Crocodile scales are shaped by tissue growth, not genetics

Crocodiles, with their intricate head scales and striking physical features, exemplify the awe-inspiring morphological diversity found across life on Earth.

This profound variety, particularly in reptiles, sparks curiosity about the forces that shape such traits. Why do these differences occur? Genetics, you may presume, and you wouldn’t be entirely incorrect.

However, a recent study by an interdisciplinary team from the University of Geneva (UNIGE) is turning heads (and scales). As it turns out, physiological characteristics like the head scales of a crocodile have much to do with the mechanics of growing tissues instead of molecular genetics.

The unknown architects of crocodile scales

Often, the connection between an organism’s physicality and genetics feels like an obvious one. Michel Milinkovitch, a professor at the UNIGE Faculty of Science, pursued a more profound understanding of this link.

His research was focused on the evolution and development of vertebrate skin appendages such as feathers, hair, and scales.

In previous work, Professor Milinkovitch found that the development of crocodile head scales mirrors the propagation of cracks in a mechanically-stressed material.

A further exploration of this phenomenon led Milinkovitch and his team down a path of extraordinary multi-disciplinary research.

A journey into the unknown

The journey began with observing the embryonic development of the Nile crocodile. The team noted that the creature’s skin remained smooth until approximately 48 days into the expected 90-day gestation period.

Around day 51, skin folds appeared and interconnected to form irregular polygonal scales of varying sizes in different regions of the head.

To determine if the constrained growth of the skin could cause these folds and the subsequent scales, the team introduced a hormone (epidermal growth factor or EGF) known for stimulating skin growth and stiffening to the crocodile eggs. The result was a surprising change in the organization of skin folds.

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

Interestingly, once the EGF-treated crocodiles hatched, this brain-like folding simplified into a pattern of much smaller scales akin to those found in another crocodilian species – the caiman.

Skin growth and crocodile scales

To better understand how skin growth impacts scale formation, the research team utilized “light sheet microscopy” to measure the growth rate and geometries of various tissues that constitute the embryo’s head.

Using this data, the team created a 3D computer model to simulate skin growth under constraint. They manipulated the specific growth rates and stiffnesses of the tissue layers in the model to evaluate their impacts.

“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,” explained Ebrahim Jahanbakhsh, a computer engineer working with Milinkovitch.

Hence, this study offers powerful evidence that the mechanics of tissue growth can contribute to the diversity of certain anatomical structures across species, without the necessity of intricate molecular genetic factors.

In essence, it appears that nature, in its utmost wisdom, employs a multitude of tools to sculpt the astounding diversity of life that graces our planet.

Implications for evolutionary biology

This research not only reshapes our understanding of crocodile scale formation but also challenges traditional views in evolutionary biology.

By demonstrating that mechanical forces, rather than solely genetic factors, can drive the development of complex anatomical features, the study opens new doors for exploring the evolution of diverse organisms.

Such findings have far-reaching implications, extending to other species with unique morphological traits, such as the patterned scales of snakes, the carapaces of turtles, and even the feathers of birds.

The findings suggest that environmental and mechanical constraints might play a pivotal role in shaping these traits, alongside genetic blueprints.

Future research directions

Moreover, this interdisciplinary approach – blending biology, physics, and computational modeling – could inspire new methods for studying how form and function evolve in both living and extinct species.

By analyzing the physical forces acting on growing tissues, researchers may gain insights into the origins of ancient species and the environmental pressures they faced.

Ultimately, the study highlights the intricate interplay between genetics, mechanics, and evolution, reminding us that nature employs a versatile set of tools to craft the breathtaking diversity of life on Earth.

The full study was published in the journal Nature.

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