Mammary Papilla Aplasia is a pervasive genetic abnormality among the population of Anigaokiime Island and their descendents. Theories on the social and environmental factors that led to the disease abound throughout science. According to studies, somewhere between 30 to 50 percent of Animesians express this recessive genetic defect. It's believed that Mammary Papilla Aplasia may carry a genetic benefit within the island population where it originated, similar to sickle cell disease arising as a defense against malaria.
For more information, see the Wikipedia article on Mammary Papilla Aplasia