Winter is coming: calling all dragons

In Richard Attenborough’s BBC documentary The Living Planet (1984), episode “World’s Apart”, he discusses the evolution of Komodo Dragon’s in the isolated Indonesian island archipelago that comprises Komodo National Park (23:05). Attenborough states that Komodo dragons can grow up to 9 ft. in length and up to 100 lbs in weight, which is much larger than any related lizard species. He presents two possible explanations for the evolution of the dragon’s large size; first, that the species is the top predator on the island and doesn’t have competition for its food resources, and second, that the larger dragons have a positive advantage because they get a larger share of the food.

Although the exact reason for their evolution towards large size is debated, it is known that their isolation facilitated the process of change. Komodo dragons are naturally found in only one distinct location in the world, on five islands in the Komodo island archipelago of Indonesia. Komodo dragons developed in this island chain distinct from any other lizards in the world, and are currently the largest extant lizards known [2]. Because of their isolated and unique environment, the dragons quickly evolved differences to their smaller ancestor lizard species. In their 2006 study, Madsen et. al suggested one possible reason for the increased lizard size in the island environment; prey density. They observed that the island’s dragons vary greatly in size based on prey density; the denser the prey density, the larger the Komodos [2].

Not all lizards on this Indonesian island grow to be as huge as the Komodo dragon, however. Other types of monitor lizards on the island can be up to 10,000 times smaller (by body mass) that the giant Komodo [1]. Why then, of all the lizards, did the Komodo become so huge? One study found that lizards who occupy different habitats on the island experience different selection pressures. Lizards who live in terrestrial habitats, like the Komodo dragon, experience selection pressures that cause evolution of large body sizes, while other lizards who are rock-dwelling evolve to become very small [1].

This phenomenon, island gigantism, is also seen in many other island-dwelling animals: from the Galapagos tortoises to the speckled rattlesnakes off Baja California, you can expect to see many other giant animals on your next island vacation [3, 4]! 

by Sonora Martinez-Summers and Rachel Lee

The Living Planet: A Portrait of the Earth, episode “Worlds Apart,” starting at 23:05

References

1. Collar DC, Schulte II JA & JB Losos. 2011. Evolution of extreme body size disparity in monitor lizards (Varanus). Evolution: International Journal of Organic Evolution 65: 2664-2680
2. Jessop TS, Madsen T, Sumner J, Rudiharto H, Phillips JA & C Ciofi. 2006. Maximum body size among insular Komodo dragon populations covaries with large prey density. Oikos 112: 422-429.
3. Meik JM, Lawing AM, & A Pires-daSilva. 2010. Body size evolution in insular speckled rattlesnakes (Viperidae: Crotalus mitchellii). PLoS One, 5: 9524.
4. Jaffe AL, Slater, GJ, & ME Alfaro. 2011. The evolution of island gigantism and body size variation in tortoises and turtles. Biology letters, 7: 558-561.