Island gigantism

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 of island gigantism include:

Eulipotyphlans

Example	Binomial name	Native range	Current status	Continental relative
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 relative	Insular / mainland length 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[10]	Tainotherium valei	Puerto Rico	Extinct
Lesser Jamaica giant hutia	Xaymaca fulvopulvis	Jamaica	Extinct
Majorcan giant hamsters	Apocricetus darderi Tragomys macpheei	Majorca	Extinct	Apocricetus alberti[11] Cricetus kormosi[12]
Gargano giant hamster	Hattomys gargantua	Gargano Island	Extinct
St Kilda field mouse	Apodemus sylvaticus hirtensis	St Kilda	Least Concern	Wood mouse	MR ≈ 2 [13]
Hensel's field mouse	Rhagamys orthodon	Corsica and Sardinia	Extinct (Late Pleistocene)
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[14]	Garden dormouse and other Leithiinae dormice
Minorcan giant dormouse	Hypnomys mahonensis	Menorca	Extinct
Majorcan giant dormouse	Hypnomys morpheus	Majorca	Extinct
Sicilian giant dormouse	Leithia cartei	Sicily	Extinct
Maltese giant dormouse	Leithia melitensis	Malta	Extinct
Orkney vole	Microtus arvalis orcadensis	Orkney Islands	Vulnerable	Common vole and other meadow voles
Gargano giant voles	Mikrotia magna M. maiuscula M. parva	Gargano Island	Extinct (Early Pliocene)
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[15]
Congreso black rat population[16]	Rattus rattus	Isla del Congreso	Least Concern
Channel Islands deer mice	Peromyscus anyapahensis P. nesodytes	Northern Channel Islands of California	Extinct (c. 6000 BC)	North American deer mouse
Gargano giant dormouse	Stertomys laticrestatus[17]	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[18] P. varonai[18]	Cuba	Extinct (Pleistocene)
Jamaican monkey	Xenothrix mcgregori	Jamaica	Extinct
Gorilla lemur	Archaeoindris fontoynontii	Central Madagascar	Extinct (c. 350 BC)	Lorisoids
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)
Koala lemurs	Megaladapis edwardsi M. grandidieri M. madagascariensis	Madagascar	Extinct (AD 1280-1420)

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	Madagascar	Extinct (before AD 1400)

Stem birds

Example	Binomial name	Native range	Current status	Continental relative
Balaur	B. bondoc	Hateg Island	Extinct (Late Cretaceous)	Jeholornis[19]
Gargantuavis	G. philohinos	Ibero-Armorican Island	Extinct (Late Cretaceous)	Patagopteryx (?)

Ratites

Example	Binomial name	Native range	Current status	Continental relative
Kiwis	Apteryx spp.	New Zealand	Variable	Proapteryx[lower-alpha 2]
Greater elephant birds	Aepyornis hildebrandti A. maximus Vorombe titan	Madagascar	Extinct (c. AD 1000)
Lesser elephant birds	Mullerornis spp.	Madagascar	Extinct (c. AD 1260)
Moas	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 relative
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[21]
Turtle-jawed moa-nalo	Chelychelynechen quassus	Kauai	Extinct (c. AD 1000)	Dabbling ducks
Small-billed moa-nalo	Ptaiochen pau	Maui	Extinct (c. AD 1000)
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

Pangalliformes

Example	Binomial name	Native range	Current status	Continental relative
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 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	Extinct
Haitian cave rail	Nesotrochis steganinos	Hispaniola	Extinct
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 A. otidiformis	New Zealand	Extinct	Madagascan flufftail[22]
Chatham coot	Fulica chathamensis	Chatham Islands	Extinct (after AD 1500)	Red-knobbed coot and 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 relative
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 relative
Liko Cave golden eagle	Aquila chrysaetos simurgh	Crete	Extinct (Late Pleistocene)	Golden eagle
Giant crab-hawk[23]	Buteogallus borrasi	Cuba	Extinct	Great black hawk and other hawks
Giant hawk	Gigantohierax sp.	Cuba	Extinct
Titan-hawk	Titanohierax gloveralleni	Cuba, Hispaniola and the Bahamas	Extinct
Jamaican caracara	Caracara tellustris	Jamaica	Extinct	Caracaras
Eyles' 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
Haast's eagle	Hieraaetus moorei	New Zealand	Extinct (c. AD 1400)	Little eagle
Philippine eagle	Pithecophaga jefferyi	Philippines	Critically endangered	Bateleur[24]

Parrots

Example	Binomial name	Native range	Current status	Continental relative
Anakena Beach parrots	Two unnamed species	Easter Island	Extinct	Other parrots
Hercules parrot	Heracles inexpectatus	New Zealand	Extinct (Miocene)
Kakapo	Strigops habroptilus	New Zealand	Critically Endangered
Broad-billed parrot	Lophopsittacus mauritianus	Mauritius	Extinct (c. AD 1680)	Psittaculine parrots

Owls

Example	Binomial name	Native range	Current status	Continental relative
Cretan owl	Athene cretensis	Crete	Extinct (Pleistocene)	Little owl
Cuban giant owls	Ornimegalonyx spp.	Cuba	Extinct (Pleistocene)	Wood owls
Greater 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)

Caprimulgiformes

Example	Binomial name	Native range	Current status	Continental relative
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 relative
Chatham raven	Corvus moriorum	Chatham Islands	Extinct	New Zealand raven
Long-legged bunting	Emberiza alcoveri	Tenerife	Extinct (after AD 1)	Cabanis's bunting
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	Other passeriforms
Capricorn silvereye	Zosterops lateralis chlorocephalus	Capricorn and Bunker Group of the Australian Great Barrier Reef	Unknown	Silvereye

Pterosaurs

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

Iguanids

Example	Binomial name	Native range	Current status	Continental relative	Insular / mainland length or mass ratio
Tongan giant iguana[25]	Brachylophus gibbonsi	Tonga	Extinct (c. 800 BC)	South American iguanas
Fijian giant iguana [26]	Lapitiguana impensa	Fiji	Extinct (c. 1000 BC)
Angel Island chuckwalla	Sauromalus hispidus	Isla Ángel de la Guarda, Baja California	Near Threatened	Peninsular chuckwalla	MR ≈ 5 [27]
San Esteban chuckwalla	Sauromalus varius	San Esteban Island, Baja California	Endangered		MR ≈ 5 [27]

Geckos

Example	Binomial name	Native range	Current status	Continental relative	Insular / mainland length or mass ratio
Delcourt's giant gekko	Hoplodactylus delcourti	New Zealand	Extinct (c. AD 1870)	Diplodactylid geckos	LR ≈ 6.75 [lower-alpha 3]
New Caledonian giant gecko	Rhacodactylus leachianus	New Caledonia	Least Concern		LR ≈ 4.4 [lower-alpha 4] MR ≈ 60 [lower-alpha 5]
Rodrigues giant day gecko	Phelsuma gigas	Rodrigues	Extinct (c. AD 1850)	Day geckos

Skinks

Example	Binomial name	Native range	Current status	Continental relative
Vaillant's mabuya	Chioninia vaillanti	Cape Verde	Endangered	Mainland mabuyine skinks
Cape Verde giant skink	Macroscincus coctei	Cape Verde	Extinct (after AD 1900)
Mauritius giant skink	Leiolopisma mauritiana	Mauritius	Extinct (after AD 1600)	Mainland eugongyline skinks
Terror skink	Phoboscincus bocourti	Île des Pins off New Caledonia	Endangered	Mainland eugongyline skinks
Kishinoue's giant skink	Plestiodon kishinouyei	Miyako Islands and Yaeyama Islands, Japan	Vulnerable	Asian Plestiodon spp.

Wall lizards

Example	Binomial name	Native range	Current status	Continental relative
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[31]	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 relative
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[32]	Elaphe quadrivirgata	Tadanae-jima island off Tokyo	Unknown	Japanese striped snake
Island tiger snake populations	Notechis scutatus	Islands Mount Chappell, Williams, Hopkins, and the Nuyts Archipelago (South Australia)[33]	Least Concern[34]	Tiger snake
Isla Cerralvo long-nosed snake	Rhinocheilus lecontei etheridgei	Jacques Cousteau Island off Baja California Sur	Unknown	Long-nosed snake

Dubious examples

Komodo dragon (Lesser Sundas)

Galápagos giant tortoise

• The Komodo dragon of Flores and nearby islands, the largest extant lizard, 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.[35] If this is true, rather than being insular giants they would be viewed as examples of phyletic gigantism. Supporting this interpretation is evidence for a lizard in Pliocene India, Varanus sivalensis, comparable in size to komodoensis.[35] Nevertheless, given that Australia is often described as the world's largest island and that the related megalania, the largest terrestrial lizard known in the fossil record, was restricted to Australia, the perception of the largest Australasian/Indonesian lizards as insular giants may still have some validity.

• Giant tortoises in the Galápagos Islands and the Seychelles, the largest extant tortoises, as well as extinct tortoises of 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 (Aldabrachelys), North America[36] (Hesperotestudo) and South America[37] (Chelonoidis, the same genus now found in the Galápagos[38]), and on a number of other, more accessible islands.[36] In the late Pliocene they were also present in Africa.[39] ("Geochelone" laetoliensis[39]). The present situation of large tortoises being found only on remote islands appears to reflect that these islands were discovered by humans recently and have not been heavily populated, making their tortoises less subject to overexploitation.

Amphibians

Example	Binomial name	Native range	Current status	Continental relative	Insular / mainland length or mass ratio
São Tomé giant tree frog	Hyperolius thomensis[40]	São Tomé Island	Endangered	African reed frogs
Palm forest tree frog	Leptopelis palmatus[40]	Príncipe Island	Vulnerable	Red tree frog	LR ≈ 1.2 [lower-alpha 6]
Giant Fiji ground frog	Platymantis megabotoniviti[43]	Viti Levu, Fiji	Extinct	Asian platymantines
São Tomé giant grass frog	Ptychadena newtoni[40]	São Tomé Island	Endangered	Mascarene grass frog

Arthropods

Example	Binomial name	Native range	Current status	Continental relative
Coconut crab	Birgus latro	Indian Ocean islands and Polynesia[44]	Unknown	Coenobita hermit crabs
Giant wetas	Deinacrida spp.	New Zealand	Variable	South African king crickets
Giant pseudoscorpion[45]	Garypus titanius	Boatswain Bird Island	Unknown	Garypoids
Madagascar hissing cockroaches	Gromphadorhina spp.	Madagascar	Least Concern	Blaberids
Saint Helena earwig	Labidura herculeana	Saint Helena	Extinct (c. AD 1967)	Shore earwig
Wallace's giant bee	Megachile pluto	North Moluccas	Vulnerable	Callomegachile
Megalara	Megalara garuda	Mekongga Mountains, Sulawesi	Unknown	Crabronine wasps
Madagascan giant pill-millipedes	Microsphaerotherium spp. Sphaeromimus spp. Zoosphaerium spp.	Madagascar	Unknown	Indian giant pill-millipedes (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[46]	Xixuthrus heros	Viti Levu, Fiji	Vulnerable	Australasian Xixuthrus
Taveuni beetle	Xixuthrus terribilis	Taveuni, Fiji	Unknown

Gastropods

Example	Binomial name	Native range	Current status	Continental relative
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.[47] Increased leaf and seed size was also reported in some island species regardless of growth form (herbaceous, bush, or tree).[48]

Example	Binomial name	Native range	Current status	Continental relative
Campbell Island carrot	Anisotome latifolia	Campbell and Auckland Islands	Unknown	Apiaceae
Ross lily	Bulbinella rossii	Campbell and Auckland Islands	Unknown	New Zealand Maori lily
Chatham Islands korokio[48][49]	Corokia macrocarpa	Chatham Islands	Unknown	New Zealand korokio[50]
Black-eyed daisy	Damnamenia vernicosa	Auckland and Campbell Islands	Unknown	Astereae
Cucumber tree[51]	Dendrosicyos socotranus	Socotra	Vulnerable	Gourds
Coco de mer[52][51]	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
Chatham Islands nikau[48]	Rhopalostylis aff. sapida	Chatham Islands	Unknown	Nikau
Macquarie Island cabbage	Stilbocarpa polaris	Macquarie Island and New Zealand subantarctic islands	Vulnerable	Araliaceae

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

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