For reptiles, crocodiles have some pretty distinct skin. Instead of the sleek and smooth scales on snakes and lizards, crocodiles have a more lumpy and three dimensional pattern on their scaly heads. Just how its unique skin is formed has puzzled scientists–until now. The pattern of scales on their face and jaws is formed by a mechanical process of skin folding and not genetics. The findings are detailed in a study published December 11 in the journal Nature.
Polka dot patterns
Typically, animal skin appendages including hair, feathers, and scales are controlled by specific genes when an embryo is developing. There are some exceptions to this rule–including in crocodile heads.
“Crocodiles are beautiful animals with a bad reputation,” Michel Milinkovitch, a study co-author and physical biologist who helps lead the Laboratory of Artificial & Natural Evolution at the University of Geneva in Switzerland, tells Popular Science. “They are remarkable beasts for multiple reasons. One of them is that they form the sister group (i.e., are the closest relatives) of birds and dinosaurs.”
According to Milinkovitch, the fact that their body scales and head scales develop so differently also sets them apart as animals. Crocodile body scales develop from what scientists call a polka dot pattern of gene expression during the embryo’s development. This is when particular genes turn on in specific and localized regions of tissue or an organ.
[Related: Tracing the crocodiles’ curious evolutionary family tree.]
“So, at each spot of high gene expression, cells become fated to form a skin appendage–a hair, a feather or a scale, depending on the species,” explains Milinkovitch.
However, crocodile head scales are a bit different from their body scales. While taking a blood sample from a Nile crocodile, Milinkovitch was struck by this unusual pattern of scales on its jaws and face, where some of the polygons had unconnected edges. This unusual pattern couldn’t really be explained by the primary genetic understanding of how sales form.
Milinkovitch suspected that a mechanical process was at play–and not genetics. Determining the precise mechanism has eluded scientists, as crocodile embryos are pretty difficult to come by.
Fold in the skin
It took over 10 years for Milinkovitch and his colleagues to gather enough embryos to conduct this new study. Once the team had several embryos to work with, they then combined experiments with the embryos and computer simulations to generate a 3D mechanical growth model that details the patterning of head scales on a crocodile.
They discovered that the scales are self-organizing through some familiar mechanical processes including compressive folding. This folding begins when the skin is growing faster than the underlying bone and when the skin itself becomes either more elastic or stiff. This folding and physical change then produces the irregular geometric patterns in the head scales as the crocodile grows in a distinctly different way.
“Our mathematical model demonstrates that the very different patterns of head scales in different species of crocodilians are easily obtained by changing slightly the mechanical properties of the skin,” says Milinkovitch. “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.”
[Related: Say hello to the surprising crocodile relative Benggwigwishingasuchus eremicarminis.]
Staining collagen
In order to pinpoint these mechanics, the team also needed to develop new ways of staining collagen. This protein builds skin, bone, ligaments, tendons, and other connective tissues and is also crucial for protecting the body’s organs among other biological roles. That new staining technique helped Milinkovitch see what mechanical properties collagen is giving to the skin and understanding how the collagen in the skin works has some applications outside of crocodiles.
“Our technique is now being picked-up by many researchers because the 3D architecture of collagen is very very important for understanding invasiveness of cancer tumours as well as for understaffing the aging of the skin,” says Milinkovitch.
Developing new methods for something as wide-reaching as collagen staining and figuring out what is going on underneath the skin of something like a crocodile also points to an often overlooked part of embryonic development.
“It [mechanics] plays a huge role in embryonic development and researchers in biology and physics are slowly realising this,” says Milinkovitch. “Stay curious, look around you, there are so many aspects of the living world we do not understand.”
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