Bridget the lioness was born at the Oklahoma City Zoo in 1999, and she lived a fairly typical zoo lion life for most of her first 18 years. Then she grew a mane.

“At first, you’re kind of like, well, that’s a little different,” said Jennifer D’Agostino, the zoo’s director of veterinary services. “Then it kind of kept going to the point where it was like, wow, it looks like she’s growing a mane. That’s not quite right.”

Bridget’s behavior had not changed since her mane started to grow in March. She seemed unbothered by her new look, and her fellow lions, half-sister Tia and a younger male named Hubert, were unfazed. But by November, the mane looked like a thick scarf, and the zoo decided to try to untangle the mystery of this bearded lady.

The likely explanation, D’Agostino said, was testosterone — the hormone that makes male lions develop manes at around one year of age. An overproduction of that hormone is also implicated in the lush locks of wild maned lionesses in Botswana’s Okavango Delta, but the causes are not clear, said lion expert Luke Hunter. It could be that the sperm of the lionesses’ fathers was “slightly aberrant,” causing a disruption of the embryos, or that the lionesses’ mothers had abnormally high levels of male sex hormone during pregnancy, he said. Researchers who study the Botswana maned lionesses say they have never become pregnant, which can be a consequence of elevated testosterone.

“Both situations occur occasionally in humans and other mammals, but, of course, we are less likely to observe it in wild animals,” said Hunter, a biologist who is chief conservation officer for Panthera, a global wild cat conservation organization. “Whatever the cause, we know that while they are infertile, maned lionesses in the wild are otherwise perfectly capable of survival.”

The case of Bridget’s mane is different, however. She grew a mane at an age that is not quite elderly but is well beyond middle-aged for a captive lion. D’Agostino said that suggests it is caused by a tumor of some sort. But she added, “If it’s not going to affect her health in any way, the fact that she has a little bit of a mane is not that big a deal.”