After Yakei’s altercation with Nanchu, reserve workers performed what is known as a “peanut test”: providing the monkeys with peanuts and seeing who eats first. Males and females stepped aside to let Yakei eat first, a confirmation of her alpha status.
Since then, “Yakei has shown some behaviors typically seen only in dominant males, such as walking with her tail up and shaking tree branches with her body. It sounds as if she is behaving like an adult male, being more aggressive than other individuals,” said Mr. Kaigaishi, who is not affiliated with the reserve.
Although Yakei seems to be leaning into her role, she is likely to face challenges. The reign of an alpha can last from a few months to over a decade. But observers of the troop say that mating season could change things for Yakei.
Mating time at the reserve, which typically runs from November until March, is in full swing. During the breeding season, male and female macaques, which are polygamous, form pair bonds. They mate, feed, rest and travel with their partners for an average of 16 days. After this period, the bond dissolves, and the females seek new partners. Females typically mate with an average of four males each breeding season.
“Mating season generally heats things up in Japanese macaque society,” said Katherine Cronin of the Lincoln Park Zoo in Chicago, who studies animal social behavior and cognition, including of her zoo’s own Japanese macaques. “The environment becomes more competitive and tense.”
During previous breeding seasons, Yakei paired with Goro, a 15-year-old male ranked sixth within the troop. According to the reserve, the Japanese newspaper Manichi Shimbun reported, Goro bit Yakei in the face in 2019, a move that caused her bottom to flush bright red — a sign she was ready and willing to take him as a mate. However, since Yakei has claimed the troop’s top spot, Goro seems to have lost interest in her.