Insular dwarfism
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Insular dwarfism, a form of phyletic dwarfism,[1] is the process and condition of large animals evolving or having a reduced body sizeTemplate:Refn 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 various species of dwarf elephants that evolved during the Pleistocene epoch, as well as more ancient examples, such as the dinosaurs Europasaurus and Magyarosaurus. 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").Script error: No such module "Unsubst". 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. This is itself one aspect of island syndrome, which describes the differences in morphology, ecology, physiology and behaviour of insular species compared to their continental counterparts.
Possible causes
There are several proposed explanations for the mechanism which produces such dwarfism.[2][3]
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.[2]
In the tropics, small size should make thermoregulation easier.[2]
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.[3]
Among carnivores, the main factor is thought to be the size and availability of prey resources, and competition is believed to be less important.[3] 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.[4]
Differences of dwarfism and 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 stegodonts 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.[5] 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.[5]
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.[3] 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).[6] 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).[6]
It has been suggested that for dwarf elephants, competition was an important factor in body size, with islands with competing herbivores having significantly larger dwarf elephants than those where competing herbivores were absent.[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 | Time frame | Continental relative |
|---|---|---|---|---|
| File:AmpelosaurusScale.png Ampelosaurus |
A. atacis | Ibero-Armorican Island | Late Cretaceous / Maastrichtian | File:Tapuiasaurus NT.jpg Nemegtosaurids |
| File:Europasaurus skull.JPG Europasaurus |
E. holgeri | Lower Saxony | Late Jurassic / Middle Kimmeridgian | File:Giraffatitan scale.png Brachiosaurs |
| File:Magyarosaurus- human size.JPG Magyarosaurus |
M. dacus | Hațeg Island | Late Cretaceous / Maastrichtian | File:Rapetosaurus BW.jpg Rapetosaurus |
| File:Lirainosaurus.jpg Lirainosaurus[11] |
L. astibiae | Ibero-Armorican Island | Late Cretaceous | |
| File:Paludititan nalatzsensis.jpg Paludititan |
P. nalatzensis | Hațeg Island | Late Cretaceous / Maastrichtian | File:Epachtosaurus sciuttoi.jpg Epachthosaurus |
Other
| Example | Species | Range | Time frame | Continental relative |
|---|---|---|---|---|
| File:Langenburg theropod size.png Langenberg Quarry torvosaur (blue) |
Unnamed | Lower Saxony | Late Jurassic / Middle Kimmeridgian | File:Torvosaurus gurneyi.png Torvosaurus |
| File:Struthiosaurus austriacus.jpg Struthiosaurus[12] |
S. austriacus S. transylvanicus S. languedocensis |
Ibero-Armorican, Australoalpine, and Hațeg Islands | Late Cretaceous | File:Edmontonia Scale.svg Edmontonia |
| File:Telmatosaurus Scale.svg Telmatosaurus |
T. transsylvanicus | Hațeg Island | Late Cretaceous | File:Hadrosaurus Scale.svg Hadrosaurids |
| File:Thecodontosaurus Scale.svg Thecodontosaurus[9] |
T. antiquus | Southern England | Late Triassic / Rhaetian | File:Human-plateosaurus size comparison.svg Plateosaurs |
| File:Iguanodontian Sizes.svg Zalmoxes[9] (purple) |
Z. robustus Z. shqiperorum |
Hațeg Island | Late Cretaceous | File:Perot Museum Tenontosaurus.jpg 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
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Squamates
| Example | Binomial name | Native range | Status | Continental relative | Insular / mainland length or mass ratio |
|---|---|---|---|---|---|
| File:20150510-IMG 0786.jpg Madagascar dwarf chameleon |
Brookesia minima | Nosy Be island, Madagascar | Endangered | File:Brookesia species male female (Journal.pone.0031314.g010).png Madagascar leaf chameleons | |
| File:Brookesia micra on a match head.jpg Nosy Hara chameleon[17] |
Brookesia micra | Nosy Hara island, Madagascar | Vulnerable | ||
| Roxby Island tiger snake[4] | Notechis scutatus | Roxby Island, South Australia | Unknown | File:Tiger snake 2.jpg Tiger snake |
|
| Dwarf Burmese python | Python bivittatus progschai | Java, Bali, Sumbawa and Sulawesi, Indonesia | Unknown | File:Burmese python (6887388927).jpg Burmese python |
LR ≈ 0.44 Template:Efn |
| Tanahjampea reticulated python[18] | Python reticulatus jampeanus | Tanahjampea, between Sulawesi and Flores | Unknown | File:Python reticulatus сетчатый питон-2.jpg Reticulated python |
LR ≈ 0.41, males LR ≈ 0.49, females Template:Efn |
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Mammals
Pilosans
| Example | Binomial name | Native range | Status | Continental relative |
|---|---|---|---|---|
| File:Bradypus pygmaeus.jpg Pygmy three-toed sloth |
Bradypus pygmaeus | Isla Escudo de Veraguas, Panama | Critically endangered | File:Bradypus variegatus, the Brown-throated Three-toed Sloth (12687597105).jpg Brown-throated sloth |
| File:Habanocnus.JPG Acratocnus |
A. antillensis A. odontrigonus A. ye |
Cuba, Hispaniola and Puerto Rico | Extinct (c. 3000 BC) | File:Megalonyx size.svg Continental ground sloths |
| Imagocnus | I. zazae | Cuba | Extinct (Early Miocene) | |
| File:Megalocnus.jpg Megalocnus |
M. rodens M. zile |
Cuba and Hispaniola | Extinct (c. 2700 BC) | |
| File:Synocnus comes.jpg Neocnus |
Neocnus spp. | Cuba and Hispaniola | Extinct (c. 3000 BC) |
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Proboscideans
| Example | Binomial name | Native range | Status | Continental relative |
|---|---|---|---|---|
| Sulawesi dwarf elephant | Elephas celebensis | Sulawesi | Extinct (Early Pleistocene) | File:Indian-Elephant-444.jpg Asian elephant |
| Cabarruyan dwarf elephant | Elephas beyeri | Luzon | Extinct | |
| Cretan dwarf mammoth | Mammuthus creticus | Crete | Extinct | File:Mammuthus Size comparison.png Mammuthus |
| File:M. exilis skeletal.png Channel Islands mammoth |
Mammuthus exilis | Santa Rosae island | Extinct (Late Pleistocene) | File:M. columbi skeletals.png Columbian mammoth |
| File:Mammuthus lamarmorai.png | Mammuthus lamarmorai | Sardinia | Extinct (Late Pleistocene) | File:Steppe mammoth size 2.jpg Steppe mammoth |
| Saint Paul Island woolly mammoth[19][20] | Mammuthus primigenius | Saint Paul Island, Alaska | Extinct (c. 3750 BC) | File:M. primigenius.png Woolly mammoth |
| File:Elephas skeleton.JPG Siculo-Maltese elephants |
Palaeoloxodon antiquus leonardi P. mnaidriensis P. melitensis P. falconeri |
Sicily and Malta | Extinct | File:Palaeoloxodon-Species-Scale-Diagram-SVG-Steveoc86.svg Straight-tusked elephant (left) |
| Cretan elephants | Palaeoloxodon chaniensis P. creutzburgi |
Crete | Extinct | |
| File:Elephas cypriotes Tusk and Molar.jpg Cyprus dwarf elephant |
Palaeoloxodon cypriotes | Cyprus | Extinct (c. 9000 BC) | |
| Naxos dwarf elephant | Palaeoloxodon sp. | Naxos | Extinct | |
| Tilos dwarf elephant | Palaeoloxodon tiliensis | Tilos | Extinct | |
| Rhodes dwarf elephant | Palaeoloxodon sp. | Rhodes | Extinct | |
| Bumiayu dwarf sinomastodont[21] | Sinomastodon bumiajuensis | Bumiayu Island (now part of Java) | Extinct (Early Pleistocene) | File:Sinomastodon.png Sinomastodon |
| File:のんほいパーク - アケボノゾウ.jpg Japanese stegodont[22][23] |
Stegodon miensis Stegodon protoaurorae Stegodon aurorae |
Japan (Also Taiwan for S. aurorae)[24] | Extinct (Early Pleistocene) | File:Stegodon skeletal.png Chinese Stegodon |
| Greater Flores dwarf stegodont[2] | Stegodon florensis | Flores | Extinct (Late Pleistocene) | File:Stegodon’s ivory displayed at Philippine National Museum.jpg Sundaland Stegodon |
| Javan dwarf stegodonts | Stegodon hypsilophus[21] S. semedoensis[25] S. sp.[21] |
Java | Extinct (Quaternary) | |
| Mindanao pygmy stegodont[26] | Stegodon mindanensis | Mindanao and Sulawesi | Extinct (Middle Pleistocene) | |
| Sulawesi dwarf stegodont[21] | Stegodon sompoensis | Sulawesi | Extinct | |
| Lesser Flores dwarf stegodont[2] | Stegodon sondaari | Flores | Extinct (Middle Pleistocene) | |
| Sumba dwarf stegodont[27] | Stegodon sumbaensis | Sumba, Indonesia | Extinct (Middle Pleistocene) | |
| Timor dwarf stegodont[21] | Stegodon timorensis | Timor | Extinct | |
| Dwarf stegolophodont[28] | Stegolophodon pseudolatidens | Japan | Extinct (Miocene) | File:Stegolophodon latidens.JPG Stegolophodon |
Primates
| Example | Binomial name | Native range | Status | Continental relative |
|---|---|---|---|---|
| Nosy Hara dwarf lemur[29] | Cheirogaleus sp. | Nosy Hara island off Madagascar | Unknown | File:Cheirogaleus-medius.jpg Dwarf lemurs |
| File:Specimen LB1.jpg Flores Man[30] |
Homo floresiensis | Flores | Extinct (Late Pleistocene) | File:Homme de Tautavel 01-08.jpg Homo erectus |
| File:LuzonensisMolars.jpg Callao Man |
Homo luzonensis[31][32] | Luzon, Philippines | Extinct (Late Pleistocene) | |
| Modern pygmies of Flores[33] | Homo sapiens | Flores | Extant | other members of Homo sapiens |
| Early Palau modern humans (disputed)[34] | Homo sapiens | Palau | Extinct (?) | |
| Andamanese[35] | Homo sapiens | Andaman Islands | Extant | |
| File:Macaca majori.JPG Sardinian macaque[36] |
Macaca majori | Sardinia | Extinct (Pleistocene) | File:Barbary macaques of Gibraltar in search of food.jpg Barbary macaque |
| File:Red Colobus 7.jpg Zanzibar red colobus |
Piliocolobus kirkii | Unguja | Endangered | File:Udzungwa Red Colobus Stevage.JPG Udzungwa red colobus |
Carnivorans
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Non-ruminant ungulates
Bovids
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Cervids and relatives
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Plants
| Possible example | Binomial name | Native range | Status | Continental relative |
|---|---|---|---|---|
| File:El Tecolote.JPG Insular elephant cacti[42][43] |
Pachycereus pringlei | Remote islands in the Sea of Cortez (e.g. Santa Cruz, San Pedro Mártir) |
Not evaluated | File:Pachycereus pringlei cardon sahueso.JPG Mainland elephant cacti |
See also
Notes
References
External links
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- ↑ Carpenter, K. (2001) The Armored Dinosaurs. Indiana University Press, 526 pages.
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- ↑ Parker S (1984) The extinct Kangaroo Island Emu, a hitherto-unrecognised species. Bulletin of the British Ornithologists' Club 104: 19–22.
- ↑ Script error: No such module "Citation/CS1".
- ↑ Cole, Theresa L., et al. "Mitogenomes uncover extinct penguin taxa and reveal island formation as a key driver of speciation." Molecular biology and evolution 36.4 (2019): 784-797.
- ↑ Script error: No such module "Citation/CS1".
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- ↑ Schirber, Michael. Surviving Extinction: Where Woolly Mammoths Endured. Live Science. Imaginova Corporation. Retrieved 2007-07-20.
- ↑ The mammoths of Wrangel Island, north of Siberia, are no longer considered dwarfs. See: Tikhonov, Alexei; Larry Agenbroad; Sergey Vartanyan (2003). Comparative analysis of the mammoth populations on Wrangel Island and the Channel Islands. DEINSEA 9: 415–420. ISSN 0923-9308
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- ↑ Siswanto, S., & Noerwidi, S. (2014). PROBOSCIDEA FOSSIL FROM SEMEDO SITE: Its Correlation With Biostratigraphy and Human Arrival in Java. Berkala Arkeologi, 34(2).
- ↑ Script error: No such module "citation/CS1".
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- ↑ Scientist to study Hobbit morphing, abc.net.au
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- ↑ "Ancient Small People on Palau Not Dwarfs, Study Says". National Geographic News. August 27, 2008.
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