Island Gigantism: How Islands Really Do Breed Giant Creatures

Islands of giant monsters aren't just in the movies. In real life, certain species grow to incredible size on islands. Why do they grow, and what does it mean for other species survival? We'll explore the different reasons — and show why this phenomenon reveals that we could probably kill off Godzilla.

The Islands Giants

If you're not a fan of rats, try to stay away from the island of Flores. The Flores giant rat can get about four feet in length, including the tail. If snakes aren't your thing, try avoiding Mount Chappell Island in Tasmania, where the outsize tiger snake stalks its outsize prey. Those who dislike creepy crawlies would be wise to steer clear of the South Atlantic island of Saint Helena, with its hand-sized earwigs. When it comes to reptiles, galapagos tortoises aren't fearsome, but everyone knows they should avoid the Komodo dragon.

Island Gigantism: How Islands Really Do Breed Giant Creatures

Go to the fossil record and there are more disturbing beasts. The Elephant Bird of Madagascar could reach twice the height of a human and was far bigger than velociraptors. Madagascar saw a lemur the size of a gorilla. In prehistoric Minorca, a giant rabbit roamed. It was hunchbacked and small-headed, with poor eyesight and hearing. With its size, it didn't need eyesight and hearing.

This is one major advantage of growing huge. Predators drop away as a creature gains size. Similarly, the more size an animal puts on, the more things around it start looking like viable prey. As monster movies show us, it's never pleasant to share a world with an animal that has increased its range of viable prey. What are the chances that we will?

Why Islands? And Why Giants?

There is no advantage to be gained by growing, if it will only put you in competition with an already-larger species. This is one of the reasons why islands, especially, tend to tip a species towards gigantism. When an animal, like a rabbit or a rat, is being preyed on, it's more to its advantage to shrink enough to be able to hide effectively than to grow and still fight ineffectively. The dearth of predators, and competitors for food, on islands mean that a species previously kept in check can grow and grow.

At least that was J Bristol Foster's reasoning in the 1960s, when he studied island gigantism and island dwarfism. Foster made tables of different species, how often they blew up or shrunk down on islands, and came up with the Island Rule. In island mammals, small animals and omnivores tended to get bigger, while carnivores and large animals tended to shrink down. In other words, hippos and elephants got small, and the rat was king.

Island Gigantism: How Islands Really Do Breed Giant Creatures

Later, Ted Case came along and showed it was a bit more complicated than that. Foster was right in that rats tended to come out the victor when it came to islands, but Case noted the exceptions, and began to theorize as to why they occurred. The path to gigantism has common factors — lack of predators, availability of more food — but there are a lot of variables, and everything has to be right for a species to grow. First, there has to be no physical restrictions on why an animal can enlarge. There's a reason why all the largest birds were walkers, not fliers. Once an animal gets to a certain size, flying isn't an option. Burrowing isn't an option, either. Hiding stops being an easy option. Food has to be exceptionally plentiful if, evolutionarily speaking, the animal has to lose some major advantage in order to grow bigger.

Timing also has to be right. Case found that on an island in Baja California, a certain subspecies of rattlesnake had grown giant, while the others had shrunk down. On the mainland, the relative size of the two species was reversed. Why? Case's theory was the small snake got to the island first, and started enlarging. Once the largest snake on the mainland got to the island, and found itself the smaller of the two, the best evolutionary response was to shrink, rather than grow.

Unfortunately for those who fear giant lizards, territoriality is probably also a major factor in which animals get big. When all predators are removed, and a species is stuck on an island, things get Malthusian. The population will expand until it consumes all the resources and starts to die off. If that's the future for big animals, it might be better to go small, and survive on the few scraps of food available to a starving population, than get big. Unless the species is territorial. Being able to aggressively attack other animals, even animals not of their own species, allows animals to carve out a territory and reserve all the food on it for personal use. The lesson here, dispiritingly enough, is that size is a great advantage to the vicious.

Portrait of a Giant Lizard

The Komodo dragon is the sexy lizard to study, when it comes to gigantism. Envenomed, aggressive, and large enough to dine on humans, it's the giant lizard that looms large in our imaginations. But the real animal to look at when it comes to determining the causes of gigantism is the giant chuckwalla. Chuckwallas are ordinary-looking lizards in the southwestern United States that drew scientists' eyes when they displayed a variation in size, and that variation correlated to the elevation they lived at. The giant chuckwalla isn't huge, it's only about 25 percent longer than its counterparts at lower elevations — but a 25 percent increase in length gets it about double the weight of animals living nearby, and scientists wanted to know why.

Island Gigantism: How Islands Really Do Breed Giant Creatures

Ted Case himself studied the chuckwalla, and found that the strongest factor correlated with the difference in chuckwalla size was the average rainfall in the region. While that's a dull detail, more rain, particularly in dry areas, means more food. Animals that had access to better diets got bigger. What confounded Case was whether or not the food alone made the chuckwallas bigger, or whether there was some genetic component as well.

In 1999 Christopher Tracy examined this with a simple experiment. He collected juvenile chuckwallas, giant and not, from different locations, put them in identical lab habitats, and served them unlimited amounts of food. (The fool! He's killed us all!)

At first it looked like the lack of food had been the only things holding the chuckwallas at low elevations back. They grew much faster than their counterparts at higher elevations, but when they reached maturity, they stopped growing.

The giant chuckwallas hit maturity, and just kept getting bigger. They might have, at first, gotten big because they were well-fed, but over time, the giant chuckwallas revealed actual genetic differences from their smaller, lower counterparts.

Why We Could Take Godzilla

When looking at lists of gigantic island species, one thing jumps out repeatedly at the reader - the word "extinct." The giant birds that roamed New Zealand, the huge lemurs of Madagascar, and even many of the giant rats have been wiped out. Guess which species is the leading candidate for the cause of their extinction?

When sources on island gigantism stress that a lack of predators contributes greatly to gigantism, they don't specify "large" predators. Humans are predators, even if we don't match our prey in size. While there are some giant island animals that have remained in human habitats, they tend to be the smaller giants — the giant lizards, the giant rats, and the giant bugs. They live because, unlike the actual giants, they can hide. Godzilla's best chance would be to hide as well.

Island Gigantism: How Islands Really Do Breed Giant Creatures

We're also more likely to kill Godzilla Terminator-style, before it was ever born. Size is a luxury good — which we should have guessed when we noticed that most giants have private islands. Size requires, as we've seen, lots of space, a lack of predators, a food-rich environment, and a long time to keep growing even after reaching maturity. With humans populating islands and fishing out the ocean, those luxuries are scarce.

There is one thing that the Godzilla movies get exactly right. When it comes to survival, the best chance the monster has is to attack. Territoriality is one of the few factors in creating gigantism that doesn't depend on a competitor-less and predator-less environment. A large animal that attacks, and clears its own path to resources, is more likely to thrive. When Godzilla is smashing up cities and killing off people, it's following the best strategy. Either it kills us, or we kill it.

[Via Gigantism and Dwarfism on Islands, The Song of the Dodo, Population Ecology.]