Why did the urinary bladder evolve?

Question

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?

Answer 1

Here are just a few points that might apply:

• Urine is used for scent marking by some species, so the ability to store urine could be useful.
• At the opposite side, controlling the release of a strong scent would help in stealth for both predators and prey. (In addition, a single strong scent might temporarily overload a predator's sense of smell making tracking more subtle scents more difficult.)
• Flushing an excretion point (single point reduces opportunities for invasion) under some pressure could help avoid blockage and parasitic invasion/accumulation. (Providing a tube from the extraction organ to the excretion point allows more flexible (and protected) placement of the organ, but also increasing the benefit of a flushing mechanism.)
• Adding a buffer is a common technique for any pipelined operation to allow smaller resources to handle temporal variation in input and output rate. Without a such a buffer, all stages have to be sized for the maximum utilization rather than something closer to average utilization.
• Avoiding potential contamination of food and water may also be a benefit of controlled urination (or excreting might have a fertilizing or pest-deterrent aspect for plants).

The above are just somewhat reasonable speculations. Hopefully, someone with actual knowledge will provide a good answer.