Zoologists puzzled after 18-year-old lioness grows 'exceptionally rare' mane Bridget the African lion is acting "completely normal," despite her new hairdo.

 -- An 18-year-old lioness at the Oklahoma City Zoo is puzzling zoologists after she mysteriously sprouted a mane.

In late 2017, caretakers noticed that Bridget the African lion, a "beloved" longtime resident of the Oklahoma City Zoo, began to grow the extra hair around her neck and head, Gretchen Cole, an associate veterinarian at the zoo, told ABC News.

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

Curious veterinarians do not yet know what the cause for the "exceptionally rare" growth is but were able to take a blood sample from Bridget's tail without placing her under anesthesia after her caretakers spent several days training her to position it to the side.

The blood sample will be compared to a sample taken from Bridget's sister, 18-year-old Tia, who was born in the same litter as her in 1999, Cole said.

Manes typically develop in males at about a year old after they experience an increase in testosterone.

"Changes when a female develops characteristics of a male are unusual," Cole said.

Other "unique" cases in which female lions have developed a mane include a 13-year-old lioness at the National Zoo in South Africa in 2013, who had an issue with her ovaries that caused an excess production of testosterone.

In 2014, a group of five lionesses in the Botswana wildness were observed with manes. In that case, researchers believe a genetic component played a role, since the lions came from the same pride and developed the manes at a young age, according to the Oklahoma City Zoo.

While Bridget's veterinarians suspect hormones may play a role, that would also be the first factor to be ruled out once they get the results of the blood work, Cole said. Another culprit could be a benign tumor located on her adrenal or pituitary glands, which regulate hormones, the zoo said.

Bridget's age places her in the "geriatric" category of lions, Cole aid. The median lifespan for lions is 16.9 years, according to the Association of Zoos and Aquariums.

Bridget doesn't seem to notice her new hairdo and is acting "completely normal," Cole aid. She's eating and grooming the same, and, more notably, her "strong attitude" is still present.

"It's only the outward appearance of the mane that has changed," she said. "... We are trying to solve the puzzle."