Is Serum Testosterone/Estradiol Ratio, an Indicator of Nipple's Primary Function in Humans?

Objective: Retention of nipples in men has been a biological curiosity. We were intrigued by the fact that if nipples do not serve any function, why it failed to get lost (like tails) during the course of evolution in the past 5 million years. Methods: Published literature on nipple and areola in men and women related to physical dimensions, histology, functional roles, total serum testosterone and serum estradiol levels were studied. Results: Nipples have two recognized functions in humans. The primary function is nutritional (i.e., suckling the young) and present only in women. The secondary function of nipple is becoming an erogenous zone during sexual arousal. This function has been positively reported in both sexes, albeit in lower percentage among men (50-60%) in comparison to women (over 80%). Calculated serum testosterone/estradiol ratio for men varies from 142-189 among men, within an age range 29 - 72 years. In pre-menopausal women (mean age 39.9 yr), testosterone/estradiol ratio is 2. But it rises to almost 13 fold to 25.9 in post-menopausal women (mean age 56.4 yr). As is known, post-menopausal women lose the primary function of nipple during menopause. Conclusion: Related to nipple development and sex spectrum in humans, we propose a hypothesis that total testosterone/ estradiol ratio in an individual influences the nipple development during pubertal onset and the position of that individual among the 9 stages of sex spectrum. This hypothesis is testable in humans, if individuals who exhibit intermediate genetic pattern of sexes can be identified and tested. [ABSTRACT FROM AUTHOR]