Why does this chimpanzee have a swollen buttocks?

I was at the zoo today and i noticed that this one chimpanzee has a really swollen buttocks, almost hanging. I thought that it might be pregnant but i don't know.All the other chimps were normal. I want serious answers please. Thank you.|||She's in heat. It's a signal that she is ready to mate. The males see her swollen area and know it's time to get on with business of propagation. Different animals have developed different strategies to signal the correct breeding time. Female cats go about caterwalling loundly all night. Some animals send out aromas that attract the males. With the Chimpanzee the swollen buttocks area is their signal. It will return to normal after she is impegnated or her cycle passes.