Veterinarians at the Oklahoma City Zoo are baffled by an 18-year-old lioness’ new hairdo.

The lioness, Bridget, mysteriously began to grow a mane between March and November of last year, the zoo said.

While male lions typically develop a mane at one year of age due to an increase in testosterone, it is “exceptionally rare” for females to ever grow the extra hair, Gretchen Cole, an associate veterinarian at the Oklahoma City Zoo, told ABC News.

"After a while, it became obvious to everybody that Bridget was developing something a little different," Cole said.

While Bridget's mane is rare, it’s not the first time a lioness has grown a mane, according to the Oklahoma City Zoo.

A 13-year-old lioness living at the National Zoo in South Africa began to develop a mane in 2011 due to a problem with her ovaries. Her body produced too much testosterone as a result. And five lionesses living in Botswana were seen with manes in 2014, according to the Oklahoma City Zoo.

A benign tumor on Bridget’s adrenal or pituitary glands could also be the cause, according to the zoo, as those glands regulate testosterone and other hormones.

There’s also a chance that Bridget's condition could be genetic.

To determine the cause, veterinarians took Bridget’s blood and will compare it to a sample of her sister Tia’s blood. Unlike Bridget, Tia, who is also 18-years-old, has remained mane-less.

Thankfully, Bridget’s veterinarians don’t think the lioness’ quality of life will be affected by the condition. Overall, Bridget, who is considered to be “geriatric,” is in good health, Cole said, adding that big cat has retained her “strong attitude.”