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Reply	Neos says: Maybe because like dogs, urine can be used to mark territory?Then again humans don't have such good noses.  (no votes) Tags: human body.
	posted on 2008-05-29 10:24:00 | Report abuse

Reply	Richard says: i guess there must be a common ancsestor for mammals that evolved it for that reason. i'm pretty sure birds only have the one excretory system.  (no votes) Tags: human body.
	posted on 2008-05-29 10:53:00 | Report abuse

Reply	SteveS says: This is because of the nature of the waste products:1)faeces come largly from undigested food and the bacteria that grow on it. So it has technically never been inside your body (meaning never comes into contact with your bloodcirculation)2)urine is an internal waste product, it's made from metabolites from inside and around your cells, so it's from inside your body. Thats' why we need kidneys: to separate the stuff we need to stay inside from the waste that has to leave our body. So in short we need these two "waste streams" to stay happy, both internally and "externally".@Richard: Birds just have one opening to the outside, but the faeces and urine are only mixed at the last moment. The two systems are also completely present in them (a chicken also has kidneys). Even fish have kidneys.@Neos: A lot of creatures don't sent mark with urine, but with excretions from a gland near the tail.  (no votes) Tags: human body.
	posted on 2008-05-29 11:34:00 | Report abuse

Reply	Shaun says: It may have something to do with risk of infection. Animal's with only one secretory orifice (reptiles/amphibians/birds/most fish/monotremes) also lay eggs. Perhaps as the lower region of the reproductive tract in these animals accomodates eggs, rather than relatively exposed foetuses, it can be a much harsher environment - deterring any ascending infection from the shared orifice.Urine is generally close to sterile until leaving the body and as such poses a greatly reduced infection risk to the mammalian uterus.  (no votes) Tags: human body.
	posted on 2008-05-29 15:29:00 | Report abuse

Reply	Sett says: @shaun Actually you're close. If I remember correctly platipi are also monotremes just like most birds, and marsupials have a single opening for excrement, but possess a seperate genital tract, as do some birds as I understand it.  (no votes) Tags: technology, unanswered, domestic science, human body.
	posted on 2008-05-29 16:14:00 | Report abuse

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