That's a great question. People can't drink
sea water because we can't make urine that's
saltier than sea water, so we lose more water to
carry away the salt.
Marine mammals have to get water from what
they eat or from the sea.
Whales actually drink sea water. I suppose that
other marine mammals do this too. They have very
big kidneys that can get rid of the salt without
losing a lot of water. They also don't sweat (one
way we lose a lot of water) and they don't lose a
lot of water when they breathe. People lose a lot
of water just by breathing. There aren't too many
Santa Barbara days when it's so cold you can see
your breath, but you still breathe out a lot of
water. (Why can you only see your breath on
cold days?) Because the air at the surface of
the ocean has a lot of water in it, marine mammals
don't lose much water this way.
So why don't we have those big,
hard-working kidneys so that we can drink sea
water? Well, those kidneys take a whole lot
of energy to work. During most of our
evolutionary history, food was in short supply, so
people who had energy-hogging kidneys would have
been at a big disadvantage almost all the time.
I recommend the site of The Marine Mammal
Center where you can read interesting information
about mammals: here
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