Did Komodo dragons evolve to eat pygmy elephants? When Jared Diamond posed this question in a 1987 Nature article, he answered in the affirmative. The 10-foot lizards owed their origin to a diet of diminutive elephants isolated on Indonesian islands. But, thanks to research that will no doubt disappoint fans of sensational prehistoric vignettes, Diamond’s speculative story has entirely unraveled.

Diamond wasn’t the first to suggest that Komodo dragons were elephant-hunters. Walter Auffenberg – an expert on the big lizards – first proposed the connection in a 1981 monograph. But Diamond’s short article has often been cited as the standard origin story for the Komodo dragons, despite the fact that the news item contained no new research whatsoever. Diamond simply connected the dots that had been laid out.

The picture looked something like this. Sometime over 50,000 years ago, during the Pleistocene, relatively small monitor lizards swam to islands such as Flores. These places were devoid of large mammalian carnivores, and so the lizards grew to intimidating girth to fill the vacant "big predator" niche. Simultaneously, due to a common trend that is not well understood, prehistoric elephants that inhabited the islands shrank in size. Comparatively tiny elephants would have been ample fare for the inflated monitor lizards. This same trend probably occurred over and over again on multiple islands, Diamond speculated, creating a chain of habitats ruled by razor-toothed, super-sized lizards. “Had Alfred Russel Wallace been able to make his famous journey through the Malay Archipelago 50,000 years ago,” Diamond wrote, “he might have found giant monitors harassing pregnant pygmy elephants on many islands.”

Komodo dragons might have dined upon dwarfed island elephants, especially when the lizards stumbled across carcasses of the small proboscideans. But the key question is whether this interaction can really explain the evolution of the Komodo dragon and not just part of the lizard’s ecology. Despite criticism from other experts such as Peter Balfour Mitchell – who pointed out that prehistoric Komodo dragons had a variety of prey to choose from and likely had a diet that “was possibly more varied than just pygmy elephant” – Diamond reaffirmed his hypothesis in a 2001 PNAS study with Gary Burness and Timothy Flannery. Diamond and collaborators assumed that the Komodo dragon was native to Flores, and had evolved there because of peculiar interactions between physiology and land area that favored a reptilian predator with a slow metabolism over a mammalian carnivore with a more active one. As he had stated many years before, Diamond believed that the Komodo dragon was a unique product of island evolution.

But we now know that the Komodo dragon didn’t evolve on Flores or any other isolated patch of dry land in the Pacific. The monitor lizards were already quite large when they arrived on their island homes. The long record of large monitor lizards in Australasia has overturned the elephant-hunting hypothesis.

The travels of giant monitor lizards over the past four million years. While Komodo dragons appear to have originated on Australia, they eventually moved from island to island. Today's Komodo dragon populations are a relic of their previous distribution through Indonesia and Australia. Image from Hocknull et al., 2009. The travels of giant monitor lizards over the past four million years. While Komodo dragons appear to have originated on Australia, they eventually moved from island to island. Today's Komodo dragon populations are a relic of their previous distribution. Image from Hocknull et al., 2009.

In 2009, paleontologist Scott Hocknull and colleagues surveyed the fossil record of large monitor lizards from India, Indonesian islands, and Australia over the past four million years. According to their findings, the Komodo dragon wasn’t an aberrant island giant. As had already been suggested by previous studies using different lines of evidence, the lizard was just one of many large-bodied monitors that proliferated through the Indonesian archipelago. Indeed, rather than being an island oddity, the Komodo dragon probably originated on Australia sometime around 3.8 million years ago. While some populations remained on the continent until about 130,000 years ago – overlapping with the larger “ripper lizard” Varanus prisca – other dragons island-hopped to Timor and onward to Flores and Java.

Not all the island populations were equally long-lived. Komodo dragons had an ephemeral presence on Java, but the lizards have lived on Flores for the last 900,000 years or so. And the length of time the Komodo dragons have been present on Flores provides another critical piece of evidence against the idea that these monitors owe their size to hunting elephantine prey. The remains of the prehistoric lizards correspond closely to the bones of their modern relatives – the Komodo dragons on Flores have been in stasis for almost a million years. The lizards didn’t change when dwarfed elephants arrived on the islands, nor with the arrival of different human species (including the ancestors of the “hobbit” Homo floresiensis). While other forms of life came and went, including the small elephants, the Komodo dragons persisted.

The rise of the giant monitor lizards wasn’t tied to an absence of meat-eating mammals, either. Hocknull and co-authors pointed out that, while not quite as large as the Komodo dragon, another group of large monitor lizards evolved among hyenas and big cats in India over two million years ago. Likewise, the Komodo dragons that made it to Java lived alongside similar carnivorans. In fact, the earliest known giant monitor lizard lived among a variety of carnivorous mammals around 7 million years ago. Recently named Varanus amnhophilis by Jack Conrad and collaborators, this monitor had a body length (excluding the tail) of about two feet – making it larger than the vast majority of other known lizards and therefore a member of the “giant” category. Nevertheless, the lizard lived among weasels, hyenas, and pigs, all of which were large enough to have preyed on the reptile. Despite the belief that reptiles could have only become giants in the absence of carnivoran competition, the fossil record shows that huge monitors evolved and persisted alongside a variety of flesh-tearing mammals.

Regardless of whether Komodo dragons ever supped on tiny elephants, the monitors do not owe their origins to such bizarre prey. But we are left with the question of why so many lineages of monitors grew so large. Islands were not the key, nor was freedom from mammalian competition. There must be some ecological or evolutionary clue to the puzzle that still eludes us. We now have a rough idea of where these giant lizards came from, and how they came to reside on the scattered islands they continue to haunt, but the essential evidence for why they became dragons of such imposing size remains a mystery.

[This post is a follow-up to my Komodo dragon venom essay from last week. Thanks to Hanneke Meijer for reminding me of the Hocknull et al. paper, and to Victoria Herridge, Darren Naish, Ed Yong, David Quammen, and others for kicking up a discussion of Diamond's idea on Twitter.]

References:

Burness GP, Diamond J, & Flannery T (2001). Dinosaurs, dragons, and dwarfs: the evolution of maximal body size. Proceedings of the National Academy of Sciences of the United States of America, 98 (25), 14518-23 PMID: 11724953

Conrad JL, Balcarcel AM, & Mehling CM (2012). Earliest example of a giant monitor lizard (varanus, varanidae, squamata). PloS one, 7 (8) PMID: 22900001

Diamond, J. (1987). Did Komodo dragons evolve to eat pygmy elephants? Nature 326: 832

Gould GC, MacFadden BJ (2005) Gigantism, Dwarfism and Cope's Rule: “Nothing in Evolution Makes Sense without a Phylogeny”. Bulletin of the American Museum of Natural History 285(1): 219–237.

Hocknull SA, Piper PJ, van den Bergh GD, Due RA, Morwood MJ, & Kurniawan I (2009). Dragon's paradise lost: palaeobiogeography, evolution and extinction of the largest-ever terrestrial lizards (Varanidae). PloS one, 4 (9) PMID: 19789642

Mitchell, P. (1987). Here be Komodo dragons. Nature 329: 111