Research Identifies the Unexpected Factor That Causes Crocodiles' Odd Head Scales!

 What caused the crocodile to smile?

Exact mechanical processes fold the scaly leer into shape long before hatching, demonstrating that the functions that give the reptile its distinctive wrinkled snout are not precisely the same as those that give it the rest of its scales.

It's not simple to get a detailed picture of what's happening within a crocodile egg, but researchers from the University of Geneva's Laboratory of Artificial and Natural Evolution (LANE) in Switzerland have succeeded.

Michel Milinkovitch, a biological scientist and lab chief at the LANE, previously discovered that the mechanisms that give rise to the arrangements of mammal hairs, avian feathers, and reptilian scales are very different from those that give rise to the irregular polygonal head scales of crocodiles.
The arrangement of these traits, which come from enlarged placodes of the embryo's outermost epidermal layer, is dictated by Turing patterns—waves of interacting chemistry—that are encoded in the genes of the animals.

"The patterning of irregular crocodile head scales provides a fascinating exception to this paradigm," the team explains in a video that summarizes the results of the study.

Instead, a mechanical process shapes the crocodile's head scales, which the team initially ascribed to tensile tension. This means that the furrows between the scales resemble the stretch scars that humans may get during a growth spurt.

 The team has abandoned the tensile stress explanation after the new investigation showed a completely other force is at play, even though they may have been correct about the mechanical cause of scale patterning.

"Here we show that the patterning of crocodile head scales in fact emerges from compressive mechanical instabilities," according to the scientists.

The crocodile's jaw is initially smooth, but as its skin thickens, it eventually becomes wrinkled. The upper jaw has huge, elongated irregular scales formed by these early creases, while the lower jaw is filled with tiny polygons.

They discovered this by injecting epidermal growth factor (EGF), a protein that improves the stiffness and pace of growth of the epidermis, into Nile crocodile (Crocodylus niloticus) eggs. This produced embryos with head scales through a greatly accelerated version of the normal procedure.

"This 'brainy' network of skin folds partially relaxes towards a pattern of smaller polygonal head scales in hatched crocodiles," according to the researchers, "that is, highly similar to the head-scale patterns of caimans."

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EGF-treated crocodiles with reduced polygonal head scales and "labyrinthine" skin folding were permitted to hatch four weeks following treatment.

Normal Nile crocodile head-scale patterning is caused by the skin growing more quickly than the bone underneath it, as well as the mismatched stiffness (but not growth rate) of the epidermis and the dermis, according to these experiments and computer simulations that further supported the laboratory results.

A little jaw of a crocodile
Light-sheet microscopy of a baby Nile crocodile's upper jaw reveals the tiny folds created by the compared to the self-organized mechanical head-scale patterning method that the study discovered. The University of Geneva in Switzerland is home to G. Timin and M. C. Milinkovitch.

The crocodile's skin expands more quickly than the rest of its body can "fill," giving it folds more akin to a sharpei puppy's than the stretch marks on a lanky human. This is the exact opposite of the growth spurt issues that teenagers face.

It also implies that variations in embryonic skin growth, which are favored by evolution, may be the cause of the diverse head-scale patterns found in different crocodilians.

The study was released in the journal Nature.