Poison is very common in the animal kingdom (and throughout the other kingdoms of life) both as a means of defense and as a weapon for hunting, however only a tiny handful of mammals are poisonous. A few shrews have mildly venomous bites. The European mole has a toxin in its saliva which can send earthworms into a coma (allowing it to store them in the larder for later). The Solenodon, a strange burrowing nocturnal insectivore is perhaps most toxic among the mammals–with one glaring exception. At the beginning of the month we wrote about the clever echidna–a monotreme with unusual brain physiology. The echidna’s closest relative, and the world’s only other remaining monotreme, the duck-billed platypus is the planet’s most poisonous mammal by far. Not only do platypuses have bills, lay eggs, and utilize electrical sensory apparatus to hunt, but the male has a moveable poisonous spur on his hind legs which is attached to a venom-producing crural gland. Only the male platypus is capable of producing a toxic peptide cocktail and injecting it through his spurs. Female platypuses (and all echidnas) have rudimentary spurs which drop off and lack functioning crural glands. Platypus venom causes pain and hyperalgesia—which means an increased sensitivity to pain–so you shouldn’t cuddle male platypuses no matter how adorable their funny little bills may look to you.
The Stanford neuroblog (from whom I borrowed the attached image of a platypuses’ poisonous organs and appendages), notes the similarity of platypus venoms with reptile venoms, “One evolutionary curiosity: the defensin-like peptides found within the platypus venom are also found within reptile venom. However, genetic analysis in 2008 revealed that the platypus peptides evolved independently from the reptile peptides, although both were derived from the same gene family.” Its curious to think of how our ties with reptile forbears are manifested in the curious and endearing (and poisonous!) platypus.