When it comes to being mammal, dolphins fit the bill in all respects. They give birth to live young and nurse them. They're warm-blooded. They have lungs and breathe air.
They're also descendants of terrestrial mammals from the Cetartiodactyla clade, which also includes pigs, cows, hippos, camels, and other even-toed ungulates. But they left life on land some 50 million years ago.
If you've gotten to pet one at an aquarium, though, you've noticed that dolphins lack the one thing all other mammals have: hair.
What gives? How come they get to be in the mammal club?
Well, kids, get ready for this: Dolphins are born with mustaches.
Yes, mustaches. Now, I'm not talking about a full-blown Fu Manchu or even a decent handlebar, but a rim of short hairs around their rostrum (or snout). The 'stache helps newborn dolphins locate and feel their mother for the first few days of nursing and falls off after a week or so because of a natural depilatory process. It's a fact that Gillette has yet to exploit.