observation of Michel Milinkovitch, a biologist at University of Geneva and the paper’s lead author, while taking a blood sample from a Nile crocodile, per Popular Science’s Laura Baisas.

“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,” Milinkovitch tells NPR’s Jonathan Lambert. “This is so difficult to reconcile with all [that] we know about the development of feathers, hair and scales in other animals.”

Upon examination, the team found that this development process in other creatures and the one in crocodiles have “nothing to do with each other,” Milinkovitch elaborates to the Guardian’s Nicola Davis. In fact, a crocodile’s head scales even differ significantly from the scales on the rest of its body.

It took more than a decade for Milinkovitch’s team to put together enough crocodile embryos to investigate this observation, per Popular Science. They then observed the development of head scales of the Nile crocodile embryo and noted that skin folds appeared around day 51, out of the 90 total days of embryonic development, then began to form irregularly shaped scales.

The scientists suspected the crocodile’s skin was growing faster than the flesh attached beneath it, forcing the skin to fold. They tested this hypothesis by injecting crocodile eggs with a hormone that triggers skin growth and stiffening called Epidermal Growth Factor (EGF). The hormone caused a dramatic change in the skin pattern, confirming their suspicions.

“We saw that the embryo’s skin folds abnormally and forms a labyrinthine network resembling the folds of the human brain. Amazingly, when these EGF-treated crocodiles hatch, this brain-like folding has relaxed into a pattern of much smaller scales, comparable to those of another crocodilian species—the caiman,” co-authors Gabriel Santos-Durán and Rory Cooper, post-doctoral fellows in Milinkovitch’s laboratory, explain in a statement.

This indicates the uniqueness of crocodiles’ head scales is driven from mechanical processes, such as growth rate and skin stiffness, rather than gene expression.

The team then used an imaging method called light sheet fluorescence microscopy to measure the growth and shape of the layers of skin and bone on the crocodiles’ heads. Using this technique, they assessed the organization of the skin’s collagen—a fiber-shaped protein that works as the body’s “glue”—and examined what mechanical effect it had on the animals’ skin. With this information, the researchers built a computer model to simulate the crocodile’s skin development and run experiments.

“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,” Milinkovitch says to NPR. These species have distinct head patterns, but that difference, the team found, is not genetic.

“Hence, one does not need to call for many genes being modified to explain the evolution of head scale patterns in crocodilians; small evolutionary changes of the growth and mechanical properties of the skin explains it all,” he tells Popular Science.

In conducting the study, the team also developed a new technique to stain collagen fibers to analyze their impact on the skin’s mechanics. Other researchers have now started using the new method “because the 3D architecture of collagen is very, very important for understanding invasiveness of cancer tumors as well as for understaffing the aging of the skin,” Milinkovitch explains to Popular Science.

He tells the Guardian that biologists have been “obsessed with genetics” for the last five decades, so it’s satisfying for the scientists to discover a “purely mechanical” process.

The research is “quite interesting,” as Gareth Fraser, a biologist at the University of Florida who was not involved with the study, tells NPR. “It’s often nice to know that evolution is so vast, but it can also be so simple.”

Margherita Bassi | READ MORE

Margherita Bassi is a freelance journalist and trilingual storyteller. Her work has appeared in publications including BBC Travel, Discover magazine, Live Science, Atlas Obscura and Hidden Compass.