Why a bladder?

I have always had some of my most interesting ideas during the periods of drowsiness that precede and follow sleep.



A couple of days ago, during one of those periods, the following question popped into my mind: “Why do we need a bladder?”  Or, better said: “What evolutionary advantage has led to the development in mammals of a bladder?”

Reptilians and birds don’t have it.  They free themselves of their excreta via a single orifice, while mammals developed separate “way outs”.  But my early-morning question was more specific than that: we could still have two separate tracts and no bladder.

Googling around, I discovered that a couple of other people had already asked the same question, but the answers were not pertinent at all.  In fact, they were a clear indication of how stupid some of the people who answer questions on the Internet are.  There seemed to be a consensus that you need the bladder otherwise you cannot free yourself of substances that, on the long run, would kill you.  That obviously answers the question “why do we need to urinate”, not “why do we need to store urine rather than just letting it go”.

Understand me: I am very happy to have a bladder, otherwise I would be condemned to cope with a frequent trickle of urine.  Perhaps, without a bladder, humans would have developed a very different type of society.  At the very least, clothes would have been made in such a way as to allow an unhindered flow of urine, or perhaps with a built-in waterproof container.

The presence of the bladder allows mammals not to leave traces behind them (unless they want, for example to mark territories).  And that is perhaps where the evolutionary advantage is: big saurians could track proto-mammals (if there is such a thing, a doubt which reveals my ignorance in species evolution) by following their trickle of urine.  To avoid being easily chased down, mammals learned to “hold it” as long as they could and, obviously, those that could do it longer were more likely to shake off their hunters.  Over many thousands of years, this might have resulted in the evolution of a bladder, so that mammals could wander about for long periods without leaving behind persistent scents.

There is obviously no proof that my hypothesis is correct, but I am happy to have found a possible (and to me quite convincing) explanation.

Disappointingly, I remain completely at a loss concerning why it was evolutionary advantageous to separate solid and liquid excreta...