Insular dwarfism

Insular dwarfism, a form of phyletic dwarfism,[1] is the process and condition of large animals evolving or having a reduced body size[lower-alpha 1] when their population's range is limited to a small environment, primarily islands. This natural process is distinct from the intentional creation of dwarf breeds, called dwarfing. This process has occurred many times throughout evolutionary history, with examples including dinosaurs, like Europasaurus, and modern animals such as elephants and their relatives. This process, and other "island genetics" artifacts, can occur not only on islands, but also in other situations where an ecosystem is isolated from external resources and breeding. This can include caves, desert oases, isolated valleys and isolated mountains ("sky islands"). Insular dwarfism 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 (island gigantism), and large species tend to evolve smaller bodies.

Possible causes

There are several proposed explanations for the mechanism which produces such dwarfism.[3][4]

One is a selective process where only smaller animals trapped on the island survive, as food periodically declines to a borderline level. The smaller animals need fewer resources and smaller territories, and so are more likely to get past the break-point where population decline allows food sources to replenish enough for the survivors to flourish. Smaller size is also advantageous from a reproductive standpoint, as it entails shorter gestation periods and generation times.[3]

In the tropics, small size should make thermoregulation easier.[3]

Among herbivores, large size confers advantages in coping with both competitors and predators, so a reduction or absence of either would facilitate dwarfing; competition appears to be the more important factor.[4]

Among carnivores, the main factor is thought to be the size and availability of prey resources, and competition is believed to be less important.[4] In tiger snakes, insular dwarfism occurs on islands where available prey is restricted to smaller sizes than are normally taken by mainland snakes. Since prey size preference in snakes is generally proportional to body size, small snakes may be better adapted to take small prey.[5]

Dwarfism versus gigantism

The inverse process, wherein small animals breeding on isolated islands lacking the predators of large land masses may become much larger than normal, is called island gigantism. An excellent example is the dodo, the ancestors of which were normal-sized pigeons. There are also several species of giant rats, one still extant, that coexisted with both Homo floresiensis and the dwarf stegodons on Flores.

The process of insular dwarfing can occur relatively rapidly by evolutionary standards. This is in contrast to increases in maximum body size, which are much more gradual. When normalized to generation length, the maximum rate of body mass decrease during insular dwarfing was found to be over 30 times greater than the maximum rate of body mass increase for a ten-fold change in mammals.[6] The disparity is thought to reflect the fact that pedomorphism offers a relatively easy route to evolve smaller adult body size; on the other hand, the evolution of larger maximum body size is likely to be interrupted by the emergence of a series of constraints that must be overcome by evolutionary innovations before the process can continue.[6]

Factors influencing the extent of dwarfing

For both herbivores and carnivores, island size, the degree of island isolation and the size of the ancestral continental species appear not to be of major direct importance to the degree of dwarfing.[4] However, when considering only the body masses of recent top herbivores and carnivores, and including data from both continental and island land masses, the body masses of the largest species in a land mass were found to scale to the size of the land mass, with slopes of about 0.5 log(body mass/kg) per log(land area/km2).[7] There were separate regression lines for endothermic top predators, ectothermic top predators, endothermic top herbivores and (on the basis of limited data) ectothermic top herbivores, such that food intake was 7 to 24-fold higher for top herbivores than for top predators, and about the same for endotherms and ectotherms of the same trophic level (this leads to ectotherms being 5 to 16 times heavier than corresponding endotherms).[7]

Examples

Non-avian dinosaurs

Recognition that insular dwarfism could apply to dinosaurs arose through the work of Ferenc Nopcsa, a Hungarian-born aristocrat, adventurer, scholar, and paleontologist. Nopcsa studied Transylvanian dinosaurs intensively, noticing that they were smaller than their cousins elsewhere in the world. For example, he unearthed six-meter-long sauropods, a group of dinosaurs which elsewhere commonly grew to 30 meters or more. Nopcsa deduced that the area where the remains were found was an island, Hațeg Island (now the Haţeg or Hatzeg basin in Romania) during the Mesozoic era.[8][9] Nopcsa's proposal of dinosaur dwarfism on Hațeg Island is today widely accepted after further research confirmed that the remains found are not from juveniles.[10]

Sauropods

Example Species Range Timeframe Continental relatives

Ampelosaurus
A. atacisIbero-Armorican IslandLate Cretaceous / MaastrichtianNemegtosaurid sauropods

Europasaurus
E. holgeriLower SaxonyLate Jurassic / Middle KimmeridgianBrachiosaurs

Magyarosaurus
M. dacusHateg IslandLate Cretaceous / MaastrichtianRapetosaurus

Paludititan
P. nalatzensisHateg IslandLate Cretaceous / MaastrichtianEpachthosaurus

Other

Example Species Range Timeframe Continental relatives

Langenberg Quarry torvosaur
UnnamedLower SaxonyLate Jurassic / Middle KimmeridgianTorvosaurus

Telmatosaurus
T. transsylvanicusHateg IslandLate CretaceousHadrosaurids

Tethyshadros
T. insularisTrieste provinceLate CretaceousJintasaurus

Thecodontosaurus[9]
T. antiquusSouthern EnglandLate Triassic / RhaetianPlateosaurus

Lufengosaurus

Zalmoxes[9]
Z. robustus

Z. shqiperorum
Hateg IslandLate CretaceousCamptosaurus

Rhabdodon

Tenontosaurus

In addition, the genus Balaur was initially described as a Velociraptor-sized dromaeosaurid (and in consequence a dubious example of insular dwarfism), but has been since reclassified as a secondarily flightless stem bird, closer to modern birds than Jeholornis (thus actually an example of insular gigantism).

Birds

Example Binomial Name Native Range Status Continental relatives
Cozumel curassow[11]Crax rubra grisconiCozumelUnknownGreat curassow

Kangaroo Island emu[12]
Dromaius novaehollandiae baudinianusKangaroo Island, South AustraliaExtinct (c. AD 1827)Emu

King Island emu[13]
Dromaius novaehollandiae minorKing Island, TasmaniaExtinct (AD 1822)
Cozumel thrasher[11]Toxostoma gluttatum CozumelCritically endangeredTypical thrashers

Squamates

Example Binomial Name Native Range Status Continental relatives

Madagascar dwarf chameleon
Brookesia minimaNosy Be island, MadagascarEndangeredMadagascar leaf chameleons

Nosy Hara chameleon[14]
Brookesia micraNosy Hara island, MadagascarVulnerable
Roxby Island tiger snake[5]Notechis scutatusRoxby Island, South AustraliaUnknownTiger snake
Dwarf Burmese python Python bivittatus progschaiJava, Bali, Sumbawa and Sulawesi, IndonesiaUnknownBurmese python
Tanahjampea python[15] Python reticulatus jampeanusTanahjampea island, between Sulawesi and FloresUnknownReticulated python

Mammals

Pilosans

Example Binomial Name Native Range Status Continental relatives

Pygmy three-toed sloth
Bradypus pygmaeusIsla Escudo de Veraguas, PanamaCritically endangeredBrown-throated sloth

Acratocnus
A. antillensis

A. odontrigonus

A. ye
Cuba, Hispaniola and Puerto RicoExtinct (c. 3000 BC)Megalonyx
ImagocnusI. zazaeCubaExtinct (Early Miocene)

Megalocnus
M. rodens

M. zile
Cuba and HispaniolaExtinct (c. 2700 BC)

Neocnus
Neocnus spp.Cuba and HispaniolaExtinct (c. 3000 BC)

Proboscideans

Example Binomial Name Native Range Status Continental relatives

Cretan mammoth
Mammuthus creticusCreteExtinctMammuthus

Channel Islands mammoth
Mammuthus exilisSanta Rosae islandExtinct (Late Pleistocene)Columbian mammoth
Sardinian mammothMammuthus lamarmoraiSardiniaExtinct (Late Pleistocene)Steppe mammoth
Saint Paul Island woolly mammoth[16][17]Mammuthus primigeniusSaint Paul Island, AlaskaExtinct (c. 3750 BC)Woolly mammoth

Siculo-Maltese elephants
Palaeoloxodon antiquus leonardi

P. mnaidriensis

P. melitensis

P. falconeri
Sicily and MaltaExtinctStraight-tusked elephant
Cretan elephantsPalaeoloxodon chaniensis

P. creutzburgi
CreteExtinct
Cyprus dwarf elephantPalaeoloxodon cypriotesCyprusExtinct (c. 9000 BC)
Naxos dwarf elephantPalaeoloxodon sp.NaxosExtinct
Rhodes and Tilos dwarf elephantPalaeoloxodon tiliensisRhodes and TilosExtinct
Japanese stegodon[18]Stegodon auroraeJapan and Taiwan[19]Extinct (Early Pleistocene) Chinese Stegodon
Larger Flores dwarf stegodon[3]Stegodon florensisFloresExtinct (Late Pleistocene) Sundaland Stegodon
Javan dwarf stegodon[20]Stegodon hypsilophusJavaExtinct
Mindanao pygmy stegodon[21]Stegodon mindanensisMindanao and SulawesiExtinct (Middle Pleistocene)
Sulawesi dwarf stegodon[20]Stegodon sompoensisSulawesiExtinct
Lesser Flores dwarf stegodon[3]Stegodon sondaariFloresExtinct (Middle Pleistocene)
Sumba stegodon[22]Stegodon sumbaensisSumba, IndonesiaExtinct (Middle Pleistocene)
Timor dwarf stegodon[20]Stegodon timorensisTimorExtinct
Sambungmacan dwarf stegodon[20]Stegodon sp.Kalibeng Island (now part of Java)Extinct (Early Pleistocene)
Bumiayu tetralophodon[20]Tetralophodon bumiajuensisBumiayu Island (now part of Java)Extinct (Early Pleistocene)Tetralophodon

Primates

Example Binomial Name Native Range Status Continental relatives
Nosy Hara dwarf lemur[23]Cheirogaleus sp.Nosy Hara island off MadagascarUnknownDwarf lemurs

Flores Man[24]
Homo floresiensisFloresExtinct (Late Pleistocene)Homo erectus
Callao ManHomo luzonensis[25][26]Luzon, PhilippinesExtinct (Late Pleistocene)
Modern pygmies of Flores[27]Homo sapiensFloresExtantHumans
Early Palau modern humans (disputed)[28]Homo sapiensPalauExtinct (?)

Andamanese[29][30]
Homo sapiensAndaman IslandsExtant

Sardinian macaque[31]
Macaca majoriSardiniaExtinct (Pleistocene)Barbary macaque

Carnivorans

Example Binomial Name Native Range Status Continental relatives

Honshū wolf
Canis lupus hodophilaxJapan (excluding Hokkaido)Extinct (AD 1905) Gray wolf
Sardinian dholeCynotherium sardousCorsica and SardiniaExtinctXenocyon (?)
Cozumel Island coati[11]Nasua narica nelsoniCozumelCritically endangeredYucatan white-nosed coati

Zanzibar leopard
Panthera pardus pardusUnguja Island, ZanzibarCritically endangered or ExtinctAfrican leopard

Bali tiger
Panthera tigris sondaicaBaliExtinct (c. AD 1940)Tiger

Javan tiger
Panthera tigris sondaicaJavaExtinct (c. AD 1975)

Cozumel raccoon
Procyon pygmaeusCozumelCritically endangeredCommon raccoon

Channel Island fox
Urocyon littoralisChannel Islands of CaliforniaNear ThreatenedGray fox
Cozumel foxUrocyon sp.CozumelCritically endangered or Extinct

Non-ruminant ungulates

Example Binomial Name Native Range Status Continental relatives

Malagasy hippopotamuses
Choeropsis madagascariensis

Hippopotamus lalouema

H. lemerlei
MadagascarExtinct (c. AD 1000)Pygmy hippopotamus

Common hippopotamus
Bumiayu dwarf hippopotamus[20]Hexaprotodon simplexBumiayu Island (now Java)Extinct (Early Pleistocene) Asian hippopotamuses

Cretan dwarf hippopotamus
Hippopotamus creutzburgiCreteExtinct (Middle Pleistocene)European hippopotamus

Maltese dwarf hippopotamus
Hippopotamus melitensisMaltaExtinct (Pleistocene)

Cyprus dwarf hippopotamus
Hippopotamus minorCyprusExtinct (c. 8000 BC)

Sicilian hippopotamus
Hippopotamus pentlandiSicilyExtinct (Pleistocene)
Cozumel collared peccary[11]Pecari tajacu nanusCozumelUnknownCollared peccary
Philippines rhinoceros[32]Rhinoceros philippinensisLuzonExtinct (Middle Pleistocene)Javan rhinoceros

Bovids

Example Binomial Name Native Range Status Continental relatives
Sicilian bison[18]Bison priscus siciliaeSicilyExtinct (Late Pleistocene)Steppe bison
Sicilian aurochs[33]Bos primigenius siciliae[18]SicilyExtinct (Late Pleistocene)Eurasian aurochs
Cebu tamarawBubalus cebuensisCebu, PhilippinesExtinctWild water buffalo

Lowland anoa
Bubalus depressicornisSulawesi and Buton, IndonesiaEndangered

Tamaraw
Bubalus mindorensisMindoro, PhilippinesCritically endangered

Mountain anoa
Bubalus quarlesiSulawesi and Buton, IndonesiaEndangered

Balearic Islands cave goat
Myotragus balearicusMajorca and MenorcaExtinct (after 3000 BC)Gallogoral
Dahlak Kebir gazelle[34]Nanger soemmerringi ssp.Dahlak Kebir island, EritreaVulnerableSoemmerring's gazelle
Nesogoral[35]Nesogoral spp.SardiniaExtinctGallogoral

Cervids and relatives

Example Binomial Name Native Range Status Continental relatives

Cretan dwarf megacerine deer
Candiacervus spp.CreteExtinct (Pleistocene)Praemegaceros verticornis[9]
Ryukyu dwarf deer[36]Cervus astylodonRyukyu IslandsExtinctSika deer (?)

Cervus praenipponicus (?)
Jersey red deer population[37]Cervus elaphusJerseyExtinct (Pleistocene)Red deer

Corsican red deer
Cervus elaphus corsicanusCorsica and SardiniaNear Threatened
Pleistocene Sicilian deer[18]Cervus siciliaeSicilyExtinct (Late Pleistocene)

Hoplitomeryx
Hoplitomeryx spp.Gargano IslandExtinct (Early Pliocene)Pecorans
Sicilian megacerine deer[18]Megaloceros carburangelensisSicilyExtinct (Late Pleistocene)Irish elk

Key deer
Odocoileus virginianus claviumFlorida KeysEndangeredVirginia deer

Sardinian megacerine deer[9]
Praemegaceros caziotiSardiniaExtinct (c. 5500 BC)Praemegaceros verticornis

Svalbard reindeer
Rangifer tarandus platyrhynchusSvalbardUnknownReindeer

Philippine deer
Rusa mariannaPhilippinesVulnerableSambar deer

See also

Notes

  1. An example of noninsular phyletic dwarfism is the evolution of the dwarfed marmosets and tamarins among New World monkeys, culminating in the appearance of the smallest example, Cebuella pygmaea.[2]

References

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