How Tortoises Grow Their Scales: Two Systems, One Head

The Milinkovitch Lab have conducted a new study that uncovers how tortoise head scales are patterned through two distinct developmental processes —chemical reaction-diffusion and mechanical folding— operating in separate regions of the head. This dual mechanism sheds light not only on skin morphogenesis in reptiles but also on the evolutionary history of pattern formation across vertebrates.

Placodes and Reaction-Diffusion: The Role of Interacting Molecular Genetic Signals

In vertebrates, skin appendages such as scales, feathers, and hair often develop from placodes —small clusters of cells formed through well-conserved molecular signaling processes. These placodes are typically arranged via reaction-diffusion (RD) systems involving activator and inhibitor molecules that create spatially-organized spots where appendages will form.

In tortoises, our researchers now find that the lateral and peripheral regions of the head follow this classic developmental pathway. Using in-situ hybridization, they detect the expression of canonical placode markers such as β-catenin and sonic hedgehog in these regions during embryonic development. This confirms that the scales in these areas arise from chemically-patterned placodes, similar to those observed in mammals and birds.

Mechanical Folding Shapes Central Head Scales

While the spatial distribution of peripheral scales is patterned through RD, the central dorsal surface of the tortoise head follows a different path. Here, scales form not through chemical cues but via mechanical forces. Our researchers find no expression of placode markers in this area. Instead, they observe tissue features —such as unjoined polygonal scales and fine-scale skin creases— characteristic of mechanically-induced folding.

To test this, our team uses light sheet fluorescence microscopy and nanoindentation to study skin layer geometries and stiffness. Their data show that as the skin stiffens due to keratinization, and as the underlying skull ossifies, compressive stress from constrained skin growth triggers surface folding. These features closely mirror the scale formation process in crocodiles, which also rely on mechanical patterning (check our video here).

Computer Simulations Support Mechanical Origin

Our researchers built a 3D finite-element growth model based on tissue geometry and stiffness data. These simulations successfully reproduced the scale folding patterns seen in sulcata, Greek, and marginated tortoises. The ability of the model to recreate interspecies variation in head scale patterns underscores how changes in tissue growth and material properties can drive morphological diversity.

Evolutionary Implications: A Shared Heritage

Interestingly, the dual patterning of head scale seen in tortoises —chemical in the periphery, mechanical in the center— bridges characteristics of both squamate reptiles and archosaurs (crocodiles and birds). Since tortoises are the sister group to Archosaurs, our researchers' results suggest that mechanical head scale patterning likely evolved in their common ancestor and was later lost in birds.

This discovery emphasizes the versatility of developmental systems —how the interplay of chemistry and physics sculpts form— and provides fresh insight into the evolutionary dynamics of skin morphology in reptiles and beyond.

Much additional information is available in the original article:

Chemical and mechanical patterning of tortoise skin scales occur in different regions of the head
Cooper R.L., Jahanbakhsh E. & M.C. Milinkovitch
iScience 28(6), 112684