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Sure it's convenient to decide when to urinate but not essential for survival or reproduction, as I understand. But just convenience is not a drive for evolution.

Does the bladder serve any essential purpose? If not why did bladders evolve?

{ asked by Dheeraj V.S. }


Here are just a few points that might apply:

Since terrestrial animals presumably retained urinary bladders developed by their marine ancestors, benefits associated with terrestrial lifestyle would only provide selective pressure to retain such a feature. However, if somehow a line of terrestrial animals abandoned urinary bladders, it is not entirely implausible that even scent marking benefits could increase the selective pressure enough to overcome some peculiar opposing selective pressure.

Initially, the animal might apply scents by scratching at an area of skin irritated by urine release against some surface. Then this scratching might be preferentially located (i.e., a scent marking behavior is developed). Having such a behavior would then obvious bring benefits to storing a significant amount of urine and eventually to being able to squirt the urine (allowing the scratching behavior to fall away).

The above are just somewhat reasonable speculations about what selective pressures might encourage the development (and retention) of a urinary bladder. Hopefully, someone with actual knowledge will provide a better answer documenting established theory and evidence for how urinary bladders actually developed.

{ answered by Paul A. Clayton }