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I'm reading an essay on the creating of the Mammalia zoological classification (Londa Schiebinger, The American Historical Review, Vol. 98, No. 2 (Apr., 1993), pp. 382-411).

It contains the statement (page 386)

All mammals (including the whale) have hair, and it is still today considered a distinguishing characteristic of mammals.

Is that statement correct? Do all known species of whale and dolphin have hair? If so, where is it? You can't see it on the surface of the animal, is it internal like we have nostril hair?

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up vote 7 down vote accepted

Well, technically yes, but most adult dolphins do not have hair:

Unlike most mammals, dolphins do not have hair, except for a few hairs around the tip of their rostrum (beak) which they lose shortly before or after birth. The only exception to this is the Boto river dolphin, which has persistent small hairs on the rostrum.

Whales do, and depending on your definition of "hair" - that includes Baleen - which is composed of fine keratin fibers, much like normal fur/hair, which is the distinguishing characteristic of "Baleen whales" (like the Blue Whale).

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What about orca or other whales with teeth? –  Canageek Nov 4 '12 at 0:41
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Yes and yes. A lot of marine mammals lose the hair before birth or shortly after, including orcas and many whales. However, because it is present during development, they can be classified as mammals if no other traits are present. –  MCM Nov 4 '12 at 4:40
    
Orcas are dolphins by the way, not whales. –  terdon Nov 4 '12 at 13:47
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