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Apparently, some species are capable of drinking salt water, like the sea lion and otter.

But given how rare fresh water is, and how abundant salt water is, why have more land animals not evolved to drink it? Why are kidneys capable of handling such a thing so uncommon?

I understand that evolution doesn't necessarily always do what's most efficient on a macro scale, but this drives me nuts.

So many animals live less than a five minute walk from the shores of an ocean, and such water is so abundant that they'd never be thirsty.

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  • $\begingroup$ How can you state [...]how rare fresh water is, and how abundant salt water is[...] ? $\endgroup$ – mattia.b89 Feb 10 at 17:32
  • $\begingroup$ What do you mean? $\endgroup$ – Sermo Feb 10 at 17:33
  • $\begingroup$ Salt water is abundant only if you live within easy walk/crawl/flying distance of an ocean. Fresh water is common everywhere on land, except in the driest of deserts. $\endgroup$ – jamesqf Feb 10 at 18:17
  • $\begingroup$ I mentioned (in my last "paragraph") that many animals indeed DO live within such ranges. Yet virtually none of these animals are able to drink the water. $\endgroup$ – Sermo Feb 10 at 18:18
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    $\begingroup$ @Sermo: But it is a useful ability only to the part of the population that lives within range of salt water. Unless your entire population lives there AND finds it awkward to get to fresh water, as with sea lions and other marine mammals, there's really no pressure for it to evolve. $\endgroup$ – jamesqf Feb 10 at 22:39
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According to an answer here there is around 356,000 km of coastline on the planet. If we take an average human walking speed of say 5 km/h then our 5 minute walk takes us 0.417 km inland - giving approximately 150,000 km^2 of land conveniently supplied with sea water.

Now according to this page the total amount of land area is around 150,000,000 km^2 which according to this page is covered by somewhere in the vicinity of 117 million lakes [I'm certain a large portion of them are more accurately described as puddles but well go with that number for now], that gives on average a fresh water body every 1.3 km^2.

If we simplify things and imagine dividing the land area into 1.3 km^2 grid with ~1.14 km the distance separating each watering hole meaning the furthest distance to a watering hole across all the earths land area (ON AVERAGE) is around 0.570 km or just under 7 minutes walk (using our previous 5km/h)

So according to my very very rough calculations only around 0.1% of the land area is located within 5 minutes of sea water while 100% is located within 7 minutes of fresh water! I don't think that extra two minute walk is going to be that much of an evolutionary drive for any species.

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  • $\begingroup$ apparently, not even the sea mammals are drinking salt water, if they can avoid it (obtaining water from food, ice or other fresh water sources): scientificamerican.com/article/how-can-sea-mammals-drink $\endgroup$ – Frieke Feb 11 at 15:25
  • $\begingroup$ In addition, a great many mammals are smaller and/or slower than humans, reducing the land area close to salt water they have available. Smaller creatures can survive on water from rain & dew, which in many places is readily available. $\endgroup$ – jamesqf Feb 11 at 19:41

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