Island gigantism explained

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 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 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. 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] 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.^[2]

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.^[3] 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.^[4] In contrast, insular dwarfism among predators more commonly results from the imposition of constraints associated with the limited prey resources available on islands.^[4] 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.^[5]

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.^[6]

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.^[7]

Examples

Examples of island gigantism include:

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 giant shrew	Nesiotites hidalgo	Majorca and Menorca	Extinct (3000-2000 BC)	Red-toothed shrews
Sardinian giant shrew	Asoriculus similis	Sardinia and Corsica	Extinct (Holocene)
Sicilian giant shrew	Asoriculus burgioi	Sicily	Extinct (Early Pleistocene)
Deinogalerix	Deinogalerix spp.		Extinct (Late Miocene)	Moon rats

Rodents

Example	Binomial name	Native range	Current status	Continental relative	Insular / mainland length or mass ratio
	Amblyrhiza inundata		Extinct (Pleistocene)
	Clidomys osborni		Extinct (Late Pleistocene)
	Elasmodontomys obliquus		Extinct (c. 1 AD)
	Quemisia gravis		Extinct
Arboreal giant hutia^[8]	Tainotherium valei		Extinct
Lesser Jamaica giant hutia	Xaymaca fulvopulvis		Extinct
Majorcan giant hamsters	Apocricetus darderi Tragomys macpheei		Extinct	Apocricetus alberti^[9] Cricetus kormosi^[10]
Gargano giant hamster	Hattomys gargantua		Extinct
	Apodemus sylvaticus hirtensis		Least Concern		MR ≈ 2 ^[11]
	Rhagamys orthodon		Extinct (After 1300 BC)
	Canariomys bravoi		Extinct (Late Pleistocene)
	Canariomys tamarani		Extinct (before AD 1500)
Formentera black-tailed garden dormouse	Eliomys quercinus ophiusae		Rare (Introduced by humans)^[12]
Balearic giant dormice	Hypnomys spp.	Mallorca & Menorca	Extinct (Holocene)
Sicilian-Maltese giant dormice	Leithia cartei	Sicily and Malta	Extinct
	Leithia melitensis
	Microtus arvalis orcadensis		Vulnerable
	Mikrotia magna M. maiuscula M. parva		Extinct (Early Pliocene)
	Mus musculus muralis		Extinct (c. AD 1930)
	Papagomys armandvillei
	Paruromys dominator		Least Concern
Admiralty giant rat	Rattus detentus		Unknown / Likely threatened^[13]
Congreso black rat population^[14]	Rattus rattus		Least Concern
Channel Islands deer mice	Peromyscus anyapahensis P. nesodytes		Extinct (c. 6000 BC)
Gargano giant dormouse	Stertomys laticrestatus^[15]		Extinct	Glirinae dormice

Lagomorphs

Example	Binomial name	Native range	Current status	Continental relative
	Nuralagus rex		Extinct (Middle Pliocene)	Alilepus (?) Trischizolagus (?)
	Prolagus imperialis		Extinct	Pikas
	Prolagus sardus		Extinct (c. AD 1800)

Example	Binomial name	Native range	Current status	Continental relative
	Antillothrix bernensis		Extinct (before AD 1600)	Cheracebus
	Insulacebus toussaintiana		Extinct
Cuban monkeys	Paralouatta marianae^[16] P. varonai		Extinct (Pleistocene)
	Xenothrix mcgregori		Extinct
	Archaeoindris fontoynontii		Extinct (c. 350 BC)	Lorisoids
	Archaeolemur spp. Hadropithecus spp.		Extinct (before AD 1280)
Sloth lemurs	Babakotia spp. Palaeopropithecus spp.	Western and Central Madagascar	Extinct (c. AD 1500)
	Megaladapis edwardsi M. grandidieri M. madagascariensis	Madagascar	Extinct (AD 1280–1420)

Carnivorans

Example	Binomial name	Native range	Current status	Continental relative
	Megalenhydris barbaricina		Extinct (Late Pleistocene)	Otters
	Cryptoprocta ferox		Vulnerable	Mongooses
	Cryptoprocta spelaea		Extinct (before AD 1400)

Birds

Stem birds

Example	Binomial name	Native range	Current status	Continental relative
Balaur	B. bondoc		Extinct (Late Cretaceous)	Jeholornis^[17]
Gargantuavis	G. philohinos	Ibero-Armorican Island	Extinct (Late Cretaceous)	Patagopteryx (?)

Example	Binomial name	Native range	Current status	Continental relative
Kiwis	Apterygidae		Variable	Proapteryx
Greater elephant birds	Aepyornithidae		Extinct (c. AD 1700)
Lesser elephant birds	Mullerornithidae		Extinct (c. AD 1260)
Giant moas	Dinornithidae		Extinct (c. AD 1450)	Tinamous
Lesser moas	Emeidae		Extinct (c. AD 1460)
Upland moas	Megalapterygidae		Extinct (c. AD 1300)

Example	Binomial name	Native range	Current status	Continental relative
	Biziura delautouri		Extinct (after AD 1500)	Australian musk duck
	Cnemiornis calcitrans C. gracilis		Extinct	Cape Barren goose
Garganornis	G. ballmanni	Gargano and Scontrone islands	Extinct (Late Miocene)	Geese^[18]
	Chelychelynechen quassus		Extinct (c. AD 1000)	Dabbling ducks
	Ptaiochen pau		Extinct (c. AD 1000)
	Thambetochen chauliodous		Extinct (c. AD 1000)
	Thambetochen xanion		Extinct (c. AD 1000)
	Cygnus falconeri		Extinct (Middle Pleistocene)	Mute swan
	Malacorhynchus scarletti		Extinct (after AD 1500)	Pink-eared duck

Example	Binomial name	Native range	Current status	Continental relative
	Megapodius molistructor		Extinct (c. 1500 BC)	Scrubfowl
Megavitiornis	Megavitiornis altirostris		Extinct	Galliformes
Sylviornis	Sylviornis neocaledoniae		Extinct

Example	Binomial name	Native range	Current status	Continental relative
	Aphanapteryx bonasia		Extinct (c. AD 1700)	Rails
	Diaphanapteryx hawkinsi		Extinct (c. AD 1900)
	Nesotrochis debooyi		Extinct
Cuban cave rail	Nesotrochis picapicensis		Extinct
Haitian cave rail	Nesotrochis steganinos		Extinct
	Porphyrio hochstetteri		Endangered
	Porphyrio mantelli		Extinct (before AD 1900)
Adzebills	Aptornis defossor A. otidiformis		Extinct	Madagascar flufftail^[19]
	Fulica chathamensis		Extinct (after AD 1500)	Red-knobbed coot and other coots
	Fulica newtonii		Extinct (c. AD 1700)
	Fulica prisca		Extinct (after AD 1280)
	Porphyrio coerulescens		Extinct (c. AD 1730)	Purple swamphens

Example	Binomial name	Native range	Current status	Continental relative
	Natunaornis gigoura		Extinct	Crowned pigeons
	Caloenas canacorum		Extinct (c. 500 BC)	Nicobar pigeon
	Pezophaps solitaria		Extinct (before AD 1778)
	Raphus cucullatus		Extinct (c. AD 1662)

Example	Binomial name	Native range	Current status	Continental relative
Liko Cave golden eagle	Aquila chrysaetos simurgh		Extinct (Late Pleistocene)	Golden eagle
Giant crab-hawk^[20]	Buteogallus borrasi		Extinct	Great black hawk and other hawks
Giant hawk	Gigantohierax sp.		Extinct
	Titanohierax gloveralleni		Extinct
	Caracara tellustris		Extinct	Caracaras
	Circus eylesi		Extinct (c. AD 1000)	Swamp harrier
Gargano Island eagles	Garganoaetus freudenthali G. murivorus		Extinct (Late Miocene)	Aquila delphinensis
	Hieraaetus moorei		Extinct (c. AD 1400)	Little eagle
	Pithecophaga jefferyi		Critically endangered	Bateleur^[21]

Parrots

Example	Binomial name	Native range	Current status	Continental relative
	Heracles inexpectatus		Extinct (Miocene)	Other parrots
	Strigops habroptilus		Critically Endangered
	Lophopsittacus mauritianus		Extinct (c. AD 1680)	Psittaculine parrots

Owls

Example	Binomial name	Native range	Current status	Continental relative
	Athene cretensis		Extinct (Pleistocene)	Little owl
Cuban giant owls	Ornimegalonyx spp.		Extinct (Pleistocene)	Wood owls
Greater Gargano giant owl	Tyto gigantea		Extinct (Late Miocene)	Barn owls
	Tyto pollens		Extinct (before AD 1600)
	Tyto riveroi		Extinct
Lesser Gargano giant owl	Tyto robusta		Extinct (Early Pliocene)

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

Passeriforms

Example	Binomial name	Native range	Current status	Continental relative
	Corvus moriorum		Extinct	New Zealand raven
	Emberiza alcoveri		Extinct (after AD 1)	Cabanis's bunting
	Hemignathus vorpalis		Extinct (after AD 1000)	Finches
Tasmanian superb fairywren	Malurus cyaneus cyaneus		Least Concern	Superb fairywren
Kangaroo Island superb fairywren	Malurus cyaneus ashbyi		Least Concern
	Pachyplichas yaldwyni		Extinct	Other passeriforms
	Troglodytes troglodytes hirtensis		Unknown	Eurasian wren
	Zosterops lateralis chlorocephalus		Unknown	Silvereye

Reptiles

Iguanids

Example	Binomial name	Native range	Current status	Continental relative	Insular / mainland length or mass ratio
Tongan giant iguana^[22]	Brachylophus gibbonsi		Extinct (c. 800 BC)	South American iguanas
Fijian giant iguana ^[23]	Lapitiguana impensa		Extinct (c. 1000 BC)
	Sauromalus hispidus		Near Threatened		MR ≈ 5 ^[24]
	Sauromalus varius		Endangered		MR ≈ 5

Geckos

Example	Binomial name	Native range	Current status	Continental relative	Insular / mainland length or mass ratio
	Gigarcanum delcourti	New Caledonia	Extinct (c. AD 1870)		LR ≈ 6.75
	Rhacodactylus leachianus		Least Concern		LR ≈ 4.4 MR ≈ 60
	Phelsuma gigas		Extinct (c. AD 1850)

Skinks

Example	Binomial name	Native range	Current status	Continental relative
	Chioninia vaillanti		Endangered	Mainland mabuyine skinks
	Macroscincus coctei		Extinct (after AD 1900)
	Leiolopisma mauritiana		Extinct (after AD 1600)	Mainland eugongyline skinks
	Phoboscincus bocourti		Endangered	Mainland eugongyline skinks
	Plestiodon kishinouyei		Vulnerable	Asian Plestiodon spp.

Example	Binomial name	Native range	Current status	Continental relative
	Gallotia auaritae		Critically endangered	Mediterranean sandrunner lizards
	Gallotia bravoana		Critically endangered
Tenerife giant lizard^[25]	Gallotia goliath		Extinct (c. AD 1500)
	Gallotia simonyi		Critically endangered
	Gallotia stehlini		Least Concern

Snakes

Example	Binomial name	Native range	Current status	Continental relative
	Crotalus mitchellii angelensis		Least Concern	Speckled rattlesnake
Tadanae-jima striped snake population^[26]	Elaphe quadrivirgata		Unknown	Japanese striped snake
Island tiger snake populations	Notechis scutatus	Islands Mount Chappell (Tasmania); Williams, Hopkins, and the Nuyts Archipelago (all South Australia)^[27]	Least Concern^[28]	Tiger snake
Isla Cerralvo long-nosed snake	Rhinocheilus lecontei etheridgei		Unknown	Long-nosed snake

Dubious examples

• 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.^[29] 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 V. komodoensis. 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), Europe^[30] (Titanochelon), Madagascar (Aldabrachelys), North America^[31] (Hesperotestudo) and South America^[32] (Chelonoidis, the same genus now found in the Galápagos^[33]), and on a number of other, more accessible islands of Oceania and the Caribbean.^[31] In the late Pliocene they were also present in Africa ("Geochelone" laetoliensis^[34]). 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^[35]		Endangered
Palm forest tree frog	Leptopelis palmatus	Príncipe Island	Vulnerable		LR ≈ 1.2
Giant Fiji ground frog	Platymantis megabotoniviti^[36]		Extinct
São Tomé giant grass frog	Ptychadena newtoni	São Tomé Island	Endangered

Arthropods

Example	Binomial name	Native range	Current status	Continental relative
	Birgus latro	Indian Ocean islands and Polynesia^[37]	Vulnerable	Coenobita hermit crabs
	Deinacrida spp.		Variable	South African king crickets
Giant pseudoscorpion^[38]	Garypus titanius		Critically Endangered	Garypoids
Hissing cockroaches	Gromphadorhini spp.		Unknown	Blaberids
	Labidura herculeana		Extinct (c. AD 1967)	Shore earwig
	Megachile pluto		Vulnerable	Callomegachile
Megalara	Megalara garuda		Unknown	Crabronine wasps
	Microsphaerotherium spp. Sphaeromimus spp. Zoosphaerium spp.		Unknown	Indian giant pill-millipedes (Arthrosphaera)
Orsonwelles	Orsonwelles spp.		Unknown	Money spiders
	Thaumatogryllus conanti		Unknown	Tree crickets
Giant Fijian long-horned beetle^[39]	Xixuthrus heros		Endangered	Australasian Xixuthrus
	Xixuthrus terribilis		Unknown

Flora

In addition to size increase, island plants may also exhibit "insular woodiness".^[40] The most notable examples are the megaherbs of New Zealand's subantarctic islands. Increased leaf and seed size was also reported in some island species regardless of growth form (herbaceous, bush, or tree).^[41]

Example	Binomial name	Native range	Current status	Continental relative
	Anisotome latifolia		Unknown	Apiaceae
	Bulbinella rossii		Naturally Uncommon	New Zealand Maori lily
Chatham Islands korokio^[42]	Corokia macrocarpa		Unknown	New Zealand korokio^[43]
	Damnamenia vernicosa		Naturally Uncommon	Astereae
Cucumber tree^[44]	Dendrosicyos socotranus		Vulnerable	Gourds
Coco de mer^[45]	Lodoicea maldivica		Endangered	Borassoid palms
	Pleurophyllum criniferum		Unknown	Cineraria
Silver-leaf daisy	Pleurophyllum hookeri		Unknown
	Pleurophyllum speciosum		Naturally Uncommon
	Stilbocarpa polaris		Vulnerable	Araliaceae

See also

• Deep-sea gigantism
• Island tameness
• Insular dwarfism
• Megafauna
• Pleistocene extinctions

External links

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

Notes and References

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2. 10.1098/rsbl.2010.1084. 21270022. The evolution of island gigantism and body size variation in tortoises and turtles. Biology Letters. 7. 4. 558–561. 2011-01-26. Jaffe . A. L.. Slater . G. J.. Alfaro . M. E. . 3130210.
3. Barahona. F.. Evans. S.E.. Mateo. J.A.. Garcia-Marquez. M.. Lopez-Jurado. L.F.. Endemism, Gigantism and Extinction in Island Lizards: The Genus Gallotia on the Canary Islands. Journal of Zoology. March 2000. 250. 3. 373–388. 10.1017/s0952836900003101. 10553/19918. free.
4. Raia . P. . Meiri, S. . The island rule in large mammals: paleontology meets ecology . . 60 . 8 . 1731–1742 . August 2006 . 10.1111/j.0014-3820.2006.tb00516.x . 17017072 . 26853128 .
5. 10.1111/jzo.12066. Evolution on a desert island: Body size divergence between the reptiles of Nevada's Anaho Island and the mainland around Pyramid Lake. Journal of Zoology. 291. 4. 269–278. 2013-08-27. Keehn . J. E.. Nieto . N. C.. Tracy . C. R.. Gienger . C. M.. Feldman . C. R..
6. Lomolino . M. V. . Body size evolution in insular vertebrates: generality of the island rule . . 32 . 10 . 1683–1699 . 2005-09-05 . 10.1111/j.1365-2699.2005.01314.x . 2005JBiog..32.1683L . 2027.42/146565. 85866114 . free.
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8. 10.1111/j.1469-7998.2006.00170.x . 270 . 4 . A new genus and species of giant hutia (Tainotherium valei) from the Quaternary of Puerto Rico: an extinct arboreal quadruped? . Journal of Zoology . 585–594 . Turvey . S. T.. 2006 .
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14. Morphometric analysis of the black rat, Rattus rattus, from Congreso Island (Chafarinas Archipelago, Spain). Jacint Ventura. María José López-Fuster. May 2000 . Orsis. 15. 91–102.
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16. Domo de Zaza, an Early Miocene Vertebrate Locality in South-Central Cuba, with Notes on the Tectonic Evolution of Puerto Rico and the Mona Passage . MacPhee, R.D.E., Iturralde-Vinent, M.A., and Gaffney, E.S. . American Museum Novitates . 1–42 . 3394 . February 2003 . 10.1206/0003-0082(2003)394<0001:DDZAEM>2.0.CO;2. 2246/2820 . 55615855 .
17. Web site: Late Cretaceous Animals of Romania's Haţeg Island--a More Complex View.
18. Pavia. M.. Meijer. H. J. M.. Rossi. M. A.. Göhlich. U. B.. The extreme insular adaptation of Garganornis ballmanni Meijer, 2014: a giant Anseriformes of the Neogene of the Mediterranean Basin. Royal Society Open Science. 4. 1. 2017-01-11. 160722. 10.1098/rsos.160722. 28280574. 5319340. 2017RSOS....460722P.
19. Web site: African Origins for the Enigmatic Adzebill.
20. Web site: Naish . Darren . Darren Naish . Titan-hawks and other super-raptors . Tetrapod Zoology blog . ScienceBlogs LLC . 2008-01-28 . 2011-03-02.
21. Lerner. Heather R.L.. Mindell. David P.. Phylogeny of eagles, Old World vultures, and other Accipitridae based on nuclear and mitochondrial DNA. Molecular Phylogenetics and Evolution . 37. 2. 2005. 327–346. 10.1016/j.ympev.2005.04.010. 15925523. 2005MolPE..37..327L.
22. Pregill . G. K. . Steadman, D. W. . South Pacific Iguanas: Human Impacts and a New Species . . 38 . 1 . 15–21 . March 2004 . 1566081 . 10.1670/73-03A . 85627049 .
23. Pregill . G. K. . Worthy, T. H. . A New Iguanid Lizard (Squamata, Iguanidae) from the Lare Quaternary of Fiji, Southwest Pacific . Herpetologica . 59 . 1 . 57–67 . March 2003 . 0018-0831 . 10.1655/0018-0831(2003)059[0057:ANILSI]2.0.CO;2 . 85804786 .
24. Petren. K.. Case. T.J.. A Phylogenetic Analysis of Body Size Evolution and Biogeography in Chuckwallas (Sauromalus) and Other Iguanines . Evolution. 51. 1. 1997. 206–219. 10.1111/j.1558-5646.1997.tb02402.x. 28568786. 22032248.
25. Maca-Meyer . N. . Carranza, S. . Rando, J. C. . Arnold, E. N. . Cabrera, V. M. . Status and relationships of the extinct giant Canary Island lizard Gallotia goliath (Reptilia: Lacertidae), assessed using ancient mtDNA from its mummified remains . Biological Journal of the Linnean Society . 80 . 4 . 659–670 . 2003-12-01 . 10.1111/j.1095-8312.2003.00265.x .
26. Current Herpetology. 21. 1. 43–50. June 2002. The Herpetological Society of Japan . Early Growth of Elaphe quadrivirgata from an Insular Gigantic Population . Akira Mori . Masami Hasegawa . 10.5358/hsj.21.43 .
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