“Do you think God gets stoned? I think so — look at the platypus.”
Venom isn’t very special in the animal kingdom, but our anthropocentric mindsets tend to focus more on large mammals than anything else, so to us it seems pretty mystical. Only a dozen or so mammals deliver venom, almost all of which deliver it via a bite for defense or predation. The platypus is unique in that it is so far the only animal known to use venom for a purpose other than defense or predation.
Only the male platypus has venom. And the male platypus only seems to have potent venom seasonally. The season when they have a lot of venom is unsurprisingly mating season, as the males actually use their venom, injected via venomous spurs on their hind legs, for intraspecific competition with other platypus males to keep territories and mates. While technically the echidna has venom, it can’t erect it’s spurs, and simply excretes a milky secretion.
Their venom, though nonlethal, causes excruciating pain for hours or days and is essentially nonresponsive to morphine. Only nerve-blocking agents (or antivenom) can provide relief.
A 2010 study found 83 peptides in platypus venom, many of which resemble venom genes from snakes, sea stars, and spiders. The platypus and reptiles have independently co-opted the same genes for venom usage making the platypus venom a cool example of molecular convergent evolution.
And just so the monotremes can continue to follow their pattern of general nonconformity and being surprisingly different from each other, the echidna venom gland transcriptome looks very different from the platypus one. You can read this post on their weird sex chromosomes for more.
The venom induces Ca2+ influx in cells, which results in neurotransmitter release. Defensin-Like peptides (defensins being immune proteins that usually defend the host from microbes), C-type natriuretic peptides (OvCNPs), nerve growth factor (OvNGF), and hyaluronidase have also been found. These peptides cause muscle relaxation, inflammation by promoting histamine release, and form ion channels in the lipid membranes of cells. The venom also contains a D-amino acid (as opposed to just all L-amino acids, which is the isomer previously thought to be the only conformation manufactured by cells).
First venomous animals were mammals
The platypus having venom and laying eggs isn’t even that weird, as it seems to be that that was the norm for the ancestors of mammals. Euchambersia mirabilis, a therocephalian therapsid from the end of the Permian (~255 mya), which were some of the “almost-mammals” (the term “mammal-like reptile” is horribly outdated and silly but for some reason people still use it), was determined to have venom glands. Venom glands which appeared way before snakes and lizards evolved them, and actually millions of years before any snakes even existed.
So while venom in mammals is very rare now, it may actually be an ancestral characteristic. Venom relatively expensive to have as it requires some method of injection into another animal, a gland, and then the making of proteins. It’s also suspected to be expensive because the loss of venom in animals that are no longer under pressure to produce any, is very common. Venom has a weak phylogenetic signal—similar types of venom are not necessarily found near each other on a phylogenetic tree, so genetically it seems not very “difficult” for various venoms to arise.
Monotreme venom as diabetes treatment?
The hormone, glucagon-like peptide-1 (GLP-1), is secreted in the gut, stimulating the release of insulin to lower blood glucose. But GLP-1 typically degrades within minutes in humans.
People with type 2 diabetes can’t maintain a normal blood sugar balance, but maybe they could if they had a less rapidly degrading GLP-1.
However in the platypus, there’s conflicting functions of the GLP-1. Not only is it a regulator of blood glucose in the gut, it is also in their venom. This conflict between the two different functions has resulted in the evolution of a dramatically changed GLP-1 system. GLP-1 in monotremes is resistant to the rapid degradation that occurs in other animals, and degrades by a completely different mechanism.
The function of GLP-1 in the venom seems to have resulted in the evolution of a stable form of GLP-1 in monotremes. Stable GLP-1 molecules can potentially be used as a type 2 diabetes treatment.
Both platypus and echidnas have evolved the same long-lasting form of the hormone GLP-1 despite echidnas not having spurs.
- Kita, Masaki, David Stc. Black, Osamu Ohno, Kaoru Yamada, Hideo Kigoshi, and Daisuke Uemura. “Duck-Billed Platypus Venom Peptides Induce Ca2 Influx in Neuroblastoma Cells.” Journal of the American Chemical Society50 (2009)
- Enkhjargal Tsend-Ayush, Chuan He, Mark A. Myers, Sof Andrikopoulos, Nicole Wong, Patrick M. Sexton, Denise Wootten, Briony E. Forbes, Frank Grutzner. Monotreme glucagon-like peptide-1 in venom and gut: one gene – two very different functions. Scientific Reports, 2016
- Julien Benoit, Luke A. Norton, Paul R. Manger, Bruce S. Rubidge. Reappraisal of the envenoming capacity of Euchambersia mirabilis (Therapsida, Therocephalia) using μCT-scanning techniques. PLOS ONE, 2017