Insular dwarfism

The skeleton of a dwarf elephant from the island of Crete.

Insular dwarfism, a form of Phyletic dwarfism , is the process and condition of the reduction in size of large animals – almost always mammals – when their gene pool is limited to a very small environment, primarily islands. The intentional breeding of insular dwarfism is called dwarfing.

This effect has made itself manifest many times throughout natural history, including dinosaurs, like Europasaurus, and modern animals such as elephants and human beings.

There are several proposed explanations for the mechanism which produces such dwarfism, which are often considered likely to be co-contributing factors, including an evolved gene-encoded response to environmental stress, as well as a selective process where only the smaller of the animals trapped on the island survive, as food declines to a borderline level. The smaller animals need fewer resources, and so are more likely to get past the break-point where population decline allows food sources to replenish enough for the survivors to flourish.

Examples

Among the most famous examples of insular dwarfism are:

• The Channel Islands Mammoth which lived on the prehistoric island of Santa Rosae in the California Channel Islands, and the small woolly mammoths of Saint Paul Island off Alaska, and Wrangel Island north of Siberia.
• Dwarf elephants in the recent natural history of Malta, Crete, Cyprus, and Sicily.
• Dwarf stegodons (elephant relatives) from the recent natural history of Flores, Sulawesi, Sumba and Timor.
• Dwarf ground sloths in the recent natural history of Cuba, Hispaniola, and Puerto Rico.
• A (disputed) species of hominin called Homo floresiensis, from fossils found on Flores Island, Indonesia.
• Small-Bodied humans from Palau, Micronesia, similar in size to the Flores hominins.
• Dinosaurs, including the recently validated Europasaurus, on Mesozoic islands such as Haţeg Island.

There are also proposed instances of this process occurring among plant life, the appearance of dwarf sequoia / redwood trees being one such proposal.

This process, and other "island genetics" artifacts, can occur not only on traditional islands, but also in other situations where an ecosystem is isolated from external resources and breeding. This can include caves, desert oases, and isolated valleys. An example of this is the "pygmy" people of Africa, such as the Mbuti, who evolved a small size while in genetic and ecological isolation in a dense jungle region.

There is an inverse form of this process, island gigantism, wherein small animals, lacking the predators of their normal homes, may become "gigantic" when breeding in isolation. An excellent example is the dodo, the ancestors of which were normal-sized pigeons.

Additional examples

Carnivora

• Channel Island fox in California, United States, and the enigmatic, nearly extinct Cozumel fox
• Honshū Wolf in Japan
• Bali Tiger
• Cozumel Island Raccoon
• Sardinian Dhole
• Sardinian Lynx

Ungulates

• The strange, tiny Balearic Islands Cave Goat (Myotragus balearicus) in Majorca and Minorca and its close relative Nesogoral, from Sardinia which became extinct after human settlement
• Philippine water buffalo, Tamaraw, Anoa, the extinct Bubalus cebuensis and other dwarf bovids from South East Asia
• Several prehistoric species of dwarf hippopotamus from the Pleistocene of Mediterranean islands (such as Hippopotamus minor) and Madagascar
• Dwarf deer species on the Philippines (Philippine Sambar, extant), and Crete (Candiacervus ropalophorus), Ryukyu Islands of Japan (Cervus astylodon) and Gargano promontory of South Italy (Hoplitomeryx)(all extinct)
• Key Deer

Other

• The tiny flightless Inaccessible Island and Laysan Rails
• The dwarf nodosaurid Struthiosaurus from Europe, and the dwarf allosaur from South Australia

See also

• Island gigantism
• Foster's rule
• Deep-sea gigantism
• Pleistocene extinctions
• Baron Franz Nopcsa

External links

• Strange world of island species October 31, 2004 The Observer