Hormones give pubic hairs their curl.
Sex hormones make pubic hair follicles an oval shape, according to Andrew Thompson, author of "Why Skies are Blue and Parrots Talk: Answers to the Questions You've Always Wanted to Ask" (John Blake, 2005).
Oval-shaped follicles give rise to similarly shaped hairs. The oval shape puts a curly kink in each hair.
Conversely, Thompson says straight hair sprouts from round follicles, which are less likely to curl pieces of hair.
The purpose of curly kinky pubic hairs remains a mystery to scientists, however.