"There is no known biological function for breasts in men," explains Dr. Eldad Zacksenhaus. "We can only speculate on this issue." The associate professor of U of T's Department of Medicine says most of what is known about breast development in human embryos has been learned by studying mice. For example, both female and male mouse embryos sprout mammary buds when they are just a few days old. On the 13th day, however, a male embryo's testes begin to secrete androgens (testosterone) to suppress the mammary glands from developing further and to stop development of the nipples. Female mice embryos have five pairs of mammary glands but, unlike males, these embryos develop nipples.
female mouse mammary gland

Zacksenhaus says less is known about mammary gland development in human embryos but, at birth, both sexes have these glands and nipples. Underneath their newly formed skin, they also have the foundations of a rudimentary epithelium - a complex web of cells (epithelial cells) that form ducts branching out from their nipples. When girls enter puberty, estrogen and other hormones cause their breasts to grow and develop fatty tissue. During this stage, the epithelial ducts in girls' breasts branch out further. During pregnancy, hormones such as progesterone and prolactin (lactating hormones) signal these ducts to produce milk that is secreted through the mother's nipples.

All humans begin life in the womb as females. If no Y chromosome is present in the foetus, then the embryo will continue to develop as and be born as a female. If there is a Y chromosome present in the embryo, the male sex hormone testosterone restricts the full development of breasts to just nipples, the labia fuse to become the ******* and clitoris develops fully to become a *****. If the Y chromosome prevails in producing a male, this is not done without a fight. Male babies are weaker as a result than female ones, occounting for the slightly higher death rate in male babies.