Sexual differentiation begins during gestationwhen the gonads are formed. Cengage Learning. Biologists today distinguish between "male-to-male combat" and "mate choice", usually female choice of male mates.
Progestin-estrogen treatment of adult, castrated feminized males produced stimulation of the glandular tissue similar to that seen after treatment of castrated female animals. When administered to gravid rats during pregnancy an anti-androgenic steroid [cyproterone acetate] induced development of nipples in male fetuses.
As puberty begins and sex hormone levels rise, differences appear, though some changes are similar in males and females. Thus, this boy demonstrates that breast development may occur in postpubertal males if the programing of the pubertal sex differentiation of the mammary gland anlagen is disturbed by an enzyme defect which causes a failure of fetal testicular testosterone production.
These changes take place only after the puberty, and it involves complex mechanisms based on hormonal influence inside the bodies of animals. Breasts are usually classed as a secondary sexual characteristic even though breastfeeding could be classed as a later stage of the adult female body's participation in the reproductive cycle.
The category of primary sexual characteristics covers all organs in the body that play a role in reproduction. However, without well-developed sex organs, secondary characteristics will never operate.
See also: Mate choice. Male birds and fish of many species have brighter coloration or other external ornaments. Clinics in Rheumatic Diseases. Elsevier Health Sciences. These nipples and associated glandular tissues develop after birth as in normal female animals.
British Journal of Obstetrics and Gynaecology.