Sexual swelling
![Red colored baboon buttocks](/Images/godic/202502/10/Baboon_buttocks2853.jpg")
![Sexual swelling in female hamadryas baboon.](/Images/godic/202502/10/Sexual_swelling_in_female_Hamadryas_baboon2853.jpg")
Sexual swellings are enlarged areas of the perineal skin occurring in some female primates that vary in size over the course of the menstrual cycle. Thought to be an honest signal of fertility, male primates are attracted to these swellings; preferring, and competing for, females with the largest swellings.