From Jared Diamond’s Why is Sex Fun
Chapter 3- Why Don’t Men Breast-feed Their Babies?
If a developing embryo has a Y chromosome the bipotential gonad develops into a testicle in the 7th week. If no Y chromosome is present the gonad becomes an ovary in the 13th week. A female is produced if there is only one Y chromosome, and a male is produced if there is one Y and two X chromosomes. A woman is the default condition.
If a testicle is present it will produce testosterone, which gets converted into dihydrotestosterone. These androgens convert some all-purpose embryonic structures into the glans penis, penis shaft, and scrotum. These same structures would otherwise develop into the clitoris, labia minora, and labia majora. Embryos start out with two sets of ducts, the Mullerian and Wolffian. In the absence of testes the Wolffian ducts atrophy, and Mullarian ducts develop into a uterus, fallopian tubes, and interior vagina. If testes are present, the Mullerian ducts atrophy and the Wolffian ducts develop into the seminal vesicles, vas deferens, and epididymis.
There are a number of different sorts of male pseudohermaphrodies. One type lacks the cell receptor, which binds testosterone and dihydrotestosterone, and develops into a normal looking, and frequently quite attractive appearing “female”.
Sexual dimorphism varies quite a lot between species. In some species males and females are very much alike, in others quite different.
Human females who are not pregnant and even virgins sometimes lactate, and men have been known to lactate after starvation or when hormones are administered. Lactation has been described in male goats and in male fruit bats.
Diamond thinks that men don’t lactate because of “evolutionary commitment”, that is it is difficult for an organism to change after millions of years of doing things the same way. He seems to suggest that male lactation could evolve sometime in the future for human males.