Island gigantism, or insular gigantism, is a biological phenomenon in which the size of an animal species isolated on an island increases dramatically in comparison to its mainland relatives. Island gigantism is one aspect of the more general "island effect" or "Foster's rule", which posits that when mainland animals colonize islands, small species tend to evolve larger bodies, and large species tend to evolve smaller bodies (insular dwarfism). This is itself one aspect of the more general phenomenon of island syndrome which describes the differences in morphology, ecology, physiology and behaviour of insular (island) species compared to their continental counterparts. Following the arrival of humans and associated introduced predators (dogs, cats, rats, pigs), many giant as well as other island endemics have become extinct (e.g. the dodo and Rodrigues solitaire, giant flightless pigeons related to the Nicobar pigeon). A similar size increase, as well as increased woodiness, has been observed in some insular plants such as the Mapou tree (Cyphostemma mappia) in Mauritius which is also known as the "Mauritian baobab" although it is a member of the grape family (Vitaceae).
Possible causes
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, reptiles or smaller carnivorans, 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.
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. Small herbivores may also benefit from the absence of competition from missing types of large herbivores.
Benefits of large size that have been suggested for island tortoises include decreased vulnerability to scarcity of food and/or water, through ability to survive for longer intervals without them, or ability to travel longer distances to obtain them. Periods of such scarcity may be a greater threat on oceanic islands than on the mainland.
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.
Territorialism may favor the evolution of island gigantism. A study on Anaho Island in Nevada determined that reptile species that were territorial tended to be larger on the island compared to the mainland, particularly in the smaller species. In territorial species, larger size makes individuals better able to compete to defend their territory. This gives additional impetus to evolution toward larger size in an insular population.
A further means of establishing island gigantism may be a founder effect operative when larger members of a mainland population are superior in their ability to colonize islands.
Island size plays a role in determining the extent of gigantism. Smaller islands generally accelerate the rate of evolution of changes in organism size, and organisms there evolve greater extremes in size.
Examples Mammals Many rodents grow larger on islands, whereas carnivorans, proboscideans and artiodactyls usually become smaller.
Eulipotyphlans Example Binomial name Native range Current status Continental relative Balearic shrew Nesiotites hidalgo Mallorca and Menorca Extinct (3000–2000 BC)Red-toothed shrews Sardinian shrew Asoriculus similis Sardinia and Corsica Extinct (Holocene)Sicilian shrewAsoriculus burgioiSicilyExtinct (Early Pleistocene) Deinogalerix Deinogalerix spp. Gargano Island Extinct (Late Miocene) Moon rats
Rodents Example Binomial name Native range Current status Continental relative Insular / mainlandlength or mass ratio Blunt-toothed giant hutia Amblyrhiza inundata Anguilla and Saint Martin Extinct (Pleistocene) Neotropical spiny rats Larger Jamaican giant hutia Clidomys osborni Jamaica Extinct (Late Pleistocene) Plate-toothed giant hutia Elasmodontomys obliquus Puerto Rico Extinct (c. 1 AD) Twisted-toothed mouse Quemisia gravis Hispaniola Extinct Arboreal giant hutia Tainotherium valei Puerto Rico Lesser Jamaica giant hutia Xaymaca fulvopulvis Jamaica Mallorcan giant hamsters Apocricetus darderi Mallorca Apocricetus albertiTragomys macpheeiCricetus kormosiFreudenthal, M. (1985). Cricetidae (Rodentia) from the Neogene of Gargano (Prov. of Foggia, Italy). Rijksmuseum van Geologie en Mineralogie. Gargano giant hamster Hattomys gargantua Gargano Island St Kilda field mouse Apodemus sylvaticus hirtensis St Kilda Least Concern Wood mouse MR ≈ 2 Hensel's field mouse Rhagamys orthodon Corsica and Sardinia Extinct (after 1300 BC) Tenerife giant rat Canariomys bravoi Tenerife Extinct (Late Pleistocene) African rufous-nosed rats Gran Canaria giant rat Canariomys tamarani Gran Canaria Extinct (before AD 1500) Formentera black-tailed garden dormouse Eliomys quercinus ophiusae Formentera Rare (Introduced by humans) Garden dormouse and other Leithiinae dormice Balearic giant dormice Hypnomys spp. Mallorca & Menorca Extinct (Holocene) Sicilian-Maltese giant dormice Leithia cartei Sicily and Malta Extinct Leithia melitensis Orkney vole Microtus arvalis orcadensis Orkney Islands Vulnerable Common vole and other meadow voles Gargano giant voles Mikrotia magna Gargano Island Extinct (Early Pliocene) M. maiusculaM. parva St Kilda house mouse Mus musculus muralis St Kilda Extinct (c. AD 1930) House mouse Flores giant rat Papagomys armandvillei Flores Near Threatened North African black rat and other true rats Sulawesi giant rat Paruromys dominator Sulawesi Least Concern Admiralty giant rat Rattus detentus Manus Island Unknown / Likely threatened Congreso black rat population Rattus rattus Isla del Congreso Least Concern Channel Islands deer mice Peromyscus anyapahensis Northern Channel Islands of California Extinct (c. 6000 BC) North American deer mouse P. nesodytes Gargano giant dormouse Stertomys laticrestatus Gargano Island Extinct Glirinae dormice
Lagomorphs Example Binomial name Native range Current status Continental relative Minorcan giant lagomorph Nuralagus rex Minorca Extinct (Middle Pliocene) Alilepus (?)Trischizolagus (?) Prolagus imperialis Gargano Island Extinct Pikas Sardinian pika Prolagus sardus Corsica, Sardinia and Tavolara Extinct (c. AD 1800)
Primates Example Binomial name Native range Current status Continental relative Hispaniola monkey Antillothrix bernensis Hispaniola Extinct (before AD 1600) Cheracebus Haitian monkey Insulacebus toussaintiana Southwestern Haiti Extinct Cuban monkeys Paralouatta marianae Cuba Extinct (Pleistocene)P. varonai Jamaican monkey Xenothrix mcgregori Jamaica Extinct Gorilla lemur Archaeoindris fontoynontii Central Madagascar Extinct (c. 350 BC) Lorisoids Baboon lemurs Archaeolemur spp. Madagascar Extinct (before AD 1280)Hadropithecus spp. Sloth lemurs Babakotia spp. Western and Central Madagascar Extinct (c. AD 1500)Palaeopropithecus spp. Koala lemurs Megaladapis edwardsi Madagascar Extinct (AD 1280–1420)M. grandidieriM. madagascariensis
Carnivorans Example Binomial name Native range Current status Continental relative Sardinian giant otter Megalenhydris barbaricina Sardinia Extinct (Late Pleistocene) Otters Fossa Cryptoprocta ferox Madagascar Vulnerable Mongooses Giant fossa Cryptoprocta spelaea Extinct (before AD 1400) Cryptoprocta bevatabe Cryptoprocta bevatabe Madagascar Extinct (before BC 6300)
Gondwanatherians Example Binomial name Native range Current status Continental relative Vintana Vintana sertichi Madagascar Extinct (Late Cretaceous) South American, India, African and Antarctic gondwanatheres. Adalatherium Adalatherium hui
Birds Stem birds Example Binomial name Native range Current status Continental relative Balaur B. bondoc Hațeg Island Extinct (Late Cretaceous) Jeholornis Gargantuavis G. philohinos Ibero-Armorican Island Patagopteryx (?)
Ratites Example Binomial name Native range Current status Continental relative Kiwis Apterygidae New Zealand Variable Proapteryx Greater elephant birds Aepyornithidae Madagascar Extinct (c. AD 1700) Lesser elephant birds Mullerornithidae Extinct (c. AD 1260) Giant moas Dinornithidae New Zealand Extinct (c. AD 1450) Tinamous Lesser moas Emeidae Extinct (c. AD 1460) Upland moas Megalapterygidae Extinct (c. AD 1300)
Waterfowl Example Binomial name Native range Current status Continental relative New Zealand musk duck Biziura delautouri New Zealand Extinct (after AD 1500) Australian musk duck New Zealand geese Cnemiornis calcitrans Extinct Cape Barren gooseC. gracilis Garganornis G. ballmanni Gargano and Scontrone islands Extinct (Late Miocene) Geese Turtle-jawed moa-nalo Chelychelynechen quassus Kauai Extinct (c. AD 1000) Dabbling ducks Small-billed moa-nalo Ptaiochen pau Maui Large-billed moa-nalo Thambetochen chauliodous Maui Nui O'ahu moa-nalo Thambetochen xanion O'ahu Giant swan Cygnus falconeri Sicily and Malta Extinct (Middle Pleistocene) Mute swan Scarlett's duck Malacorhynchus scarletti New Zealand Extinct (after AD 1500) Pink-eared duck
Pangalliformes Example Binomial name Native range Current status Continental relative Pile-builder megapode Megapodius molistructor New Caledonia and Tonga Extinct (c. 1500 BC) Scrubfowl Megavitiornis Megavitiornis altirostris Fiji Extinct Galliformes Sylviornis Sylviornis neocaledoniae New Caledonia and Isle of Pines
Gruiformes Example Binomial name Native range Current status Continental relative Red rail Aphanapteryx bonasia Mauritius Extinct (c. AD 1700) Rails Hawkins' rail Diaphanapteryx hawkinsi Chatham Islands Extinct (c. AD 1900) Antillean cave rail Nesotrochis debooyi Puerto Rico and Virgin Islands Extinct Cuban cave rail Nesotrochis picapicensis Cuba Haitian cave rail N. steganinos Hispaniola South Island takahē Porphyrio hochstetteri South Island, New Zealand Endangered North Island takahē Porphyrio mantelli North Island, New Zealand Extinct (before AD 1900) Adzebills Aptornis defossor New Zealand Extinct Madagascar flufftailA. otidiformis Chatham coot Fulica chathamensis Chatham Islands Extinct (after AD 1500) Red-knobbed cootand other coots Mascarene coot Fulica newtonii Mauritius and Réunion Extinct (c. AD 1700) New Zealand coot Fulica prisca New Zealand Extinct (after AD 1280) Réunion swamphen Porphyrio coerulescens Plaine des Cafres, Réunion Extinct (c. AD 1730) Purple swamphens
Pigeons Example Binomial name Native range Current status Continental relativeViti Levu giant pigeon Natunaornis gigoura Viti Levu, Fiji Extinct Crowned pigeons Kanaka pigeon Caloenas canacorum New Caledonia Extinct (c. 500 BC) Nicobar pigeon Rodrigues solitaire Pezophaps solitaria Rodrigues Extinct (before AD 1778) Dodo Raphus cucullatus Mauritius Extinct (c. AD 1662)
Birds of prey