Island gigantism

  Island gigantism is a biological phenomenon where the size of animals isolated on an island increases dramatically over generations. It is a form of natural selection in which bigger size provides a survival advantage (see Bergmann's Rule). Large size in herbivores usually makes it harder to escape or hide from predators, but on islands, these are often lacking. Thus, island gigantism is not an evolutionary trend due to fundamentally new parameters determining fitness (as in island dwarfing), but rather the removal of constraints. With the arrival of humans and associated predators (dogs, cats, rats, pigs), many giant island endemics have become extinct. As opposed to island dwarfing, island gigantism is found in most major vertebrate groups and in invertebrates.

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Examples

Examples of island gigantism include:

Mammals

• Rodents
  • Giant rabbits and shrews from Mediterranean islands (extinct)
  • Flores Giant Rats
  • Giant hutias from the West Indies (extinct)
  • Majorcan Giant Dormouse and Minorcan Giant Dormouse (extinct)

Many rodents grow larger on islands, whereas lagomorphs, carnivores, proboscideans and artiodactyls usually become smaller.

• Primates
  • the extinct giant lemur genera Archaeoindris, Palaeopropithecus and Megaladapis of Madagascar (strictly speaking, Madagascar is not an island but a mini-continent in the context of biogeography)

Birds

• Ratites
  • the elephant bird, the world's largest bird, formerly living on Madagascar (extinct)
  • the extinct moa of New Zealand.
• Waterfowl
  • Moa-nalos, extinct giant ducks from Hawaii.
• Wildfowl
  • Sylviornis neocaledoniae a huge extinct megapode-like bird from New Caledonia
  • some extinct Polynesian megapodes.
• Rails
  • the Takahe from New Zealand and related species from Melanesia and Polynesia.
• Seabirds
  • the extinct Spectacled Cormorant from Bering Island.
• Pigeons
  • Dodo and Rodrigues Solitaire from the Mascarenes
  • the extinct flightless Viti Levu Giant Pigeon.
• Birds of prey
  • the extinct Haast's Eagle and Eyles' Harrier of New Zealand.
• Parrots
  • the extinct Broad-billed Parrot from Mauritius, an undescribed huge extinct parrot from Easter Island, and the Kakapo of New Zealand.

Reptiles

• Turtles
  • Giant tortoises on the Seychelles, Galápagos Islands and formerly the Mascarenes
• Lizards
  • Komodo dragon, a rare example of a giant insular carnivore (Islands tend to offer limited food and territories, thus island carnivores are usually smaller in size than continental ones)
  • San Esteban Chuckwalla (Sauromalus varius) of islands off Baja California
  • Leiolopisma mauritiana and Macroscincus coctei, two extinct skinks from Mauritius and Cape Verde, and the rare New Caledonian skink Phoboscincus bocourti
  • the extinct Rodrigues giant day gecko and New Zealand giant gecko, and the extant New Caledonian giant gecko

Insects

• Madagascar hissing cockroach
• St. Helena Earwig
• Conant's Giant Nihoa Tree Cricket
• Lord Howe Island phasmid
• Giant weta of New Zealand

Flora

• Megaherb

In popular culture

The movie King Kong provides a fictional (and exaggerated) example of Island gigantism. The animals, bugs and plants found on Skull Island all present monstrous sizes.

See also

• Island dwarfing
• Island rule
• Deep-sea gigantism