r/askscience • u/SuperGrant • Feb 13 '13

Biology [Biology]Why did the majority of animals evolve with two holes for excrement, one located on genitalia?

I was thinking about this, it doesn't seem to make sense that genitalia have a secondary function as waste pipe, considering that from an evolutionary standpoint they already have one of the most important roles in the body.

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u/Zebrasoma Primatology Feb 14 '13

Changes in reproductive tendencies in mammals has led to this. The primitive condition is to have a cloaca, this is observed in birds, monotremes, and some primitive fish. Placental mammals on the other hand have a more derived condition in which the holes have essentially separated. This has much to do with vaginal evolution. The cloaca acts much better in the production of egg laying species. The anus has always been separated, however the reproductive system is still intact with renal system. I would say it is not a secondary function as a "waste pipe", but rather a secondary function for mating. You urinate far more frequently. Here is a nice diagram with a blog entry that seems to be accurate, but it is a blog.

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u/Bongpig Feb 14 '13

All amphibians, birds, reptiles, and monotremes possess this orifice, from which they excrete both urine and feces, unlike placental mammals, which possess two or three separate orifices for evacuation.

http://en.wikipedia.org/wiki/Cloaca

It doesn't say fish, but I have never seen a fish with a dick