Photograph by Joel Sartore, National Geographic Photo Ark
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Platypuses hunt underwater, where they swim gracefully by paddling with their front webbed feet and steering with their hind feet and beaverlike tail.

Photograph by Joel Sartore, National Geographic Photo Ark

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.

It’s a mashup that inspired Mark Anthony Libre to ask Weird Animal Question of the Week: "How did [the platypus] evolve in this unlikely fashion?”

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 shared a common ancestor, says Wes Warren of the Washington University School of Medicine in St. Louis.

Probing Platypus Evolution February 6, 2009—National Geographic researchers are trying to collect DNA samples from these odd duck-billed mammals to determine whether there are separate subspecies.

Warren led the 2008 study that found that the platypus has a "fascinating combination of reptilian and mammalian characters." (Related: "Platypus Genome Reveals Secrets of Mammalian Evolution.")

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.

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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. (Related: "Which Animals Have Barely Evolved?")

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 close kin.

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

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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,” remains a mystery, says Warren.

The platypus's milk seeps through pores in its abdomen, not through teats as in all other mammals. Another incredible adaptation 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.

Their venom is located in a spur in the males' heels—a unique method of delivery among venomous creatures. (Also see "Venomous Primate Discovered in Borneo.")

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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.

So while many things about the platypus remain mysterious. Young notes that there is "some randomness to how we acquire things over time," plus mutations and adaptations that happen more quickly.

Weird Animal Question of the Week answers your questions every Saturday. If you have a question about the weird and wild animal world, tweet me, leave me a note or photo in the comments below, or find me on Facebook.