#period sync

When male lions strike out on their own, they will challenge another male for control of his harem. If the interloper succeeds, he'll kill all the cubs, which brings the females into estrus, or heat, again. (See "Why Do Animals Sometimes Kill Their Babies?")

TIMING IS EVERYTHING

Though many other animals come into heat at the same time, fewer species go into heat when their young die. Instead, most go into heat seasonally, including most wild species of hooved mammals, which only give birth in the spring. Male deer testosterone peaks in fall, during the “rut,” when they will compete for and mate with females. Does are pregnant during the winter and give birth around May and June, when warmer weather helps fawns survive.

If lions had periods, lionesses within a pride might get them at roughly the same time. But lions don't menstruate: The only mammals that menstruate overtly the way that humans do are some other primates and a few species of bats and rodents.

A persistent myth is that people living in close proximity—such as several women sharing a dorm in college—synchronize their estrus cycles, which is evidenced by having periods around the same time. This idea, which caught on due to research by psychologist Martha McClintock in 1971, has been discredited in several studies since.

In 2006, Zhengwei Yang and Jeffrey C. Schank found the chance that a woman would share a cycle with someone living with her is about as likely as sharing it with anyone else. (Read "The Scientist Who Said Periods Weren’t a Big Deal.")

Because women have slightly different menstrual cycles, there is a good chance that, if two women spend enough time together, they will eventually match cycles.

“It’s just a mathematical property of irregular rhythms, and rhythms of different lengths,” says Schank. He adds that it’s human nature to notice when things match, but not to notice when they don’t match.