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Echidnas and the platypus are the only egg-laying mammals, known as monotremes. 
The average lifespan of an echidna in the wild is estimated around 14–16 years. When fully grown, a female can weigh up to and a male can weigh up to . The echidnas' sex can be inferred from their size, as males are 25% larger than females on average. The reproductive organs also differ, but both sexes have a single opening called a cloaca, which they use to urinate, release their faeces and to mate.
Echidnas and the platypus