What is the average size of a mammal?

The only mammals which lay eggs are the monotremes. These include the platypus and the two species of echidna, the short-beaked echidna of Australia and the long-beaked echidna of New Guinea.

In the case of the platypus, the eggs are about 11 - 15 mm in size, or about the size of a grape. Interestingly, only the left ovary is functional. Also, while the mother produces milk, she does not have teats.

The echidna lays eggs which are about the same size.

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