What Is a Platypus’s Closest Living Relative?

The closest living relatives of the platypus (Ornithorhynchus anatinus) are the echidnas (family Tachyglossidae), which comprise four species. Both the platypus and the echidnas (or spiny anteaters) are monotremes belonging to the order Monotremata, an ancient group of mammals that are oviparous (egg-laying). The name Monotremata means “one-holed,” in reference to the fact that both sexes have only one opening at the rear of the body, which is used for both reproduction and excretion. In addition to laying eggs, both animals possess electroreceptors, which they use to detect electrical signals in their environments.

Despite their shared ancient ancestry and uniqueness as egg-laying mammals, the platypus and the echidnas have evolved divergent features in order to occupy very different ecological niches. The platypus is a semiaquatic carnivore-insectivore thriving in the waterways of Australia and Tasmania. The echidnas, however, are land-dwelling insectivores found in a variety of habitats across Australia, Tasmania, and New Guinea. Whereas the platypus has a ducklike bill that it uses for feeding on invertebrates on the bottoms of rivers and streams, the echidnas have tubular beaks, similar to anteaters, for consuming insects in terrestrial habitats.