Unusual scales on crocodile heads due to skin growth rate, scientists say | Science

It sounds like a conundrum that Rudyard Kipling would have explored in his Just So Stories, but researchers say they have the answer to how crocodiles get the scales on their heads.

Many animals, from turtles to birds, have scales – hard plate-like structures that form on the skin.

But scientists say the irregular scales that develop on the heads of crocodiles as they grow inside an egg arise from a very different process to the way in which scales typically form, including elsewhere on the body of a crocodile.

“This is a completely different process, nothing to do with each other,” said Prof Michel Milinkovitch of the University of Geneva, who led the research.

Milinkovitch said scales, hair or feathers typically formed on animals where particular genes were turned on during development as a result of certain chemicals – themselves produced from genes – acting on DNA. This gives rise to a regular pattern.

The dynamics behind this complex process were worked out by Alan Turing, the mathematician and computer scientist.

But it appears the scales on a crocodile’s head are not under such genetic control. Milinkovitch and his colleagues said the scales instead developed because the skin of the face and jaw grew faster than the tissues to which it was attached, and the layers of skin itself had different levels of stiffness. The upshot is the skin folds inward, giving rise to scales with an irregular pattern.

In other words, said Milinkovitch, it is a purely mechanical process.

The team added that variations in skin growth rate and in the material properties of the skin explained why different species of crocodile showed different head-scale patterns.

“What we see is that we can produce the pattern of any species of crocodile,” said Milinkovitch.

Writing in the journal Nature, the team reported how they carried out a series of experiments with Nile crocodile embryos inside the egg.

The team found that when they injected a growth hormone to increase both the rate at which the outer layer of the crocodiles’ skin grew and its stiffness, the patterns of scales on the head changed. Indeed, under certain conditions, they resembled those of the spectacled caiman – a relative of crocodiles with very small, polygon-shaped head-scales.

The researchers then created a computer simulation of crocodile head-scale patterning based on the structures and properties of different layers of tissue found in the face and jaws of a crocodile embryo.

This model reproduced the patterning seen in real life, showing that the skin folds extended like cracks in dried mud before interconnecting with each other. The team said the 3D organisation of collagen fibres was found to be an important factor influencing the direction and shape of the resulting scales.

Milinkovitch said the study highlighted the importance of looking at how different mechanisms underpinned the 3D shapes of organisms.

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