Nature’s Designs: How Spots and Stripes Are Formed

From spots on leopards to stripes on zebras to hexagons on boxfish, nature creates an almost infinite array of dazzling patterns and stunning colors. But a full explanation for how these designs form has remained elusive—until now.

Recently, in a paper published on November 8, 2023, in the journal Science Advances, engineers at the University of Colorado Boulder showed that the same physical process that helps remove dirt from laundry could play a role in how colorful, sharp patterns develop on leopards, tropical fish, zebras and other creatures.

And, in time, that could help in the advancement of high-tech materials and new medicines.

Diffusion: fuzzy outlines

Scientists say that many biological questions are fundamentally the same one: how do organisms develop complicated patterns and shapes when everything starts off from a spherical clump of cells?

Previously, biologists demonstrated that many animals evolved to have coat patterns that attract mates or that provide camouflaging. While genes encode pattern information—such as the color of a leopard’s spots—genetics alone do not explain where, exactly, the spots will develop.

In 1952, before biologists discovered the double helix structure of DNA, Alan Turing, the mathematician credited with inventing modern computing, proposed a bold theory for how animals get their patterns.

Turing hypothesized that as tissues develop, they produce chemical agents. These agents diffuse through tissue in a process that’s like adding milk to coffee. Some of the agents react with each other, forming spots. Others inhibit the spread and reaction of the agents, forming spaces between the spots. Turing’s theory suggests that instead of complex genetic processes, this simple reaction-diffusion model could be enough to explain the basics of biological pattern formation.

But, the University of Colorado Boulder researchers reason, while Turing’s mechanism can produce patterns, diffusion doesn’t yield sharp ones like those found on leopards and zebras. For instance, when milk diffuses in coffee, it flows in all directions with a fuzzy outline.

Diffusiophoresis: sharp edges

That’s when Benjamin Alessio, the University of Colorado Boulder paper’s first author, visited the Birch Aquarium in San Diego, California. While watching an ornate boxfish, he became impressed by the sharpness of the animal’s intricate pattern: a purple dot surrounded by a distinct, hexagonal, yellow outline. Turing’s theory alone would not be able to explain the sharp lines of these hexagons, he thought. But the pattern did remind Alessio of computer simulations he had been conducting, where particles do form sharply defined stripes. Alessio wondered if the process known as diffusiophoresis plays a role in nature’s pattern formation.

Diffusiophoresis happens when a molecule moves through liquid in response to changes, such as differences in concentrations, and accelerates the movement of other types of molecules in the same environment. While it may seem like an obscure concept to nonscientists, it’s how laundry gets clean.

One recent study showed that rinsing soap-soaked clothes in clean water removes the dirt faster than rinsing soap-soaked clothes in soapy water. This is because when soap diffuses out of the fabric into water with a lower soap concentration, the movement of soap molecules draws out the dirt. When the clothes are put in soapy water, the lack of a difference in soap concentration causes the dirt to stay in place.

The movement of molecules during diffusiophoresis, as Alessio observed in his simulations, always follows a clear trajectory and gives rise to patterns with sharp outlines.

To see if diffusiophoresis may play a role in giving animals their vivid patterns, Alessio and his team ran a simulation of the purple-and-yellow, hexagonal pattern seen on ornate boxfish skin using only the Turing equations. The computer produced a picture of blurry, purple dots with a faint-black outline.

Then, the researchers modified the equations to incorporate diffusiophoresis. The result turned out to be much more like the bright, sharp, bicolor, hexagonal pattern seen on the boxfish.

The team’s theory suggests that when chemical agents diffuse through tissue as Turing described, they also drag pigment-producing cells with them through diffusiophoresis —just like soap pulls dirt out of laundry. These pigment cells form spots and stripes with a much sharper outline.

Development: from patterns to pathbreaking products

Decades after Turing proposed his groundbreaking theory, scientists have used the mechanism to explain many other patterns in biology, such as the arrangement of hair follicles in mice and the ridges in the roofs of the mouths of mammals.

But diffusiophoresis may have been underappreciated in the field of pattern formation. The University of Colorado Boulder scientists hope that their study—and more research that’s currently underway—will not only improve understanding of how patterns in nature are created but inspire others to develop innovative materials and trailblazing medicines. It could also provide an opportunity to investigate the role of diffusiophoresis in other biological processes, such as embryo formation and tumor formation.

Dreams: wishing for spots and pining for pinstripes

Spots and stripes found in nature’s feathers, fins and fur are stunningly beautiful and functionally complex. Giraffes can tell each other apart by their differently shaped spots, and a baby okapi follows its mom’s stripes to travel through the forest. Deer, emus and tigers use the patterns on their bodies to hide in their environments.

I have often wished that I had stripes. I fancy a blaze of white cascading from my chin into my neck or bands of black extending down my arms.

I’m sure the designs decorating my body would prove to be not only lovely to look at but also highly handy.

Here’s to finding your true places and natural habitats,

Candy