Nostrils weren’t always used for breathing.
Back when we were all still fish, our noses were good only for smelling – as modern fishes’ noses still are.
And we’d have had two pairs of nostrils, one set behind the other – as modern fish still do.
Water enters the piscine nose through the front pair and exits through the rear.
We humans and other tetrapods (mammals, birds, reptiles and amphibians) have kept the front pair, but the rear ones have become internal tubes that connect our nasal cavity to our throat, allowing us to breath through our nose.
For whales, breathing through nostrils sited in the customary position on the front of the face would be highly inefficient as it would require lifting the head up out of the water.
Which is why cetacean nostrils have migrated to the top of the head to become blowholes.
While baleen whales, such as blues and humpbacks, have a pair of blowholes, equivalent to our own paired nostrils, sperm whales, dolphins and other toothed whales have just one. This is derived from the left nostril; the right one opens internally.
And as anyone who has got their own nostrils to within sniffing distance of a blowhole will testify, a whale’s exhaled breath isn’t so sweet – a cocktail of farts, cabbages and rotting fish.
Main image: A blue whale's blowhole can measure up to 50cm in diameter. © Kevin Schafer/Getty