March 5, 2014 | 45
What with all the monitor-themed goodness around these parts lately (see links below), it seems only fitting that I provide a re-vamped, substantially updated version of this Tet Zoo ver 2 classic (originally published in September 2007). Here we go…
Without doubt, one of the coolest living animals on the planet is the Komodo dragon Varanus komodoensis, a giant flesh-eating lizard that kills water buffalo, eats children, has venom glands, and is impervious to bullets (ok, I made that last bit up).
Unknown to western science until 1912 (when it was ‘discovered’ by J. K. H. van Steyn van Hensbroek, and described in the same year by Peter A. Ouwens), the dragon is the largest living lizard, big adult males exceeding 3 m in total length and 70 kg in weight. As usual with big animals, there are diverse views on exactly how big it can be. Some seemingly reputable sources provide maximum lengths of 3.5 m and weights of 250 kg or so (Steel 1996) whereas others have 70 kg (or slightly more) as the greatest weight attained in the wild (Ciofi 2004). Mark Carwardine’s Guinness Book of Animal Records gives the St Louis Zoological Gardens specimen (presented in 1928 by the Sultan of Bima) as the record-holder for the species, and in 1937 this was apparently 3.1 m long and 166 kg (Carwardine 1995). The big caveat here, of course, is that captive animals tend to be much fatter than wild ones. There are, of course, stories about people seeing (and even killing) Komodo dragons 4 m, 5 m and even 7 m long. [Image below by MattKingston.]
It’s said that the Komodo’s legs and tail become proportionally shorter and stockier as it gets larger (in other words, these parts of the body exhibit negative allometry). As a juvenile it’s an excellent climber and in fact is predominantly arboreal – indeed, juveniles live such different lives from adults they effectively function as separate ‘species’ (Imansyah et al. 2008). Parthenogenesis sometimes occurs (Watts et al. 2006), though as yet we have no idea how common it is in the wild. As an adult it can dig burrows, locate carcasses that are more than 3 km away, swim from island to island, and kill mammals as big as deer, wild pigs, and even water buffaloes and humans. Using teeth and claws, it disembowels, hamstrings and slashes its prey, and it may also shake prey to break the neck.
‘Infector killers’ or envenomators?
Until recently, it was thought that dragons infect prey animals with a toxic bite, this toxicity resulting from the transmission of bacteria that are ‘stored’ in the interdenticle slots on the margins of the posterior teeth*, septicaemia and death being the result. This became known as the ‘infector killer’ hypothesis, later applied by some people to theropod dinosaurs like Tyrannosaurus (Abler 1992, Horner & Lessem 1993).
* Anterior teeth are unserrated on both anterior and posterior carinae.
However, I’m not sure it was ever a good idea, since observations indicate that dragons ordinarily kill prey by injuring them with their teeth and claws (Auffenberg 1981), as mentioned above. That is, they don’t generally bite things, release them, and then track them down over days or weeks. Furthermore, the dragons have comparatively good oral hygiene and apparently don’t leave fragments of rotting meat stuck in their teeth, as was assumed by proponents of the ‘infector killer’ idea.
Today, it’s well known that the supposed effects of septicaemia reported as a consequence of dragon bite more likely reflect envenomation, the dragon actually possessing a complicated mandibular venom gland that allow it to inject toxins and anticoagulants into the wounds of prey (Fry et al. 2009). The gland is unusually complex compared to other reptile venom glands, with a large posterior compartment, and five small, anterior ones that send off ducts that emerge between successive teeth (Fry et al. 2009). Oral venom glands are now known to be reasonably widespread in lizards (Fry et al. 2005) and a case has been made for their importance in the predation style used by Komodo dragons and other big monitors (like the giant, extinct V. priscus). However, some varanid experts are sceptical about this proposal, arguing that the venom’s importance may have been grossly over-stated.
While adult dragons today mostly eat mammals that have been introduced to Komodo, Rinca, Flores and the adjacent islands, fossils show that dragons previously lived alongside dwarf elephants (both Elephas and Stegodon), and it has even been argued that the dragon might be a specialised macropredatory giant that evolved to kill these (now extinct) endemic mammals (Diamond 1987). However, there were plenty of other small and mid-sized mammals that inhabited the same islands at the same time so “their diet was possibly more varied than just pygmy elephant” (Mitchell 1987).
Island giant or island dwarf, smarts, and sociality
Furthermore, the Komodo dragon almost certainly isn’t an example of island giantism. Instead it apparently retains the large body size of a mainland ancestor (Gould & MacFadden 2004), and fossils reveal that its current range is very much relictual, since it previously occurred on Australia in the Pliocene and Pleistocene (Hocknull et al. 2012) (for more on that, see the 2012 article Goanna-eating goannas: an evolutionary story of intraguild predation, dwarfism, gigantism, copious walking and reckless thermoregulation). In fact, rather than being an island giant, the Komodo dragon might – in part – be an island dwarf, since some of the modern populations restricted to small islands seem to have average sizes lower than those on the bigger islands. Proper study is needed to demonstrate this.
Dragons can be pugnacious beasts that might maim one another during combat (there’s a video where one dragon bites the entire tail off another individual); they also have cannibalistic tendencies. However, they also engage in appeasement behaviour, they show evidence in the wild of co-operative behaviour and have pretty clear dominance hierarchies, and they tame quickly and readily bond with humans. “It is the opinion of many keepers that dragons may be among or the most intelligent of reptiles” (Walsh et al. 2004, p. 12). As demonstrated by the awesome photos shown here, it is well documented (although poorly known) that they form ‘feeding scrums’ when attracted to a large carcass (Auffenberg 1981).
The variable, widespread dragon
Finally, the wild population of several thousand individuals is generally regarded as relatively stable. Excepting Flores, the range of V. komodoensis was declared Komodo National Park in 1980. Some of the smaller populations (such as those of Gili Motang and Flores) are regarded as particularly vulnerable to habitat loss and other threats, such as slash and burn farming (Ciofi et al. 1999).
Dragons are quite variable in genetic terms across their range, and the populations from the different islands can be recognised by eye once you know what to look for. Hien (2003) reported a new colour morph from Riung along the north-east coast of Flores. These dragons are greyish-yellow, with grey, white-speckled lower limbs. Are they just a local colour morph, or something more interesting? This population excepted, the two main populations on Flores (one in the northeast, the other in the far west) might or might not be closer to each other than the western population is to the one on Rinca. The Gili Motang and Komodo populations are apparently outside the clade formed by the Rinca and Flores animals, with the Komodo ones being the most distinct (Ciofi et al. 1999). This might show that these populations have been distinct for a long period of time (more than 10,000 years), or might indicate a complex colonisation history for the region. Anyway, no subspecies have been named… yet. Given that this modern range is very much relictual compared to the prehistoric one, we might wonder whether dragons were far more diverse genetically (and morphologically) in the past – in fact, they surely were.
The pictures used here of the group-feeding dragons were kindly supplied by Tim Isles (thanks Tim), but they’ve been used on the internet before: we lack data on who took the photographs and would be interested to get any news on this. Thanks to Hanneke Meijer for reminding me about Hien (2003).
For previous articles on varanids and other platynotan lizards, see…
Refs – -
Abler, W. L. 1992. The serrated teeth of tyrannosaurid dinosaurs and biting structures in other animals. Paleobiology 18, 161-183.
Auffenberg, W. 1981. The Behavioral Ecology of the Komodo Dragon. University of Florida Press, Gainesville.
Carwardine, M. 1995. The Guinness Book of Animal Records. Guinness Publishing, Enfield, Middlesex.
Ciofi, C. 2004. Varanus komodoensis. In Pianka, E. R., King, D. & R. A. King (eds) Varanoid Lizards of the World. Indiana University Press, Bloomington, pp. 197-204.
- ., Beaumont, M. A., Swingland, I. R. & Bruford, M. W. 1999. Genetic divergence and units for conservation in the Komodo dragon Varanus komodoensis. Proceedings of the Royal Society of London B 266, 2269-2274.
Diamond, J. 1987. Did Komodo dragons evolve to eat pygmy elephants? Nature 326, 832.
Fry, B. G., Vidal, N., Norman, J. A., Vonk, F. J., Scheib, H., Ramjan, S. F. R., Kuruppu, S., Fung, K., Hedges, S. B., Richardson, M. K., Hodgson, W. C., Ignjatovic, V., Summerhayes, R. & Kochva, E. 2005. Early evolution of the venom system in lizards and snakes. Nature 439, 584-588.
- ., Wroe, S., Teeuwisse, W., van Osch, M. J. P., Moreno, K., Ingle, J., McHenry, C., Ferrara, T., Clausen, P., Scheib, H., Winter, K. L., Greisman, L., Roelants, K., van der Weerd, L. Clemente, C. J., Giannakis, E., Hodgson, W. C., Luz, S., Martelli, P., Krishnasamy, K., Kochva, E., Hang Fai Kwok, Scanlon, D., Karas, J., Citron, D. M., Goldstein, E. J. C., Mcnaughtan, J. E. & Norman, J. A. 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 106, 8969-8974.
Gould, G. C. & MacFadden, B. J. 2004. Gigantism, dwarfism, and Cope’s rule: “nothing in evolution makes sense without a phylogeny”. Bulletin of the American Museum of Natural History 285, 219-237.
Hien, P. 2003. Erster photographischer Nachweis von ungewöhnlich gefärbten Varanus komodoensis (OUWENS 1912) bei Riung, Flores, Indonesien. Sauria 25 (1), 3-9.
Hocknull, S. A., Piper, P. J., van den Bergh, G. D., Due, R. A., Morwood, M. J., et al. 2009. Dragon’s paradise lost: palaeobiogeography, evolution and extinction of the largest-ever terrestrial lizards (Varanidae). PLoS ONE 4(9): e7241. doi:10.1371/journal.pone.0007241
Horner, J. R. & Lessem, D. 1993. The Complete T. Rex. Souvenir Press, London.
Imansyah, M. J., Jessop, T. S., Ciofi, C. & Akbar, Z. 2007. Ontogenetic differences in the spatial ecology of immature Komodo dragons. Journal of Zoology 274, 107-115.
Mitchell, P. B. 1987. Here be Komodo dragons. Nature 329, 111.
Steel, R. 1996. Living Dragons. Blandford, London.
Walsh, T., Visser, G. & Lewis, R. 2004. Komodo Dragon Husbandry Manual of the AZA/SSP & EAZA/EEP: accessed online, 2005, at http://www.varanusweb.com/species_content/Kd-HDL-2004.pdf.
Watts, P. C., Buley, K. R., Sanderson, S., Boardman, W., Ciofi, C. & Gibson, R. 2006. Parthenogenesis in Komodo dragons. Nature 444, 1021-1022.