These monkeys have a 'puberty switch' they flip when the right male comes along
Female geladas delay sexual maturation when dad runs the troop, and wait for an eligible male to take over
Sexual maturity in humans and other primates is usually determined by age, provided living conditions like health and nutrition are met. Geladas, a sister species to baboons, do not follow that model. They have the capacity to delay or advance sexual maturity depending on whether a viable mate is present. This seems to be an adaptation to avoid inbreeding.
Geladas are a species of monkey related to baboons, and live only in the highlands of Ethiopia. They're an unusual monkey as their diet consists almost entirely of grass.
They live in herds of about 1200 animals, socially organized into much smaller family groups composed of one dominant breeding male and several females who may possibly include his daughters.
In a new study, Jacinta Beehner, a professor of anthropology and psychology at the University of Michigan, and her colleagues, gathered long term data on 80 female geladas over a period of 15 years. They tracked date of sexual which is visible in the monkeys with the swelling of a patch of skin on the chest, and by looking at hormone levels in fecal samples.
On average geladas would normally sexually mature at about four-and-a-half years old. However females in a group led by their father would suppress sexual maturity well past this time as an inbreeding avoidance mechanism. When a new male arrives to oust the existing male, all the females in the group who have delayed sexual maturity quickly become mature.
In addition, they found the arrival of the new breeding male triggers sexual maturity early in females as young as three-and-a-half years. Geladas live to be about 20.