Generations of girls were told to “eat your crusts, they'll make your hair curl” – at least in eras when curls were in fashion. Leaving aside the sexist assumption that curly hair is just for one gender, the idea that eating more heavily baked bread will change the curvature of keratin doesn't have a lot of evidence. So what makes some hair curl and others not? A study of merino wool fibers provides an answer.

Human hair is too thick to examine the cell structure easily, but the fine curly wool of merino sheep is an easier matter. Dr Duane Harland of New Zealand's AgResearch considered it the perfect material to study to test two competing theories of hair curvature.

According to one theory, cells divide more often on one side of the hair, thus making it longer than the other and therefore forcing curls. The alternative theory also relies on hairs' convex side being longer, but proposes the cells on that side are elongated.

After studying multiple merino hairs using high-powered microscopes, Harland and co-authors report in the Journal of Experiment Biology that “in all cases, the orthocortical cells close to the outside of curvature were longer than paracortical cells close to the inside of the curvature, which supports the theory that curvature is underpinned by differences in cell type length.” The authors think the same is probably true for all mammals.

Case closed? There are the same number of cells on each side of the hair, but some are longer than others? Not so fast. The authors also note that it would be expected that the amount of curliness should correlate with cell proportions. Oddly, however, this is not the case.

Instead, the authors report, that “the absolute length of cells of each type, and proportion of cells, varies from fiber to fiber, and only the difference between the length of the two cell types is important.” The authors argue cell length is shaped in the follicle, but can be influenced thereafter through external factors such as the formation of chemical bonds that alter cells' shape.

We think of straight or curly hair as being a largely aesthetic matter, but the flexibility of fur is one of the things that has helped mammals conquer the world. The paper gives the example of deer, where “straight high-diameter guard hairs scaffold a mass of narrow diameter curly underhairs.” The combination provides protection against the weather and some parasites.

Wild sheep have a similar hair structure to deer, but generations of farmers have bred merinos to have thin curly hairs both in their under and overlayers, producing excellent woolen clothing.