In a rarely seen phenomenon in the simian world, a nine-year-old female known as Yakei has become the boss of a 677-strong troop of Japanese macaque monkeys at a nature reserve on the island of Kyushu in Japan.

Yakei’s path to the top began in April when she beat up her own mother to become the alpha female of the troop at the Takasakiyama Natural Zoological Garden in the city of Oita. While that would have been the pinnacle for most female monkeys, Yakei decided to throw her 10kg weight around among the males.

In late June, she challenged and roughed up Sanchu, the 31-year-old alpha male who had been leader of “troop B” at the reserve for five years.

Surprised wardens at Takasakiyama, where there has never been a female monkey boss in the reserve’s 70-year history, carried out a “peanut test” on June 30, putting out nuts for the group and seeing who ate first.

Sanchu backed away and gave Yakei first dibs on the treat, confirming her alpha status.

“Since then, Yakei has been climbing trees and shaking them, which is an expression of power and a very rare behavior in females,” said Satoshi Kimoto, a guide at Takasakiyama.

“She has been walking around with her tail up, which is also very unusual for a female,” Kimoto said, adding that staff at the reserve were at a loss as to the causes of Yakei’s dominant antics.