Are echidnas really platypuses?

By Tim Low 23 June 2017
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Do echidnas exist because a platypus once deserted the water and evolved spines? Many experts think so.

PLATYPUS FOSSILS GO BACK more than 60 million years, while the oldest-known echidna fossils are only a quarter that age. A molecular study in 2009 concluded that echidnas diverged from platypuses about 32 million years ago, and perhaps only 19 million years ago. The scientific paper explaining this talks of “a recent, amphibious ancestry for echidnas”. A 2003 molecular study made similar findings.

Something that separates monotremes (egg-laying mammals) from other mammals is their ability to detect electric currents. Platypuses usually hunt with their eyes, ears and nostrils closed, finding freshwater crustaceans, tadpoles and insect larvae by sensing the weak electric currents produced by their prey’s muscle contractions.

Related: Why do echidnas swim?

Echidnas too have electroreception although it is rudimentary. One echidna was trained to dip its snout into two water troughs, through either of which a weak electric current was passed, signalling access to food. The echidna learnt to press a pedal attached to the trough that had electricity running through its water.

Nearly all the animals known to have electroreception, including some sharks, rays, catfish and axolotls, are aquatic. Why do echidnas have this electric sense when they get all their food on land? They may be able to detect electric currents produced by ants and termites in moist soil, although ants and termites avoid wet soil. New Guinea has three species of large long-beaked echidnas that feed on worms and beetle larvae in rainforest in damp soil that probably suits the transmission of electrical currents. The first platypuses to leave the water may have fed on worms in water-saturated soil along stream edges.

Echidna electroreception could be something that natural selection is eliminating because in most situations it is not helpful. Platypuses have about 40,000 electroreceptors on the tips of their bills, compared to 2000 in New Guinea echidnas and only 400 in Australia’s echidnas.

One hot summer’s day I came upon an echidna resting in a shallow pool and looking very much at home.  I can’t help wondering, was it really a spiny platypus that I saw?