Saturday, 20 May 2017

Komodo Dragons. A peaceful morning in the Komodo National Park and an acrimonious debate on reptilian venoms

Last September’s Expedition Cruise from Darwin through the Lesser Sunda islands of East Timor and Indonesia included stops at Rinca and Komodo. Ever since my fourth cousin once removed visited Komodo in 1956 I have wanted to see Komodo Dragons alive and in their natural habitat. They did not disappoint. We saw very large males (hanging out around the kitchen of the ranger station on Rinca hoping for a hand out), females and juveniles (but not the hatchlings which apparently take to the trees to avoid their predatory parents). One female was digging out the nest mound of a megapode, the Orange-footed Scrubfowl (Megapodius reinwardt) in which to lay her own eggs, September being the egg-laying time in their breeding cycle.

When we got back I had a chance to look at and think about the various hypotheses that have been advanced as to how Komodo Dragons—and, possibly, some other monitor lizards—kill their prey. The highly publicised but never properly tested proposal that Komodo Dragons are venomous in the sense that venomous snakes are venomous, i.e. toxins delivered by injection having a very or fairly rapid systemic effect on the prey, seems to be in the process of being discarded in the course of some pretty acrimonious arguments, along with the associated hypothesis of a single, early origin of venom in reptilian evolution—the Toxicofera hypothesis. The other idea, that pathogenic bacteria harboured in the mouth of dragons causes sepsis in prey animals that are bitten but escape the initial attack, doesn’t seem that convincing or special given the likelihood of infection from a bite from any animal, as any postman will testify, especially if that postman were to immerse his bitten ankle in fetid water, as non-native water buffalo do when bitten by a Dragon. If I were to bite a Timor Deer on the leg, there is every likelihood an infected wound that could impede mobility would ensue. 

Given the opposition to the Toxicofera hypothesis and the contentious nature of the evidence for Komodo Dragons being venomous, it seems a pity that the BBC repeated last November the screening of an episode from its 2011-12 season of the Natural World Series called Komodo—Secrets of the Dragon that was devoted virtually entirely to that proposition and its main protagonist. Had this been a programme about a topic other than science-led natural history the BBC would have been falling over itself to show someone with the opposite view, however perverse. As it is, the viewing public in Britain has been left with the impression that the venomous nature of the Dragon is accepted. At the very least the BBC should have been aware of what was going on (by reading Wikipedia for example) and before rescreening it should have added an annex to the original programme. That annex could have  explained the opposing views and evidence—some of which made news media reports*—that have been accumulating since 2009. Adam Hargreaves (Oxford), Abigail Tucker (King’s, London) and John Mulley (Bangor), who produced a devastating critique** of the toxicoferan hypothesis in 2015, should have been consulted and involved. But let’s leave the BBC and its usually excellent (but sometimes spectacularly poor) natural history programmes and return to the mouth of the dragon.

None of the criticisms of the venom hypothesis imply that the composition and quantity of saliva are not of selective advantage to the despatch of prey, subsequent swallowing and digestion or protection of the oral cavity against infection. Nor, indeed, do they refute the idea or evidence that saliva may have local beneficial—to the Dragon—effects on inflicted wounds, like the application of a secreted anticoagulant, for example.

The drooling mouths of some of the male Dragons we saw were impressive. The only comparable example I could think of was my late mother-in-law’s Boxer dogs given the slightest hint of finding something edible. With no obvious sign of food or feeding activity, is Dragon saliva being used for some other purpose like scent marking?

The accumulated data that I have found suggests to me that Komodo Dragons, as originally thought, kill their prey by sheer brute force from wounds inflicted by a very large mouth with very big teeth. In one study†, 17 attacks on large prey were observed, 12 were fatal. Of the 5 that escaped with injuries to their limbs and rump, 1 was quickly attacked and killed by a second Dragon, 2 died within hours, one fled being pursued by other Dragons and one limped away without being pursued. The eventual fate of the two that possibly survived is not known.

One may though ask how, if the Komodo Dragon originally preyed upon the now extinct dwarf elephant, Stegodon, of Flores, as suggested in 1987 by Jared Diamond, those beasts (smaller than a domestic water buffalo) were killed? Would biologically-active substances in saliva have been of selective advantage?

Nowhere have I found (but I am not have looked in the right place) any discussion of the selective benefits of a slow death of the prey to the predator. A snake with a fast-acting neurotoxin can quickly track and swallow its prey without that investment in metabolically expensive venom being lost to another snake or any old predator happening to find the corpse. But let’s say a Dragon does inject toxins with a small bite and the animal runs away to die. There is no guarantee that the investment in venom would pay off. With lots of other Dragons around, the prey could be lost entirely or the original killer would get only a small share in a communal but competitive feeding session. Surely, if monitor lizards are venomous at all then why have they not evolved a more effective venom to ensure a quick kill?

The venomous dragon and other monitor lizards hypothesis has been around now for ten years. I find it sad that so little has been done at the whole animal and tissue levels to test it. I found some of the original observations and experiments‡ unconvincing. But given the captive populations of Dragons and a local abundance on Komodo and Rinca, a rigorous examination of the whole question cannot be beyond the bounds of practical realisation or funding.

*e.g. Zimmer C. 2009. Chemicals in Dragon’s Glands Stir Venom Debate. New York Times, 18 May 2009. Yong E. 2015. A Venomous Fight Among Reptile Scientists. The Atlantic, 2 November 2015

*Hargreaves AD, Tucker, AS, Mulley JF. 2015. A critique of the toxicoferan hypothesis. In, Gopalakrishnakone P (ed), Malhotra M (ed). A critique of the toxicoferan hypothesis. In Evolution of Venomous Animals and Their Toxins: Toxicology. Springer Netherlands, p 1-15. DOI: 10.1007/978-94-007-6727-0_4-1

†Bull JJ, Jessop TS, Whiteley M. 2010. Deathly drool: evolutionary and ecological basis of septic bacteria in Komodo Dragon mouths. PLoS ONE 5(6): e11097. doi:10.1371/journal.pone.0011097

‡Fry BG Wroec S, Teeuwisse W, van Osch MJP, Moreno K, Ingle J, McHenry C, Ferrara T, Clausen P, Scheib H, Winter KL, Greisman L., Roelants K, van der Weer L, Clemente CJ, Giannakis E, Hodgson WC, Luz S, Martelli P, Krishnasamy K, Kochva E, Kwok H, Scanlon D, Karas J, Citron DM, Goldstein EJC, Mcnaughtan JE, Norman JA. 2009. A central role for venom in predation by Varanus komodoensis (Komodo Dragon) and the extinct giant Varanus (Megalania) priscus. Proceedings of the National Academy of Sciences of the USA 106, 8969-8974 doi 10.1073 pnas.0810883106