The Scales of Experience

Scientists had long thought that crocodiles’ random and unique head scales arose from their genes. However, recently, they uncovered a surprising alternative explanation: The distinctive shapes are actually driven by physical forces.

In a new study, researcher Michel Milinkovitch and his colleagues found that the irregular scales on crocodile heads, as opposed to their bodies, follow a different pattern of formation, one that is purely mechanical. And now they know how it works.

“This is a completely different process, nothing to do with each other,” Milinkovitch told the Guardian.

During development, the skin on a crocodile embryo’s head grows faster than the underlying tissues, causing it to buckle and fold, said researchers. By day 51 of the Nile crocodile’s 90-day incubation period, these folds begin forming irregular polygonal scales.

The team tested this process by injecting Nile crocodile eggs with Epidermal Growth Factor (EGF), a hormone that increases skin growth and stiffening.

Their findings showed that treated crocodiles developed head-scale patterns resembling those of their relatives, the spectacled caiman.

“We saw that the embryo’s skin folds abnormally and forms a labyrinthine network resembling the folds of the human brain,” explained co-authors Gabriel Santos-Durán and Rory Cooper in a press release.

But researchers didn’t stop there.

Using advanced imaging and computer modeling, they created a 3D simulation of crocodile head-scale development to understand the effects of changing the specific growth rates and stiffness of the tissue layers.

“By exploring these different parameters, we can generate the different head scale shapes corresponding to Nile crocodiles both with or without EGF treatment, as well as the spectacled caiman or the American alligator,” Ebrahim Jahanbakhsh, another author of the study, said in the press release.

Milinkovitch emphasized that the study is a departure from genetics-focused biology.

The authors noted that findings not only shed light on crocodile evolution but also highlight the relationship between physical forces and biological development – showing us that sometimes, nature’s artistry doesn’t need a genetic blueprint.

“For the last 50 years or more, biology has been obsessed with genetics,” Milinkovitch told the Guardian. “So here, what is nice is that we have a process which is purely mechanical.”

Watch how it works here.