Proof Of God's Sense Of Humour

 So please - before you think about hurting someone over this trifle of a film, remember: even God has a sense of humor. Just look at the Platypus.
-Opening credits to Kevin Smith's film Dogma.

In all fairness to the talents of God and evolution, these are some pretty hilarious and fascinating creatures.

First off, they are, despite appearances, mammals.  They're in Order (remember Linnaean taxonomy? Kingdom, Phylum, Class, Order, Family, Genus, Species) Monotremata along with just the Echidnas in Class Mammalia, and the monotremes are defined as egg-laying mammals.  All told, there are five species of monotremes, four of which are echidnas and just one species of platypus.

Secondly, they're venomous.  One of only eight species of mammals known to be venomous (nine if you count the slow loris, but I don't), and the only one that actively injects venom (the way a snake does).  The rest 'just' have venomous saliva, so their bite is venomous, but they don't technically inject it through a needle-like delivery system.  Plus, all the other species of venomous mammals are small.  Mouse to rat-sized small.  The platypus is a fairly big animal, roughly rabbit or housecat sized.  The males (and only the males) have a smallish spur on their back legs that can be used to inject a venom.  How dangerous is this venom?  It's killed dogs, and in it causes what is called "extreme pain" and swelling in people.  Nobody's ever died from a platypus sting, which is good, because nobody's figured out how to make an antivenom for it yet.

The next thing that makes them awesome is that beak of theirs.  It looks like a duck's, and while it does have bones in it, it's mostly soft and sensitive.  Better than that though, is that it's got electro-receptors in it.  Like sharks, they can detect the bioelectric field that living things put out.  They use it to track down prey, such as insects, crayfish and shrimp, because when a platypus dives, it closes not just its eyes, but also its nose and ears.  Saying it can't smell under water would be wrong, it has a secondary sense of smell that relies on vomeronasal receptors instead of olfactory receptors, and it's unusually acute (a thousand receptors compared to just hundreds for most mammals), but even without the vomeronasal senses it would still be able to find prey.  Which researchers think it has to do, since their prey can and do hide in the mud at the bottom of the ponds and rivers where it lives.

The platypus: Proof of God's sense of humour, and of the wonders of evolution (particularly convergent evolution).

One last fun fact for the road: Female platypuses (not platypi) have no nipples, just two smooth milk secreting patches of skin on their belly.

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Thanks to Martin O'Halloran for pointing out both a spelling mistake and an omission regarding Linnaean taxonomy.

Sources

Image from LearnAnimals.com
Wikipedia article on venomous mammals
Australian Platypus Conservancy website
AustralianFauna.com platypus factsheet
Wikipedia article on the platypus
Protein Spotlight article on platypus venom
Cosmos magazine article on platypus venom
Cosmos magazine article on the platypus genome