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.
The mane is more Santa Claus beard than king of the savanna, but her keepers were nonetheless puzzled when it began sprouting last March.
"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 behaviour had not changed. Her appetite remained healthy. She seemed unbothered by her new look, and her fellow lions, half-sister Tia and a younger male named Hubert, were similarly 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, lion expert Luke Hunter said in an email. 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 organisation. "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, perhaps on her ovaries, adrenal gland or pituitary gland. The first investigative step would be a blood test to compare Bridget's testosterone level with that of a mane-free lioness - in this case, with a stored sample from Tia, who also is 18.
That requires a blood sample, and getting a blood sample from a lion is not so easy. On the rare occasions it's done, blood is drawn under anesthesia, D'Agostino said, but that comes with risks that the zoo did not want to take with a lion of Bridget's age. Keepers had been planning to train the zoo's big cats to put up with blood draws, so they "brought her to the front of the line," D'Agostino said.
Over a couple months, zoo staff used a standard training technique called operant conditioning, during which an animal is rewarded for performing the desired behaviour. When she lay down in a special crate, a keeper clicked a clicker and Bridget got her favourite treat of horsemeat (imported from Canada; the United States no longer has horse slaughterhouses). When she went further into the crate, she got another click and a hunk of meat. More rewards followed when she allowed staff to use a hook to pull her tail through a little opening, then hold her tail, then press it with a blunt-tipped needle, and, finally, when she permitted them to insert the needle into a vein in her tail.
"She did fantastic, because she's just so smart. She caught on really fast," D'Agostino said. "Once she had her meat, she had no problem."
So far, the zoo has drawn only enough blood to do routine bloodwork, all of which has come back normal, D'Agostino said. Caretakers plan to try again this week to get a sample for a hormone test.
"Every time we do this, we don't always get a huge volume of blood," and it's important not to drag out the process, she said. "We don't want it to become a negative experience."
If the levels suggest a tumour, zoo staff will then try to determine its location. Treatment could involve hormone supplements to suppress the testosterone, or surgery as a last resort, D'Agostino said. But that would happen only if Bridget's medical team believes it is necessary or would improve her quality of life, she said.
"If it's not going to affect her health in any way," D'Agostino said, "the fact that she has a little bit of a mane is not that big a deal."