Lionesses in the Okavango Delta of Botswana have grown manes and are roaring, and showing other male-like behaviours.
One of the lionesses has begun mounting females as well as scent-marking more frequently and even killing cubs, which are all behaviours observed much more frequently in males.
Nobody is sure exactly what causes these changes but theories encompass genetic and hormonal explanations.
Geoffrey D Gilfillan, of the University of Sussex, first reported five lionesses with manes at the Moremi Game Reserve in Botswana in March 2014.
For the last two years he has been studying the behaviour of one of them, called SaF05. She had an underdeveloped mane and was larger than most females.
"While SaF05 is mostly female in her behaviour – staying with the pride, mating males – she also has some male behaviours, such as increased scent-marking and roaring, as well as mounting other females," he told New Scientist.
"Although females do roar and scent-mark like males, they usually do so less frequently," he said. "SaF05, however, was much more male-like in her behaviour, regularly scent-marking and roaring."
Gilfillan said the aggressive behaviour went as far as attacking other prides in a fight over a zebra.
"A neighbouring pride stole the zebra from SaF05, but in return SaF05 killed two of their cubs," he said.
While the phenomenon appears to be peculiar to the Moremi region, with regular sightings of another maned female named Martina being reported between the late 1990s and 2002, similar effects have been observed in captive animals.
In 2011, a lioness at the National Zoological Gardens of South Africa developed a mane. Tests revealed high levels of testosterone due to a problem in her ovaries, and once they were removed she reverted to a typical lioness.
The role of testosterone in the case of the Botswana lionesses is also backed by observations of their reproductive success, said Kathleen Alexander at Virginia Tech in Blacksburg.
"While some of the maned lionesses were observed mating, none of them became pregnant, suggesting they are infertile, a known consequence of high levels of androgens such as testosterone in females," she said. "The behavioural changes suggest this is likely the case."
Luke Hunter, president and chief conservation officer at the global wild cat conservation organisation Panthera, said that there must be a genetic element to the condition.
"Given all five known maned females come from the Okavango region, there must be a genetic component in this population underlying the phenomenon," he said.
"I don't think this is anything to be concerned about," said Hunter. "Although the females are apparently infertile, they otherwise appear to live long, healthy lives.
"And from a conservation perspective, there is nothing to suggest the pattern is increasing or will ever be anything more than a rare, local phenomenon."