OKLAHOMA CITY — Lab results have revealed the answer to a mystery at an Oklahoma zoo: Just what caused a female lion to sprout a mane.

The Oklahoma City Zoo says in its March newsletter that testing at the University of Tennessee found the African lioness named Bridget has an elevated level of androstenedione, a hormone that can contribute to developing male features.

Veterinarians compared Bridget’s blood to samples from her sister, who has no mane. Bridget’s blood also contained a higher level of cortisol, which regulates metabolism and the immune system.

The zoo says the results likely mean the 18-year-old lioness has a benign tumor that’s producing the hormones, but that her health is excellent.

The zoo says female lions with manes are not unheard of, but they are rare.