Island gigantism or insular gigantism is a biological phenomenon in which the size of animals isolated on an island increases dramatically in comparison to their mainland relatives. Island gigantism 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. With the arrival of humans and associated predators (dogs, cats, rats, pigs), many giant island endemics have become extinct.
Large mammalian carnivores are often absent on islands because of insufficient range or difficulties in over-water dispersal. In their absence, the ecological niches for large predators may be occupied by birds or reptiles, which can then grow to larger-than-normal size. For example, on prehistoric Gargano Island in the Miocene-Pliocene Mediterranean, on islands in the Caribbean like Cuba, and on Madagascar and New Zealand, some or all apex predators were birds like eagles, falcons and owls, including some of the largest known examples of these groups. However, birds and reptiles generally make less efficient large predators than advanced carnivorans.
Since small size usually makes it easier for herbivores to escape or hide from predators, the decreased predation pressure on islands can allow them to grow larger.[a] Small herbivores may also benefit from the absence of competition from missing types of large herbivores.
Thus, island gigantism is usually an evolutionary trend resulting from the removal of constraints on the size of small animals related to predation and/or competition. Such constraints can operate differently depending on the size of the animal, however; for example, while small herbivores may escape predation by hiding, large herbivores may deter predators by intimidation. As a result, the complementary phenomenon of island dwarfism can also result from the removal of constraints related to predation and/or competition on the size of large herbivores. In contrast, insular dwarfism among predators more commonly results from the imposition of constraints associated with the limited prey resources available on islands. As opposed to island dwarfism, island gigantism is found in most major vertebrate groups and in invertebrates.
There are several possible benefits animals affected by gigantism experience, including the ability to go long distances without food and water, as seen in the Giant Tortoise. If resources are limited, then the environment will select for those who are able to survive these conditions (the larger animals) and the population will continue to push towards the larger size. 
Examples of island gigantism include:
- Deinogalerix (extinct)
- Flores giant rat
- Tenerife giant rat and Gran Canaria giant rat (both extinct)
- Giant hutias of the West Indies (extinct)
- Majorcan giant dormouse and Minorcan giant dormouse (both extinct)
- Formentera black-tailed garden dormouse, descendant of garden dormice introduced from the continent by Neolithic humans
- St Kilda field mouse, descendant of wood mice introduced by humans
- Birds of prey
- Giant tortoises in the Galápagos Islands, Seychelles, and formerly the Mascarenes are often considered examples of island gigantism. However, during the Pleistocene, comparably sized or larger tortoises were present in Australia (Meiolania), southern Asia (Colossochelys atlas), Madagascar (Dipsochelys) and North and South America, as well as on a number of other, more accessible islands. In the late Pliocene they were also present in Africa. The present situation of large tortoises being only found on remote islands may reflect that these islands were discovered by humans fairly recently and have not been heavily populated, making their tortoises less subject to overexploitation.
- The Komodo dragon and a similar (extinct) giant monitor lizard from Timor are examples of giant insular carnivores. Since islands tend to offer limited food and territory, their mammalian carnivores (if present) are usually smaller than continental ones. These cases involve ectothermic carnivores on islands too small to support much mammalian competition. However, these lizards are not as large as their extinct Australian relative Megalania, and it has been proposed based on fossil evidence that the ancestors of these varanids first evolved their large size in Australia and then dispersed to Indonesia. If this is true, rather than being insular giants they would be viewed as examples of phyletic gigantism. Nevertheless, given that Australia is sometimes viewed as the world's largest island, the former view may not be entirely invalid.
- The Angel Island chuckwalla (Sauromalus hispidus) and the San Esteban chuckwalla (Sauromalus varius) of islands off Baja California
- The extinct giant (1.5 m long) iguana Lapitiguana from Fiji.
- Leiolopisma mauritiana and Macroscincus coctei, two extinct skinks from Mauritius and Cape Verde, the Solomon Islands skink 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
- Four extant and one extinct species of lacertid lizard of the genus Gallotia in the Canary Islands
- Tiger snake populations on Mount Chappell Island (Tasmania) and Williams Island, Hopkins Island and islands of the Nuyts Archipelago (South Australia). On these islands the available prey is restricted to larger sizes than commonly taken by mainland snakes; restricted seasonal availability of prey also appears to contribute to gigantism.
- Conant's giant Nihoa tree cricket
- Giant pill-millipedes of Madagascar
- Giant wetas of New Zealand
- Saint Helena earwig
- Taveuni and giant Fijian long-horned beetles of Fiji
Island plants often also exhibit "insular woodiness".
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