We may never know why Britney Spears shaved her head.

But she's hardly the first female celebrity to go hair-free. Nor even the first singer. The difference: Most of those women lost their locks for their art.

Historically, shaved heads have had practical purposes. In ancient Egypt men and women shaved their heads and wore wigs because it helped protect against lice, among other things.
 

Forcing a woman to shave her head also has been used as a means of control and humiliation: Thousands of women lost their hair in Nazi concentration camps.

These days, a shaved head is often a sign that a person is undergoing chemotherapy, which can cause hair loss, or is making a some kind of political or militant statement, said David Shmagin, stylist and manager of Robert Kree salon in New York, where clients include Drew Barrymore and Sarah Jessica Parker.

"It's a strength thing and to show they don't have to care about (their hair) because they are focused on other things," Shmagin said of women who cut their hair voluntarily.

Remember the nearly bald Sinead O'Connor tearing up a picture of Pope John Paul II on live television in 1992, exhorting viewers to "Fight the real enemy"?

Most celebrities who go without hair do so when the script calls for it.

Natalie Portman, who had long brown hair, shaved her head for "V for Vendetta," which was released last year.

Portman kept the hairstyle for several months and debuted it at the Cannes Film Festival during the premiere of "Revenge of the Sith," in which she played Darth Vader's love interest (who did have hair).

Demi Moore shaved her hair on camera while depicting the first female Navy SEAL in "G.I. Jane" in 1997. Sigourney Weaver shaved her locks for "Alien3" in 1992.

Persis Khambatta made bald beautiful as Lt. Ilia in "Star Trek: The Motion Picture." And Canadian supermodel Eve Salvail graced the cover of Elle magazine and the world's top runways (including for Chanel and Versace) sans hair.

Musicians have been more likely to shave their heads as personal fashion statements.

Erykah Badu got rid of her hair in 2001, saying she wanted to "make room for new things." India.Arie went hairless to the 2003 Grammy awards, explaining that her shaved head indicated "spiritual cleansing."

But unquestionably, bald women have not earned the same level of acceptance as purposely bald men have in American society.

Boston hairstylist Mario Russo of Salon Mario Russo said in 26 years of hairstyling, he has never fielded a request from a woman to shave her head, unless she was undergoing chemotherapy.

Bald women are still fair game to make fun of, too.

In the "Seinfeld" episode "The Beard," the follicly-challenged George tests out his new toupee on a blind date, yet rejects her when she turns out to be bald.

Stylist Shmagin said he suspected Spears' haircut was partly in reaction to her recent split from husband Kevin Federline.

Stylists don't like to do "breakup haircuts," he said, which may explain why Spears had hers done at a San Fernando Valley tattoo parlor.

Not long after her head-shaving and tatooing, she checked herself into a treatment facility. She left after less than a day.

Still, he said Spears "had been torturing her hair for so long" by wearing hair extensions and recently reportedly dying her locks herself, and may find that when her hair grows back, it looks better than ever.

Shmagin said he doesn't think Spears' haircut will be emulated since no one shaved their head after actress Portman did it "and she pulled it off well."

"No one wants to be like Britney, and that's so sad. We can all laugh and joke but I think once she shaved her head you really start feeling bad for her. Sweetheart, what happened? We want to know answers."