Testosterone relax in the fetal male rat happens prior to the phase of mammogenesis whereby the teat is created whereas other types halt the same procedure after the teat is formed.

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According come the exact same page, masculine rats and horses (and mice) space unique among non-monotreme mammals in not having actually nipples.

Male humans have nipples since there"s no evolutionary press for them not to. (Why execute men have nipples?) Then, why to be there an option for males to lack nipples in the case of mice, rats, and also horses?

(Apparently, male horses lack nipples since "female horses have actually their nipples between their hind legs, and also in males, that genuine estate is used for other purposes". (Straight Dope))

Evolutionarily, why do male rats and also horses lack nipples?


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Male mice lack nipples too. Mouse are frequently used for embryonic research study as castle are small and give birth quickly. That is believed that masculine mice do build nipples, but that castle regress during development (Wysolmerski, 1998).

In general, the is believed that mammalian organisms develop as females by default once there is no male (Y) chromosome existing (Hughes, 2004). Notably, X_ individuals and also XY people with certain Y-chromosome deletions likewise develop together females. Basically, the is testosterone the suppresses female features. Hence, likewise boys through testosterone production, however without the proper receptors also develop as females.

Hence, men have nipples since females do (Simons, 2003).

The advance of nipples is a pretty complex process (Wysolmerski, 1998). Disturbances in the process cause regression. Indeed, in mice, the tissues common for nipples do develop in males throughout embryogenesis, but degenerate in ~ a few days, leaving no trace of nipples at birth.

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Hence, given the mouse example, i think that every mammalian male types develops through default nipples somewhere in advance like mice, but that advancement is stopped at assorted stages throughout species. The later in advance it halts, the an ext it each other female nipples. As soon as it is stopped very early after gestation, lock regress completely, choose in mice.

Why is there a distinction between varieties as to when the nipple breakthrough halts? due to the fact that the nipple developmental sequence is complicated (Wysolmerski, 1998), and because evolution typically prefers losing things over getting them, it is likely that through happenstance among many critical genes was lost in males, but selective push was simply too little to it is in of any far-reaching advantage over males v slightly better-developed nipples. However from below on, it is guessing.

References- Hughes, N Engl J Med (2004); 351(8): 248-50- Lawrence, Nature News, August 1999- Simons, Sci Am, September 2003- Wysolmerski et al., Development (1998); 125: 1285-94