Insular dwarfism, a form of phyletic dwarfism, is the process and condition of the reduction in size of large animals over a number of generations[a] when their population's range is limited to a 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. 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, isolated valleys and isolated mountains ("sky islands"). Insular dwarfism is one aspect of the more general "island rule", which posits that when mainland animals colonize islands, small species tend to evolve larger bodies, and large species tend to evolve smaller bodies.
Possible causes of insular dwarfism
One is a selective process where only smaller animals trapped on the island survive, as food periodically declines to a borderline level. The smaller animals need fewer resources and smaller territories, 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. Smaller size is also advantageous from a reproductive standpoint, as it entails shorter gestation periods and generation times.
Among herbivores, large size confers advantages in coping with both competitors and predators, so a reduction or absence of either would facilitate dwarfing; competition appears to be the more important factor.
Among carnivores, the main factor is thought to be the size and availability of prey resources, and competition is believed to be less important. In tiger snakes, insular dwarfism occurs on islands where available prey is restricted to smaller sizes than are normally taken by mainland snakes. Since prey size preference in snakes is generally proportional to body size, small snakes may be better adapted to take small prey.
Dwarfism versus gigantism
The inverse process, wherein small animals breeding on isolated islands lacking the predators of large land masses may become much larger than normal, is called island gigantism. An excellent example is the dodo, the ancestors of which were normal-sized pigeons. There are also several species of giant rats, one still extant, that coexisted with both Homo floresiensis and the dwarf stegodons on Flores.
The process of insular dwarfing can occur relatively rapidly by evolutionary standards. This is in contrast to increases in maximum body size, which are much more gradual. When normalized to generation length, the maximum rate of body mass decrease during insular dwarfing was found to be over 30 times greater than the maximum rate of body mass increase for a ten-fold change in mammals. The disparity is thought to reflect the fact that pedomorphism offers a relatively easy route to evolve smaller adult body size; on the other hand, the evolution of larger maximum body size is likely to be interrupted by the emergence of a series of constraints that must be overcome by evolutionary innovations before the process can continue.
Factors influencing the extent of dwarfing
For both herbivores and carnivores, island size, the degree of island isolation and the size of the ancestral continental species appear not to be of major direct importance to the degree of dwarfing. However, when considering only the body masses of recent top herbivores and carnivores, and including data from both continental and island land masses, the body masses of the largest species in a land mass were found to scale to the size of the land mass, with slopes of about 0.5 log(body mass/kg) per log(land area/km2). There were separate regression lines for endothermic top predators, ectothermic top predators, endothermic top herbivores and (on the basis of limited data) ectothermic top herbivores, such that food intake was 7 to 24-fold higher for top herbivores than for top predators, and about the same for endotherms and ectotherms of the same trophic level (this leads to ectotherms being 5 to 16 times heavier than corresponding endotherms).
Among the most famous examples of insular dwarfism are:
- Dinosaurs, including the recently validated Europasaurus, on Mesozoic islands such as Haţeg Island, Romania. Other dwarf dinosaurs, mainly from Haţeg Island, include Magyarosaurus, Rhabdodon, Balaur bondoc and Telmatosaurus. Also, the dwarf nodosaurid Struthiosaurus from Europe.
- Dwarf ground sloths in the recent natural history of Cuba, Hispaniola, and Puerto Rico. (There is also a living dwarf tree sloth on Isla Escudo de Veraguas.)
- 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 or mammoths in the recent natural history of Malta, Crete, Cyprus, Sardinia, Sicily and several of the Cyclades and Dodecanese islands.
- Dwarf stegodonts (elephant relatives) from the recent natural history of the Philippines, Flores, Sulawesi, Sumba and Timor.
- A recently confirmed separate species of hominid called Homo floresiensis, from fossils found on Flores Island in Indonesia.
- Small-bodied humans from Palau, Micronesia, similar in size to the Flores hominins (disputed).
- Bali tiger †
- Channel Island fox in California, United States, and the enigmatic, nearly extinct Cozumel fox
- Cozumel raccoon
- Honshū wolf † in Japan
- Sardinian dhole †
- Zanzibar leopard, possibly extinct
- Key deer of the Florida Keys
- Extinct red deer populations in Jersey
- Philippine sambar
- Candiacervus ropalophorus †, Crete
- Cervus astylodon †, Ryukyu Islands of Japan
- Hoplitomeryx †, the Gargano promontory (the former Gargano Island) of South Italy
- Philippine water buffalo, tamaraw, anoa, the extinct Bubalus cebuensis † and other dwarf bovids from Southeast Asia
- 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
- Several extinct species of dwarf hippopotamus from the Pleistocene of Mediterranean islands (such as Hippopotamus minor) † and Madagascar
- Brookesia micra, a less than 3.0 cm long chameleon from the islet of Nosy Hara off the northern tip of Madagascar, one of the "smallest amniote vertebrates in the world"
- Tiger snakes of Roxby Island, South Australia
- Reticulated pythons of Tanahjampea island (P. r. jampeanus), between Sulawesi and Flores in Indonesia
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- Deep-sea gigantism
- Foster's rule
- Island gigantism
- Pleistocene extinctions
- Baron Franz Nopcsa
- Island tameness
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