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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2$\begingroup$ The urinary bladder is not unique to mammals. $\endgroup$– kmmFeb 25, 2013 at 12:55
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10$\begingroup$ 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. $\endgroup$– biojlFeb 25, 2013 at 15:06
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$\begingroup$ 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. $\endgroup$– shigetaFeb 25, 2013 at 17:28
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2$\begingroup$ @rg255: working with mice every day I can assure you they do not. $\endgroup$– nicoFeb 26, 2013 at 8:15
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1$\begingroup$ This is good explanation of a possible role of the bladder link.springer.com/article/10.1007/s42452-019-1692-9 $\endgroup$– ThejamesomaticJan 19, 2020 at 13:28
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3 Answers
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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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1$\begingroup$ 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. $\endgroup$– kmmFeb 26, 2013 at 21:19
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$\begingroup$ 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. $\endgroup$ Aug 14, 2013 at 14:38
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$\begingroup$ @JackAidley Does the edit address your concern? (Note that all of the "above are just ... speculations", though I think they are "somewhat reasonable".) $\endgroup$– user1858Aug 15, 2013 at 20:31
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$\begingroup$ @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. $\endgroup$ Aug 15, 2013 at 23:54
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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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1$\begingroup$ do you have references for these? $\endgroup$– user3795Aug 14, 2013 at 7:37
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1$\begingroup$ perhaps this is better as a comment on the accepted answer? maybe ask @Paul-a-clayton to add them in if valid $\endgroup$– rg255Aug 14, 2013 at 11:20
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$\begingroup$ @GriffinEvo Craig didn't have 50 rep. $\endgroup$ Jan 15, 2015 at 5:05
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$\begingroup$ I'll let you keep the rep and not delete your answer but please put well structured, referenced answers from now on. $\endgroup$ Jan 15, 2015 at 11:02
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Apparently, the urinary bladder is not unique to mammals, and is even found in fish, which one might presume can urinate at any time or even better - expel ammonia out of their gills, so why do they need a bladder?
I found the following article trying to understand what the bladder does in freshwater fish: https://jeb.biologists.org/content/155/1/567.
Apparently they concluded that the bladder is important because the kidneys let too much useful stuff (like Na and Cl ions) out, and letting it sit in the bladder for 30 minutes (as measured in the trouts in that research) allows some of these ions to be re-absorbed by the cells lining the bladder. I don't know if this is why the bladder evolved (which would have been easy - just as an enlarged cavity in front of the kidney holding some urine before it is expelled), or why couldn't a better kidney - absorbing more salts - have evolved instead. I also don't know if mammals still use the urinary bladder in this way. But once it has evolved, it is here to stay, unless it has a negative survival contribution. And as other answers suggested, it probably has positive contribution, if anything - such as avoiding leaving a strong trail of scent leading to you, or avoiding soiling your den with urine.
answered Jan 20, 2020 at 14:13