Insular gigantism is when animals on isolated islands grow larger due to the lack of predators and competition. This is caused by the removal of constraints rather than new selection pressure. Examples include giant tortoises, Komodo dragons, Flores giant rats, Madagascar hissing cockroaches, and the Lord Howe Island stick insect. Many extinct bird species, such as the dodo, moa, Haast’s eagle, and elephant bird, also exhibited island gigantism.
Insular gigantism is a biological phenomenon in which animals living on isolated islands tend to grow larger due to lack of predators and competition that would otherwise be present on land. Rather than being caused by new selection pressure, as is thought to be the case with the complementary phenomenon of island dwarfism, island gigantism is caused by the removal of constraints. The result is organisms that swell to a heavy size. Insular gigantism is an apparent counterexample to Bergmann’s rule, a generality that states that animals living near the equator tend to be smaller. Some islands with species exhibiting island gigantism are found near the equator, but still have unusually large animals.
There are numerous interesting examples of insular gigantism, mostly occurring between relatively small animals to begin with. There are the famous giant tortoises, which are found on the Seychelles and Galápagos Islands. These icons of the island’s unusual fauna are members of one of the oldest surviving orders of reptiles. One individual, Harriet the Tortoise, who was once a pet of Charles Darwin, is the longest-living vertebrate known, having been born in 1830 and only died in 2005, 175 years later. Scientists have analyzed the internal organs of these turtles and found that they look the same whether the animal is old or young, causing speculation that these turtles exhibit senescence (increased mortality with age) as most do of the other animals.
Another less friendly giant island reptile is the 2-3 m (6.5-10 ft) Komodo dragon of Indonesia, found only on the islands of Komodo, Rinca, Flores, Gili Motang and Gili Dasami. The Komodo dragon is an ambush and scavenger predator that can kill numerous small animals within its range, making it the dominant predator on the islands where it inhabits. A reptile taking the place of the apex predator is an ecological oddity, as mammalian predators began routinely occupying this position long ago, after the extinction of the dinosaurs. The Komodo dragon is a reminder of a bygone evolutionary era. A rare example of island gigantism in a carnivore, the Komodo dragon is by far the largest living species of lizard.
Island gigantism is rarer among mammals, but an example is the Flores giant rat, found on Flores Island in Indonesia, measuring 41-45 cm (1.3-1.5 ft) with a tail length of 33 -70 cm, at least twice the size of the more familiar Brown Rat. Numerous other gigantic mammals, mostly rodents, existed at one point but are now extinct. These include the giant hutia, a West Indian rodent about the size of an American black bear, the giant dormouse of Majorca and Menorca, the giant lemurs of Madagascar, and giant rabbits and shrews from various isolated Mediterranean islands.
Other examples of island gigantism can be found among insects. The Madagascar hissing cockroach, found in Madagascar off the southeast coast of Africa, is the largest living cockroach and the only known insect capable of hissing by forcing air through its spiracles (breathing holes ) rather than rubbing the appendages together. Surprisingly, Madagascar hissing cockroaches are kept as beloved pets around the world and can live up to five years. New Zealand’s giant weta, whose genus name, Deinacrida, is Greek for “terrible grasshopper,” is among the heaviest insects on Earth, weighing more than a sparrow. While the weta is a familiar site to New Zealand locals, its sheer appearance is enough to make a visitor scream in shock.
The Lord Howe Island stick insect is another interesting example of insular gigantism. This half-foot-long stick insect, called the “walking sausage” for its appearance, was thought to have gone extinct in the 1930s, only to be rediscovered at Ball’s Pyramid, the world’s tallest and most isolated sea stack between New Zealand and Australia. A population of 20-30 individuals has been found under a single Melaleuca shrub. A part of the population was taken into captivity and raised, now numbering 50 individuals and thousands of eggs. Researchers hope to reintroduce the bugs to nearby Howe Island following the completion of a campaign to eliminate invasive rats on the island.
Many of the beneficiaries of the island’s gigantism are birds, the vast majority of which have become extinct due to humans and the species we have introduced, especially black rats. Probably the most famous is the dodo, which lived on the island of Mauritius in the Indian Ocean until it became extinct between 1650 and 1700. The dodo is sometimes thought of as a giant pigeon or dove, as it is closely related to both species. It has not had a chance to survive humans and new predators introduced to Mauritius. Over the past several hundred years, many thousands of island bird species have similarly gone extinct, often when mice raid their nests and eat the eggs faster than they can hatch.
Some of the largest birds of recent times were island giants, notably the moa and Haast’s eagle of New Zealand and the elephant bird of Madagascar. The moa bird and the elephant both exceeded three meters (10 ft) in height, while Haast’s eagle had wingspans of 2.6 to 3 m (8 to 10 ft). The elephant bird’s weight is thought to have approached half a ton (1,000 pounds). Both of these amazing creatures went extinct around the 16th century and possibly a few hundred years earlier, likely wiped out by western settlers and the species they introduced. The moa and the elephant were both ratites, flightless birds that include such surviving species as the cassowary, ostrich, rhea, emu, and kiwi, all but the last one being quite large by bird standards. Haast’s eagle is probably the largest eagle that ever lived and would have been capable of killing humans with a single strike. While it likely preyed primarily on moa, Haast’s eagle became extinct recently enough to have had encounters with humans and may have actually been exterminated by us, as we would probably have considered it a threat. Haast’s eagle’s display of island gigantism is another rare example of predatory island gigantism.
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