Island gigantism

The large Haast's eagle and moa from New Zealand (both extinct)

Island gigantism or insular gigantism is a biological phenomenon in which the size of an animal 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. With the arrival of humans and associated predators (dogs, cats, rats, pigs), many giant as well as other island endemics have become extinct.

Possible causes

Diagram displaying the change in size of weta species in two ecosystems. The size and population of wetas are affected by predation. Rats introduced on the mainland began to prey on wetas, reducing their population; wetas shrank in response. On an island isolated from predation, such as Little Barrier Island, wetas have a dense population and have grown to a massive size. Insular species of giant wetas are the only ones not facing extinction. As wetas grow over time, bird predation declines.

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. 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.^{[1][lower-alpha 1]} 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.^[4]

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.^[5] 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.^[6] In contrast, insular dwarfism among predators more commonly results from the imposition of constraints associated with the limited prey resources available on islands.^[6] 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.^[7]

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.^[8]

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.^[9]

Examples

Examples of island gigantism include:

Mammals

Many rodents grow larger on islands, whereas carnivores, proboscideans and artiodactyls usually become smaller.

Eulipotyphlans

Example	Binomial name	Native range	Current status	Continental relatives
Corsican giant shrew	Asoriculus corsicanus	Corsica	Extinct (before 500 BC)	Red-toothed shrews
Balearic giant shrew	Asoriculus hidalgo	Majorca and Menorca	Extinct
Sardinian giant shrew	Asoriculus similis	Sardinia	Extinct
Deinogalerix	Deinogalerix spp.	Gargano Island	Extinct (Late Miocene)	Moon rats

Rodents

Example	Binomial name	Native range	Current status	Continental relatives
Blunt-toothed giant hutia	Amblyrhiza inundata	Anguilla and Saint Martin	Extinct (Pleistocene)	Neotropical spiny rats
St Kilda field mouse	Apodemus sylvaticus hirtensis	St Kilda	Least Concern	Old World field mouse
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)
Larger Jamaican giant hutia	Clidomys osborni	Jamaica	Extinct (Late Pleistocene)	Neotropical spiny rats
Plate-toothed giant hutia	Elasmodontomys obliquus	Puerto Rico	Extinct (c. 1 AD)
Formentera black-tailed garden dormouse	Eliomys quercinus ophiusae	Formentera	Rare[10]	Garden dormouse
Minorcan giant dormouse	Hypnomys mahonensis	Menorca	Extinct	Leithiinae dormice
Majorcan giant dormouse	Hypnomys morpheus	Majorca	Extinct
Sicilian giant dormouse	Leithia cartei	Sicily	Extinct
Maltese giant dormouse	Leithia melitensis	Malta	Extinct
Gargano giant voles	Mikrotia magna M. maiuscula M. parva	Gargano Island	Extinct (Early Pliocene)	Meadow voles
Flores giant rat	Papagamoys armandvillei	Flores	Near Threatened	True rats
Sulawesi giant rat	Paruromys dominator	Sulawesi	Least Concern
Twisted-toothed mouse	Quemisia gravis	Hispaniola	Extinct	Neotropical spiny rats
Admiralty giant rat	Rattus detentus	Manus Island	Unknown / Likely threatened[11]	True rats
Congreso Island black rat population[12]	Rattus rattus	Congreso Island, one of the Chafarinas Islands	Least Concern	North African black rat
Arboreal giant hutia[13]	Tainotherium valei	Puerto Rico	Extinct	Neotropical spiny rats
Lesser Jamaica giant hutia	Xaymaca fulvopulvis	Jamaica	Extinct

Lagomorphs

Example	Binomial name	Native range	Current status	Continental relatives
Minorcan giant lagomorph	Nuralagus rex	Menorca	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 relatives
Hispaniola monkey	Antillothrix bernensis	Hispaniola	Extinct (before AD 1600)	Titis
Gorilla lemur	Archaeoindris fontoynontii	Central Madagascar	Extinct (c. 350 BC)	Galagos, pottos and lorises
Baboon lemurs	Archaeolemur spp. Hadropithecus spp.	Madagascar	Extinct (before AD 1280)
Sloth lemurs	Babakotia spp. Palaeopropithecus spp.	Western and Central Madagascar	Extinct (c. AD 1500)
Haitian monkey	Insulacebus toussaintiana	Southwestern Haiti	Extinct	Titis
Koala lemurs	Megaladapis edwardsi M. grandidieri M. madagascariensis	Madagascar	Extinct (AD 1280-1420)	Galagos, pottos and lorises
Cuban monkeys	Paralouatta marianae[14] P. varonai[14]	Cuba	Extinct (Pleistocene)	Titis
Jamaican monkey	Xenothrix mcgregori	Jamaica	Extinct

Carnivorans

Example	Binomial name	Native range	Current status	Continental relatives
Sardinian giant otter	Megalenhydris barbaricina	Sardinia	Extinct (Late Pleistocene)	Otters
Fossa	Cryptoprocta ferox	Madagascar	Vulnerable	Mongooses
Giant fossa	Cryptoprocta spelaea	Madagascar	Extinct

Birds

Stem birds

Example	Binomial name	Native range	Current status	Continental relatives
Balaur	B. bondoc	Hateg Island	Extinct (Late Cretaceous)	Jeholornis[15]
Gargantuavis	G. philohinos	Southern France	Extinct (Late Cretaceous)	Patagopteryx (?)

Ratites

Example	Binomial name	Native range	Current status	Continental relatives
Kiwis	Apteryx spp.	New Zealand	Variable	Proapteryx[lower-alpha 2]
Larger elephant birds	Aepyornis spp. Vorombe spp.	Madagascar	Extinct (c. AD 1000)
Lesser elephant birds	Mullerornis spp.	Madagascar	Extinct (c. AD 1260)
Moa	Anomalopteryx didiformis Dinornis spp. Emeus crassus Euryapteryx spp. Megalapteryx didinus Pachyornis spp.	New Zealand	Extinct (before AD 1445)	Tinamous

Waterfowl

Example	Binomial name	Native range	Current status	Continental relatives
New Zealand musk duck	Biziura delautouri	New Zealand	Extinct (after AD 1500)	Australian musk duck
New Zealand geese	Cnemiornis calcitrans C. gracilis	New Zealand	Extinct	Cape Barren goose
Garganornis	G. ballmanni	Gargano and Scontrone islands	Extinct (Late Miocene)	Geese[17]
Turtle-jawed moa-nalo	Chelychelynechen quassus	Kauai	Extinct (c. AD 1000)	Ducks
Small-billed moa-nalo	Ptaiochen pau	Maui	Extinct (c. AD 1000)
Maui Nui large-billed moa-nalo	Thambetochen chauliodous	Maui Nui	Extinct (c. AD 1000)
O'ahu moa-nalo	Thambetochen xanion	O'ahu	Extinct (c. AD 1000)
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

Galliformes

Example	Binomial name	Native range	Current status	Continental relatives
Pile-builder megapode	Megapodius molistructor	New Caledonia and Tonga	Extinct (c. 1500 BC)	Scrubfowl
Noble megapode	Megavitiornis altirostris	Fiji	Extinct	Galliformes
New Caledonian giant megapode	Sylviornis neocaledoniae	New Caledonia and Isle of Pines	Extinct

Gruiformes

Example	Binomial name	Native range	Current status	Continental relatives
Red rail	Aphanapteryx bonasia	Mauritius	Extinct (c. AD 1700)	Rails
Adzebills	Aptornis defossor A. otidiformis	New Zealand	Extinct	Trumpeters
Hawkins's rail	Diaphanapteryx hawkinsi	Chatham Islands	Extinct (c. AD 1900)	Rails
Chatham coot	Fulica chathamensis	Chatham Islands	Extinct (after AD 1500)	Coots
Mascarene coot	Fulica newtonii	Mauritius and Réunion	Extinct (c. AD 1700)	Eurasian coot
New Zealand coot	Fulica prisca	New Zealand	Extinct (after AD 1280)	Coots
Antillean cave rail	Nesotrochis debooyi	Puerto Rico and Virgin Islands	Extinct	Rails
Cuban cave rail	Nesotrochis picapicensis	Cuba	Extinct
Haitian cave rail	Nesotrochis steganinos	Hispaniola	Extinct
Réunion swamphen	Porphyrio coerulescens	Plaine des Cafres, Réunion	Extinct (c. AD 1730)	Purple swamphens
South Island takahē	Porphyrio hochstetteri	South Island, New Zealand	Endangered	Rails
North Island takahē	Porphyrio mantelli	North Island, New Zealand	Extinct (before AD 1900)

Suliformes

Example	Binomial name	Native range	Current status	Continental relatives
Spectacled cormorant	Phalacrocorax perspicillatus	Komandorski Islands	Extinct (c. AD 1850)	Double-crested cormorant

Pigeons

Example	Binomial name	Native range	Current status	Continental relatives
Viti Levu giant pigeon	Natunaornis gigoura	Viti Levu, Fiji	Extinct	Crowned pigeons
Rodrigues solitaire	Pezophaps solitaria	Rodrigues	Extinct (before AD 1778)	Nicobar pigeon
Dodo	Raphus cucullatus	Mauritius	Extinct (c. AD 1662)

Birds of prey

Example	Binomial name	Native range	Current status	Continental relatives
Liko Cave golden eagle	Aquila chrysaetos simurgh	Crete	Extinct (Late Pleistocene)	Golden eagle
Giant crab-hawk[18]	Buteogallus borrasi	Cuba	Extinct	Great black hawk
Jamaican caracara	Caracara tellustris	Jamaica	Extinct	Caracaras
Eyles's harrier	Circus eylesi	New Zealand	Extinct (c. AD 1000)	Swamp harrier
Gargano Island eagles	Garganoaetus freudenthali G. murivorus	Gargano Island	Extinct (Late Miocene)	Aquila delphinensis
Giant hawk	Gigantohierax sp.	Cuba	Extinct	Hawks
Haast's eagle	Harpagornis moorei	New Zealand	Extinct (c. AD 1400)	Little eagle Booted eagle
Titan-hawk	Titanohierax gloveralleni	Cuba, Hispaniola and the Bahamas	Extinct	Hawks

Parrots

Example	Binomial name	Native range	Current status	Continental relatives
Anakena Beach parrots	Two unnamed species	Easter Island	Extinct	Parrots
Broad-billed parrot	Lophopsittacus mauritianus	Mauritius	Extinct (c. AD 1680)	Psittaculine parrots
Kakapo	Strigops habroptila	New Zealand	Critically Endangered	Parrots

Owls

Example	Binomial name	Native range	Current status	Continental relatives
Cretan owl	Athene cretensis	Crete	Extinct (Pleistocene)	Little owl
Cuban giant owls	Ornimegalonyx spp.	Cuba	Extinct (Pleistocene)	True owls
Larger Gargano giant owl	Tyto gigantea	Gargano Island	Extinct (Late Miocene)	Barn owls
Andros Island barn owl	Tyto pollens	Andros Island, Bahamas	Extinct (before AD 1600)
Rivero's barn owl	Tyto riveroi	Cuba	Extinct
Lesser Gargano giant owl	Tyto robusta	Gargano Island	Extinct (Early Pliocene)

Storks

Example	Binomial name	Native range	Current status	Continental relatives
Flores flightless stork[19]	Leptostilos robustus	Flores	Extinct (Late Pleistocene)	Greater adjutant Lesser adjutant

Caprimulgiformes

Example	Binomial name	Native range	Current status	Continental relatives
New Zealand owlet-nightjar	Aegotheles novazelandiae	New Zealand	Extinct (c. AD 1200)	Australian owlet-nightjar
New Caledonian owlet-nightjar	Aegotheles savesi	New Caledonia	Critically endangered

Passeriforms

Example	Binomial name	Native range	Current status	Continental relatives
Chatham raven	Corvus moriorum	Chatham Islands	Extinct	New Zealand raven
Long-legged bunting	Emberiza alcoveri	Tenerife	Extinct (after AD 1)	Cabanis's bunting Yellowhammer
Giant nukupu'u	Hemignathus vorpalis	Hawaii	Extinct (after AD 1000)	Finches
Tasmanian superb fairywren	Malurus cyaneus cyaneus	Tasmania	Least Concern	Superb fairywren
Kangaroo Island superb fairywren	Malurus cyaneus ashbyi	Kangaroo Island	Least Concern
Stout-legged wren	Pachyplichas yaldwyni	South Island of New Zealand	Extinct	Passeriforms
Capricorn silvereye	Zosterops lateralis chlorocephalus	Capricorn and Bunker Group of the Australian Great Barrier Reef	Unknown	Silvereye

Reptiles

Pterosaurs

Example	Binomial name	Native range	Current status	Continental relatives
Hatzegopteryx	H. thambema	Hateg Island	Extinct (Late Cretaceous)	Quetzalcoatlus

Iguanids

Example	Binomial name	Native range	Current status	Continental relatives
Tongan giant iguana[20]	Brachylophus gibbonsi	Tonga	Extinct (c. 800 BC)	South American iguanids
Fijian giant iguana [21]	Lapitiguana impensa	Fiji	Extinct (c. 1000 BC)
Angel Island chuckwalla	Sauromalus hispidus	Isla Ángel de la Guarda, Baja California	Near Threatened	Peninsular chuckwalla
San Esteban chuckwalla	Sauromalus varius	San Esteban Island, Baja California	Endangered

Geckos

Example	Binomial name	Native range	Current status	Continental relatives
Delcourt's giant gekko	Hoplodactylus delcourti	New Zealand	Extinct (c. AD 1870)	Diplodactylid geckos
Rodrigues giant day gecko	Phelsuma gigas	Rodrigues	Extinct (c. AD 1850)	Day geckos
New Caledonian giant gecko	Rhacodactylus leachianus	New Caledonia	Least Concern	Diplodactylid geckos

Skinks

Example	Binomial name	Native range	Current status	Continental relatives
Vaillant's mabuya	Chioninia vaillanti	Cape Verde	Endangered	Mabuyas
Mauritius giant skink	Leiolopisma mauritiana	Mauritius	Extinct (after AD 1600)	New Zealand Leiolopisma skinks
Cape Verde giant skink	Macroscincus coctei	Cape Verde	Extinct (after AD 1900)	Mabuyas
Terror skink	Phoboscincus bocourti	Île des Pins off New Caledonia	Endangered	Skinks

Wall lizards

Example	Binomial name	Native range	Current status	Continental relatives
La Palma giant lizard	Gallotia auaritae	La Palma	Critically endangered	Mediterranean sandrunner lizards
La Gomera giant lizard	Gallotia bravoana	Gomera	Critically endangered
Tenerife giant lizard[22]	Gallotia goliath	Tenerife	Extinct (c. AD 1500)
El Hierro giant lizard	Gallotia simonyi	El Hierro	Critically endangered
Gran Canaria giant lizard	Gallotia stehlini	Gran Canaria	Least Concern

Snakes

Example	Binomial name	Native range	Current status	Continental relatives
Angel de la Guarda Island speckled rattlesnake	Crotalus mitchellii angelensis	Isla Ángel de la Guarda off Baja California	Least Concern	Speckled rattlesnake
Tadanae-jima striped snake population[23]	Elaphe quadrivirgata	Tadanae-jima island off Tokyo	Unknown	Japanese striped snake
Island tiger snake populations	Notechis scutatus	Mount Chappell Island (Tasmania) and Williams Island, Hopkins Island and islands of the Nuyts Archipelago (South Australia)[24]	Least Concern[25]	Tiger snake
Isla Cerralvo long-nosed snake	Rhinocheilus lecontei etheridgei	Jacques Cousteau Island off Baja California Sur	Unknown	Long-nosed snake

Dubious examples

• 
  The Komodo dragon of Flores and nearby islands is the largest living lizard.
• 
  The largest tortoises in the world are found on the Seychelles and Galápagos Islands (example of the latter shown).

• The Komodo dragon and a similar (extinct) giant monitor lizard from Timor have been regarded as 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.^[26] 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 still be valid.
• Giant tortoises in the Galápagos Islands, the Seychelles, and formerly the Mascarenes and Canary Islands are often considered examples of island gigantism. However, during the Pleistocene, comparably sized or larger tortoises were present in Australia (Meiolania), southern Asia (Megalochelys), Madagascar (Dipsochelys), North^[27] (Hesperotestudo) and South America^[28] (Chelonoidis, the same genus now found in the Galápagos^[29]), and on a number of other, more accessible islands.^[27] In the late Pliocene they were also present in Africa^[30] ("Geochelone" laetoliensis^[31]). 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.

Arthropods

Example	Binomial name	Native range	Current status	Continental relatives
Coconut crab	Birgus latro	Indian Ocean islands and Polynesia[32]	Unknown	Coenobita hermit crabs
Giant wetas	Deinacrida spp.	New Zealand	Variable	South African king crickets
Lord Howe Island stick insect[33][34]	Dryococelus australis	Lord Howe Island	Critically endangered	Phasmatid stick insects
Giant pseudoscorpion[35]	Garypus titanius	Boatswain Bird Island off Ascension Island	Unknown	Garypoid pseudoscorpions
Madagascar hissing cockroaches	Gromphadorhina spp.	Madagascar	Least Concern	Blaberid cockroaches
Saint Helena earwig	Labidura herculeana	Saint Helena	Extinct (c. AD 1967)	Labidura riparia
Giant pill-millipedes of Madagascar	Microsphaerotherium spp. Sphaeromimus spp. Zoosphaerium spp.	Madagascar	Unknown	Giant pill-millipedes of India (Arthrosphaera)
Orsonwelles	Orsonwelles spp.	Hawaii	Unknown	Money spiders
Conant's giant Nihoa tree cricket	Thaumatogryllus conanti	Nihoa	Unknown	Tree crickets
Giant Fijian long-horned beetle[36]	Xixuthrus heros	Viti Levu, Fiji	Vulnerable	Australasian Xixuthrus species
Taveuni beetle	Xixuthrus terribilis	Taveuni, Fiji	Unknown

Gastropods

Example	Binomial name	Native range	Current status	Continental relatives
Kauri land snails	Paryphanta spp. Powelliphanta spp.	New Zealand	Near Threatened	Other rhytidids

Flora

In addition to size increase, island grass plants may also exhibit "insular woodiness". The most notable examples are the megaherbs of New Zealand's subantarctic islands.^[37]

Example	Binomial name	Native range	Current status	Continental relatives
Campbell Island carrot	Anisotome latifolia	Campbell and Auckland Islands	Unknown	Apiaceae
Ross lily	Bulbinella rossii	Campbell and Auckland Islands	Unknown	New Zealand Maori Lily South African Yellow Cat-tail
Black-eyed daisy	Damnamenia vernicosa	Auckland and Campbell Islands	Unknown	Astereae
Coco de mer[38]	Lodoicea maldivica	Seychelles	Endangered	Borassoid palms
	Pleurophyllum criniferum	Antipodes, Auckland and Campbell Islands	Unknown	Cineraria
Silver-leaf daisy	Pleurophyllum hookeri	Macquarie Island, Auckland and Campbell Islands	Unknown
Campbell Island daisy	Pleurophyllum speciosum	Campbell and Auckland Islands	Unknown
Macquarie Island cabbage	Stilbocarpa polaris	Macquarie Island and New Zealand subantarctic islands	Vulnerable	Araliaceae

See also

• Deep-sea gigantism
• Foster's rule
• Insular dwarfism
• Island tameness
• Megafauna
• Pleistocene extinctions

Notes

References

External links

• PBS NOVA: Why Do Islands Breed Giants (And Sometimes Dwarfs)?