It is a special deformation of sweat glands. Although only women have mammary glands, we are all the same in the embryo, regardless of gender. In the development, the embryo has always been a female model, until six weeks later, the sex chromosome in the male embryo did not appear. At this time, male embryos begin to develop their sexual characteristics. Therefore, men have nipples and some breast tissues, and men may even get breast cancer. In some cases, men will have breast hyperplasia. Abnormal hyperplasia of male mammary glands is called "male mammary dysplasia". This disease can be caused by anabolic steroids. So, if Barry Bonds returns to the "Old Star Competition" in a few years, and he has drooping huge breasts, then I think everyone will know about the steroid debate.