Insular dwarfism

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Insular dwarfism, a form of Phyletic dwarfism,^[1] is the process and condition of the reduction in size of large animals – typically mammals but also dinosaurs – when their population's gene pool is limited to a very small environment, primarily islands. This natural process is distinct from the intentional creation of dwarf breeds, called dwarfing.

This process has occurred many times throughout evolutionary history, with examples including dinosaurs, like Europasaurus, and modern animals such as elephants and their relatives.

There are several proposed explanations for the mechanism which produces such dwarfism, which are often considered likely to be co-contributing factors. One explanation is an evolved gene-encoded response to environmental stress. Another is a selective process where only smaller 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.

Contents 1 Examples 2 Additional examples 3 See also 4 References 5 External links

[edit] Examples

Among the most famous examples of insular dwarfism are:

• Dinosaurs, including the recently validated Europasaurus, on Mesozoic islands such as Haţeg Island. Other dwarf dinosaurs mainly from Haţeg Island, Romania, include Magyarosaurus, Rhabdodon and Telmatosaurus.
• A dwarf form of crocodile from the Early Cretaceous Period is Bernissartia, which grew no bigger than a domestic cat.
• Dwarf ground sloths in the recent natural history of Cuba, Hispaniola, and Puerto Rico.

• 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.^[2]
• Dwarf stegodons (elephant relatives) from the recent natural history of Flores, Sulawesi, Sumba and Timor.^[3]
• A recently confirmed separate species of hominid called Homo floresiensis, from fossils found on Flores Island in Indonesia.^[4]
• Small-bodied humans from Palau, Micronesia, similar in size to the Flores hominins.^[5]

There are also proposed instances of this process occurring among plant life, the appearance of dwarf sequoia / redwood trees being one such proposal.^{[citation needed]}

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.

There is also an inverse process, island gigantism, wherein small animals breeding on isolated islands lacking the predators of large land masses may become much larger than normal. An excellent example is the dodo, the ancestors of which were normal-sized pigeons. There are also several species of giant rats, some extinct and some still extant, that coexisted with both Homo floresiensis and the dwarf stegodons on Flores.

[edit] 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
• Goats from the Juan Fernández Islands, in Chile
• 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

[edit] See also

Wikinews has related news: Bones of "small-bodied humans" found in cave

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

[edit] References

[edit] External links

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