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© Copyright 1998, Jim Loy
What is the difference between a dolphin and a porpoise? I heard this question asked, on Johnny Carson. And, Johnny decided that a dolphin was a mammal, and a porpoise was a fish. Wrong. They are both mammals, and are very closely related. The porpoises have a rounded head, while the dolphins have a protruding snout. They are smaller members of the toothed whale family, to which killer whales and sperm whales belong.
To complicate things, there are a couple of fish which are also called dolphins. I assume that these are sometimes called "dolphin fish" to avoid confusion. But, the well-known dolphins are those cute mammals, like Flipper (who is a bottle-nosed dolphin).
Mammals have hair, breath air, give birth to live babies (platypuses and echidnas lay eggs), and feed milk to their young. Whales breath air (they can hold their breaths for long times), give birth to live babies, and feed milk to their young. Where is their hair? Evidently, they have a small amount of hair around their blow-holes, those holes, in the top of their heads, that they breath through.
The toothed whales are among the most intelligent of all animals. The killer whale (orca) apparently has a brain mass to body mass ratio (a rough measure of intelligence) which is greater than that of humans. Incidentally, while toothed whales are predators, there is no record of any killer whale ever having killed a human being.
Addendum:
I received email asking for the difference between a dolphin and a whale. Here is my response:
Dolphins, and their relatives (which feed on fish), are small toothed whales, closely related to the large toothed whales, the killer whale and the sperm whale (which feed on fish and other animals). The other even larger whales are called baleen whales (which feed on krill (tiny shrimp)), which have baleen (whale bone but not really bone) in their mouths, which acts as a sieve to filter food out of the water. These two kinds of whales (toothed and baleen) are the two groups of the order Cetacea (whale).
The sperm whale hunts giant squid. The giant squid was long thought to be a myth, but numerous dead giant squids have washed up onto shore around the world. This has been well-documented. Also, the circular marks from the suckers on their arms have often been seen on the skin of sperm whales, apparently the result of life and death struggles between these two giants. And so, the giant squid exists, at great depths of the oceans, although they are almost never seen (except by sperm whales).