Testosterone release in the fetal male rat happens prior to the phase of mammogenesis wherein the teat is developed whereas other types halt the same process after the teat is formed.
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According come the exact same page, male rats and horses (and mice) are unique amongst non-monotreme mammals in not having actually nipples.
Male humans have actually nipples since there"s no evolutionary push for them no to. (Why perform men have actually nipples?) Then, why to be there selection for males to lack nipples in the case of mice, rats, and horses?
(Apparently, male horses lack nipples since "female horses have their nipples between their hind legs, and also in males, that actual estate is used for various other purposes". (Straight Dope))
Evolutionarily, why execute male rats and horses absence nipples?
evolution mammals anatomy
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Male mice lack nipples too. Mice are commonly used for embryonic research as castle are small and blee quickly. It is thought that masculine mice do develop nipples, however that castle regress during development (Wysolmerski, 1998).
In general, that is assumed that mammalian organisms develop as females through default as soon as there is no male (Y) chromosome existing (Hughes, 2004). Notably, X_ individuals and also XY individuals with particular Y-chromosome deletions additionally develop as females. Basically, it is testosterone that suppresses female features. Hence, additionally boys through testosterone production, but without the ideal receptors additionally develop together females.
Hence, men have actually nipples due to the fact that females do (Simons, 2003).
The development of nipples is a pretty complex process (Wysolmerski, 1998). Interruptions in the process cause regression. Indeed, in mice, the tissues usual for nipples do build in males during embryogenesis, however degenerate in ~ a few days, leaving no trace of nipples at birth.
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Hence, provided the computer mouse example, ns think that every mammalian male types develops by default nipples what in breakthrough like mice, however that development is stopped at various stages across species. The later on in breakthrough it halts, the more it each other female nipples. When it is stopped very early after gestation, they regress completely, choose in mice.
Why is there a distinction between varieties as to when the nipple breakthrough halts? since the nipple developmental sequence is complex (Wysolmerski, 1998), and also because evolution commonly prefers losing things over obtaining them, the is likely that through happenstance among many an essential genes was lost in males, but selective push was simply too small to it is in of any far-ranging advantage over males with slightly better-developed nipples. But from right here on, the 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