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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?

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The urinary bladder is not unique to mammals. –  kmm Feb 25 '13 at 12:55
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Well a Naïve answer for a Naïve question: It's not very "convenient" to leak urine constantly and create a beautiful path leading directly to you when you have lots of predators around looking for you. It's very "convenient" to be able to mark your territory if you're a predator. –  biojl Feb 25 '13 at 15:06
    
The benefits of adaptations can be very small and still be widespread. It doesn't have to be life or death 100% of the time. –  shigeta Feb 25 '13 at 17:28
    
apparently mice urinate all the time/as they move –  GriffinEvo Feb 25 '13 at 17:49
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@rg255: working with mice every day I can assure you they do not. –  nico Feb 26 '13 at 8:15

2 Answers 2

up vote 12 down vote accepted

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.)
  • Flushing could also reduce contact with skin. Urine might act as an irritant and a nutrient source for parasites.
  • 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).

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.

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To get rid of excessive liquid as well. –  Chase Yuan Feb 26 '13 at 3:01
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I'd add to this excellent answer that some desert tortoises store usable water in the bladder. They can reabsorb it from the bladder somehow. –  kmm Feb 26 '13 at 21:19
    
I doubt the validity of your first point; I do not see how urine could be used for scent marking until it has evolved. I think it's a case of exaptation of an existing feature for a new purpose. –  Jack Aidley Aug 14 '13 at 14:38
    
@JackAidley Does the edit address your concern? (Note that all of the "above are just ... speculations", though I think they are "somewhat reasonable".) –  Paul A. Clayton Aug 15 '13 at 20:31
    
@PaulA.Clayton: Not really. There are grades of speculation, aren't there? And we're free to disagree on what its reasonable and what is not. The rest of your points are good, I just disagree that the first in plausible. –  Jack Aidley Aug 15 '13 at 23:54

A few observations to add to Paul's:

Urinating on yourself in winter could be a fatal thermoregulatory mistake.

Urinating on your substrate could hinder locomotion.

Having a continual slick of high osmolarity fluid on your skin would be damaging to the epithelium and work at odds to the action of the kidneys.

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do you have references for these? –  user3795 Aug 14 '13 at 7:37
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perhaps this is better as a comment on the accepted answer? maybe ask @Paul-a-clayton to add them in if valid –  GriffinEvo Aug 14 '13 at 11:20

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