Why do only some animals have periods?

The menstrual cycle plays an essential role in human reproduction. However, most other Animals don't experience menstruation.

So, which other species have periods, and what's the evolutionary point of bleeding periodically?

According to Deena Emera, an evolutionary biologist at the Buck Institute for Research on Aging, scientists know of around 15 maMMAl species that have a menstrual cycle. Most of these are primates, including our closest living relatives chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). Scientists have also discovered menstrual cycles in a few species of bats, elephant shrews and most recently spiny mice (Acomys cahirinus).

Because these animals aren't all closely related, the trait likely evolved convergently, meaning there must be some evolutionary benefit to it, Emera told Live Science.

Related: Are you genetically more similar to your mom or your dad?

Beyond these creatures, there are other Animals that periodically bleed through their reproductive organs. Owners of unspayed dogs may know the unfortunate experience of finding blood on their favorite couch and realizing their Pet has gone into heat, also called estrus. However, the bleeding that dogs experience comes from a different source than in menstruating Animals.

In animals that bleed while in estrus, an increase in the hormone estrogen while the animal is fertile causes the blood vessels inside the vagina to dilate. This results in small amounts of blood leaking out of the vessels and getting expelled.