Veterinarians have been left baffled after a female lion suddenly sprouted a mane at an American zoo.

Bridget, an 18-year-old African lioness living at Oklahoma City Zoo began to develop the usually-masculine trait in March last year.

The cause of the shaggy growth around the lion’s jaw is so far unexplained, although zoo staff said they were analysing blood samples in an attempt to solve the mystery.

"After a while, it became obvious to everybody that Bridget was developing something a little different," Gretchen Cole, an associate veterinarian at the zoo told ABC News.

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

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

Manes generally develop in male lions at around the age of one due to a heightened level of testosterone production.

While it is extremely rare for lionesses to develop manes, it is not unheard of and scientists have observed several cases in the wild in recent years.

In 2011, Emma, a 13-year-old lioness at the National Zoo in South Africa began growing a mane similar to that of an adolescent male.

Zoologists discovered the growth was related to a problem with her ovaries, which caused excessive testosterone production and the mane receded once the issue was resolved.

A group of five lionesses sporting manes were discovered in Botswana during 2014, as well as exhibiting typically male behaviour such as roaring.

Scientists believe that as the lions came from the same pride, the trait may have been passed on genetically and could even serve as an advantage by making it appear to competitors as if there are more males protecting the group.

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Veterinary staff at the zoo say Bridget’s mane is unlikely to affect the quality of her life and that no other outward changes to her health or behaviour have been observed.

They believe blood results could reveal the rare phenomenon has been caused by hormone imbalance.

However, the growth may have also been sparked by a benign tumour on the lioness’s adrenal or pituitary gland, causing irregularities in the way her body regulates testosterone.