What Exactly Makes Hair Curly?

If you’re a curly-haired person, or just a coveted straight-haired person, you may be wondering what causes some strands to spin and others to just stay flat. According to a 2018 study published in the Journal of Experimental Biology, it turns out there are two theories.

The first hypothesis states that the curl can be explained by a higher number of hair cells on the convex side of the follicle – the outer edge of the curl – and a lower number of hair cells on the concave side – the inner edge. The relatively low number of cells on the inner edge creates a shorter edge, which pulls the follicle inward, creating a curl.

Another theory suggests that the difference in cell length between the convex and concave sides of the hair strand can explain the curl. Similar to the first theory, the difference in relative size between the elongated cells on the outside and the compact cells on the inside produces the curl.

Curly hair is thought to keep mammals warmer than straight hair alone. In fact, straight and curly hair are intertwined to form a last line of defense against heat loss.

“The typical mammalian hair structure is that of a forest and shrub,” Says study lead researcher Duane Harland, a senior scientist at AgResearch, one of New Zealand’s largest corporate government research organizations. Straight hairs affixed to it “create a space near the skin,” and “finer, curly hairs fill that space and trap air,” Harland told Live Science.

However, Harland’s study only addressed the curly hair in Merino sheep. It’s difficult to determine if insulation is also what drives some humans to develop curly hair.” The short answer is that no one specifically understands human hair,” said Harland.” Our social aspects and ability to develop technology that replaces functions that were originally covered by biology, such as hats, make it difficult to be sure.”

Still, we can learn a lot about the origins and biology of our own hair follicle function from our more furry friends, because if you go back far enough, our hair evolved from the same genetic origins.” Mammalian hair is ancient,” says Harlan. It may have developed before dinosaurs, Harlan notes. (According to a 2016 study in the journal Scientific Reports, an analysis of fossilized skulls of 29 ancient mammal relatives, therapsids, suggests that these mammals had hair in their predecessors.)

To test both theories, Harland and his colleagues used sophisticated microscopy techniques to zoom in on wool fibers and measure differences in the number and size of cells on the inside and outside of the curves. Almost immediately, the team found evidence that dispelled the idea that the number of cells was causing the curls.

“We found evidence that contradicts the theory that curling results in more cells on the side of the fiber closest to the outside,” he says. That’s because, in all cases, he found that the cells on the outside of the curve were longer, “which supports the theory that curvature is supported by differences in cell type length,” Harland says.

Alas, it’s not that simple. The story, as science often does, doesn’t end there.” We haven’t quite gotten to the bottom of the curls yet,” says Harland. His study only looked at a single cross-section of sheep’s wool hair fibers under a microscope. That snapshot of the hair could be caused by bending, he says, but it could also have some twisting forces that confuse the results.

Even if those cross-sections are a good representation of the hair strands as a whole, that doesn’t mean that the other theory – the one that says the number of cells is the cause – is wrong. It may be that the hairs of different animals are curled for different reasons.

“It would be pure hubris to claim that this condition is universal,” Harlan points out.

“So there’s room for further discoveries,” he says.” If other scientists out there are busy replicating and building on our research, that’s great. Maybe they’ll find out we got something wrong, if anything, hopefully it’s just something small, but that’s science.”