How the Venomous, Egg-Laying Platypus Evolved

The odd Australian mammal has an intriguing family tree.

If there was a poster animal for diversification, it would have to be the platypus. It looks like an otter that’s gone trick-or-treating as a duck.

The platypus is an Australian mammal with some weirdly reptilian traits, like egg laying.

While we think of mammals and reptiles as very different, at one time they were more closely related, says Wes Warren of the Washington University School of Medicine in St. Louis.

Warren led the 2008 study that found that the platypus has genetic similarities to reptiles, birds, and mammals.

Evolutionary Split

Mammal-like reptiles diverged from the lineage they shared with birds and reptiles about 280 million years ago.

Around 80 million years later, the monotremes—or egg-laying mammals—split off from the mammalian lineage, says Rebecca Young, a biologist at the University of Texas at Austin.

Venomous males have sharp stingers on the heels of their rear feet and can use them to deliver a strong blow.

All that remains of that branch of the family tree is the platypus and four species of echidna. 

This split happened before the evolution of the placenta, Young says, “so in that sense they are somewhere between a lizard and what we think of as a human-like placental mammal,” retaining some reptilian and mammalian features.

Though monotremes' fossil record is limited, some skulls have been found, such as the extinct Obdurodon dicksoni. The creature, which lived during the Miocene period (about 23.3 million to 5.3 million years ago), has a similar snout to the modern-day platypus, but is likely not directly related.

While modern platypuses are down under, fossil evidence also shows that an ancient platypus lived in South America.

A zookeeper cradles rare twin platypus babies, which are known as puggles, at Healesville Sanctuary in Australia.

Your Electric Bill

But why platypuses “stopped evolving and losing these components that make a mammal a mammal,” such as fur, remains a mystery, says Warren.

For instance, the platypus's milk seeps through pores in its abdomen, not through teats as in all other mammals. Another unmammal-like trait is how they forage for food. Platypuses close their eyes, ears, and noses underwater and find prey by sensing electric currents with their ducklike bills.

Another reptilian similarity is their venom, which males inject into prey via a spur in their heels—a unique method of delivery among venomous creatures.

These bottom feeders scoop up insects, larvae, shellfish, and worms in their bill along with bits of gravel and mud. Platypuses do not have teeth, so the bits of gravel help them to "chew" their meal.

Warren led a 2010 study that found 83 toxins in platypus venom, which contains genes that resemble the venom genes of other animals, including snakes, starfish, and spiders. It's likely an example of convergent evolution, in which unrelated species evolve similar traits.

Speaking of evolution, the platypus is a good reminder that the process can be random, with mutations and adaptations that happen along the way, Young says.

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