A platypus spur being displayed, I imagine, very carefully. (Source: Wikipedia. Image by Unknown)
At the midpoint of the semester I feel the need to let off some steam, and so this post will be all about the many violent ways to deliver venom. The venomous animal kingdom offers an impressive weapons collection. Everyone knows about fangs. Bah. Biting is seriously boring. In this armoury there’s barbs, beaks, harpoons, nematocysts, pincers, proboscises, spines, sprays, spurs and stingers (Casewell et al 2013). I’m going to take a look at two of my favourites.
Platypus duels
What a strange creature is the platypus. Forget the duck beak and the beaver tail – the males of the species bear a pretty serious venom-delivery system. The pointy end is a hollow spur on each rear ankle, fed through ducts by kidney-shaped venom glands (Grant & Temple-Smith 1998). In humans the venom produces immediate local effects that are typically described as “torturous”, “agonising” and “excruciating”, words that are likely embellished with colourful adjectives I cannot repeat on this blog. The venom is strong enough to kill rabbits when injected into their bloodstream (Ligabue-Braun et al 2012). Since platypuses don’t hunt rabbits – they much prefer benthic invertebrates that they don’t even kill with venom (McLachlan‐Troup et al 2010) – why do they have spurs and venom at all? And what selection pressure makes the venom so strong?
The best answer to the first question so far seems to be “for the ladies”. Female platypuses are born with spurs but lose them later (Ligabue-Braun et al 2012). In males the glands, along with the testes, increase in size for the breeding season. During this time they are often found with puncture mark injuries, especially near the tail (Grant & Temple-Smith 1998). Based on these observations Grant, Temple-Smith and others have suggested that males may use the spurs to fight each other for the right to mate with females. Why the venom needs to be so potent for this purpose is a mystery.
To help answer this question, I would love to know what effect the venom has on another platypus. A battle to the death would be, literally, overkill. Inflicting a little bit of pain or temporary paralysis would be enough to give one male the advantage over the other. If platypuses are not killing each other in these duels, does this mean they are resistant to the venom? Increased venom resistance would give a male an advantage over another in a fight. Stronger venom would negate the advantage. Selection then favours even higher venom resistance – and on it goes. If males really are fighting over females, an intraspecific evolutionary arms race among platypuses could explain the strength of the venom.
End of speculation. It’s time to remind you that this post is about violence with a video of a platypus fight. Is this an example of two males attacking each other with their spurs?
Snails bearing ballistic weapons
Cone snails (Conus) are a genus of around 600 marine gastropods (Reece et al 2012). The weapon of choice for some in this group is a harpoon loaded with powerful toxins. The hunting strategy that revolves around this harpoon is quite appalling, in the most fascinating of ways. The snail either digs into the sediment or crawls about on it at a sedate, snail-appropriate pace, gently waving around a thin tube called a siphon, which it uses to sniff out a juicy fish. The snail presses the tip of its proboscis against the fish’s scales. Driven by a ballistic mechanism, the harpoon is released in less than a millisecond, drilling into the fish’s flesh and releasing the venom (Schultz et al 2004). The hapless fish is quickly paralysed. The cone snail stretches its mouth open to cavernous, cartoonish proportions, engulfs the fish whole and draws the entire appendage back under its shell as if the fish never existed. The harpoon is a modified radula, the rasping organ that many molluscs use to scrape up food, and designed for one-time use only (Reece et al 2012). It actually looks like a harpoon, too (Schultz et al 2004). The process of evolution produces traits and strategies that are stranger than we can imagine – by building on things that have come before.
Those are just two of the remarkable venom-delivery systems to be found on the planet. There are many more. And just to remind you that this post was about violence, here is a final video, of a cone snail attack.
References
Casewell, NR, Wüster, W, Vonk, FJ, Harrison, RA & Fry, BG 2013, ‘Complex cocktails: the evolutionary novelty of venoms’, Trends in Ecology and Evolution, vol. 28, no. 4, pp. 219-229.
Grant, TR & Temple-Smith, PD 1998, ‘Field biology of the platypus (Ornithorhynchus anatinus): historical and current perspectives’, Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, vol. 353, no. 1372, pp. 1081-1091.
Ligabue-Braun, R, Verli, H & Carlini, CR 2012, ‘Venomous mammals: a review’, Toxicon: Official Journal of the International Society on Toxinology, vol. 59, no. 7-8, pp. 680.
McLachlan‐Troup, TA, Dickman, CR & Grant, TR 2010, ‘Diet and dietary selectivity of the platypus in relation to season, sex and macroinvertebrate assemblages’, Journal of Zoology, vol. 280, no. 3, pp. 237-246.
Platypus fight, video, thetynevalley, 19 November 2009, viewed 20 April 2014, <http://www.youtube.com/watch?v=F7H7sR7VEEI >.
Platypus spur, digital image, Wikipedia, viewed 20 April 2014, <Duplicated genes, digital image, Wikipedia, viewed 5 April 2014, <http://en.wikipedia.org/wiki/File:EvolutionOfDuplicateGenes.png >.
Reece, JB, Meyers, N, Urry, LA, Cain, ML, Wasserman, SA, Minorsky, PV, Jackson, RB & Cooke, BN 2012, Campbell biology, Pearson Australia, Frenchs Forest, NSW.
Schulz, JR, Norton, AG & Gilly, WF 2004, ‘The projectile tooth of a fish-hunting cone snail: Conus catus injects venom into fish prey using a high-speed ballistic mechanism’, The Biological Bulletin, vol. 207, no. 2, pp. 77-79.
World’s weirdest: killer cone snail, video, NatGeoWild, 8 June 2012, viewed 20 April 2014, <http://www.youtube.com/watch?v=zcBmMPJrrKk >.
The Venom Evolution Files 
Tasmin
23 April 2014 at 1:12 pm
I saw my first wild platypus on the weekend Very cool video. There definitely does seem to be lots of kicking with the back legs going on. The question I have, is how easy is it to tell the difference between a male and a female from a distance? The cone snail is also quite amazing. Is there any mention of how quickly that harpoon can be ejected? Really awesome information this week!
toxinguide
23 April 2014 at 8:44 pm
You’re absolutely right, Tasmin. You can’t tell if these are males or not and it would be hard to reliably tell the difference from a distance. I do find the hypothesis quite intriguing and it would be great to find out the reason for this odd adaptation. I read somewhere in those papers that echidnas have residual spurs. I’m really quite interested in this now, it’s bugging me. 🙂
According to Schulz et al (2004), who filmed Conus catus, it takes between 240 to 295 ms between the proboscis first touching the fish to release of the harpoon, which moves at least three metres per second when released. They say “at least” because even at 1000 frames per second, the camera’s recording rate was too slow.