Why are platypuses mammals?

Mammals come in lots of shapes, and are classified not only by the fact that most of them give birth to live young. Mammals are also characterized by the presence of sweat glands, for instance, which other animals do not have. Mammals are warm-blooded, have hair instead of feathers or scales, have three middle-ear bones, and possess a developed reasoning center in their brains.

And there there’s the platypus. Not only is it an egg-laying mammal, it’s also produces venom, something only a few other mammals do, so it really bucks many trends. The echnidna also lays eggs, and together, the platypi and echidna share the distinction of being the only egg-laying mammals, which are called monotremes.

So why are they mammals? Because they share so many of the other traits of mammals – hair, sweat glands, big brains, et cetera – that they are given a pass for laying eggs, just like the whales and dolphins are given a pass for being largely hairless and just like the pangolin is given a pass for having scales instead of fur.

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