Insular dwarfism

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Skeleton of Malta's extinct Palaeoloxodon falconeri, the smallest known species of elephant. Adult males measured less than one meter in shoulder height and weighted about 305 kg. Females were considerably smaller.

Insular dwarfism, a form of phyletic dwarfism,[1] is the process and condition of the reduction in size of large animals over a number of generations[a] 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 traditional 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 rule", which posits that when mainland animals colonize islands, small species tend to evolve larger bodies, 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]

Example Species Range Timeframe Continental relatives
Europasaurus skull.JPG
Europasaurus
E. holgeri Lower Saxony Late Jurassic / Middle Kimmeridgian Brachiosaurs
Magyarosaurus- human size.JPG
Magyarosaurus
M. dacus Hateg Island Late Cretaceous / Maastrichtian Rapetosaurus
Paludititan nalatzsensis.jpg
Paludititan
P. nalatzensis Hateg Island Late Cretaceous / Maastrichtian Epachthosaurus
Telmatosaurus sketch v2.jpg
Telmatosaurus
T. transsylvanicus Hateg Island Late Cretaceous Hadrosaurids
Tethyshadros insularis.JPG
Tethyshadros
T. insularis Trieste province Late Cretaceous Jintasaurus
Thecodontosaurus antiquus skeleton.png
Thecodontosaurus[9]
T. antiquus South England Late Triassic / Rhaetian Plateosaurus

Lufengosaurus
Zalmoxes dichotomy.jpg
Zalmoxes[9]
Z. robustus

Z. shqiperorum
Hateg Island Late Cretaceous Camptosaurus

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 grisconi Cozumel Unknown Great curassow
Baudin emus.jpg
Kangaroo Island emu[12]
Dromaius baudinianus Kangaroo Island, South Australia Extinct (c. 1827 AD) Emu
Dromaius peroni.jpg
King Island emu[13]
Dromaius novaehollandiae minor King Island, Tasmania Extinct (1822 AD)
Cozumel thrasher[11] Toxostoma gluttatum Cozumel Critically endangered Typical thrashers

Squamates

Example Binomial Name Native Range Status Continental relatives
20150510-IMG 0786.jpg
Madagascar dwarf chameleon
Brookesia minima Nosy Be Island, Madagascar Endangered Madagascar leaf chameleons
Brookesia micra on a match head.jpg
Nosy Hara chameleon[14]
Brookesia micra Nosy Hara Island, Madagascar Vulnerable
Roxby Island tiger snake[5] Notechis scutatus Roxby Island, South Australia Unknown Tiger snake
Tanahjampea python[15] Python reticulatus jampeanus Tanahjampea Island, between Sulawesi and Flores Unknown Reticulated python

Mammals

Pilosans

Example Binomial Name Native Range Status Continental relatives
Bradypus pygmaeus.jpg
Pygmy three-toed sloth
Bradypus pygmaeus Isla Escudo de Veraguas, Panama Critically endangered Brown-throated sloth
Habanocnus.JPG
Acratocnus
A. antillensis

A. odontrigonus

A. ye
Cuba, Hispaniola and Puerto Rico Extinct (c. 3000 BC) Megalonyx
Imagocnus I. zazae Cuba Extinct (Early Miocene)
Megalocnus.jpg
Megalocnus
M. rodens

M. zile
Cuba and Hispaniola Extinct (c. 2700 BC)
Synocnus comes.jpg
Neocnus
Neocnus spp. Cuba and Hispaniola Extinct (c. 3000 BC)

Proboscideans

Example Binomial Name Native Range Status Continental relatives
Cretanelephant-petermaas.jpg
Cretan mammoth
Mammuthus creticus Crete Extinct Mammuthus
Mammuthus exilis.jpg
Channel Islands mammoth
Mammuthus exilis Santa Rosae island Extinct (Late Pleistocene) Columbian mammoth
Sardinian mammoth Mammuthus lamarmorai Sardinia Extinct (Late Pleistocene) Steppe mammoth
Saint Paul Island woolly mammoth[16][17] Mammuthus primigenius Saint Paul Island, Alaska Extinct (c. 3750 BC) Woolly mammoth
Mammuthus falconeri (dwarf mammoth).jpg
Siculo-Maltese elephants
Palaeoloxodon antiquus leonardi

P. mnaidriensis

P. melitensis

P. falconeri
Sicily and Malta Extinct Straight-tusked elephant
Cretan elephants Palaeoloxodon chaniensis

P. creutzburgi
Crete Extinct
Cyprus dwarf elephant Palaeoloxodon cypriotes Cyprus Extinct (c. 9000 BC)
Naxos dwarf elephant Palaeoloxodon sp. Naxos Extinct
Rhodes and Tilos dwarf elephant Palaeoloxodon tiliensis Rhodes and Tilos Extinct
Japanese stegodon[18] Stegodon aurorae Japan and Taiwan[19] Extinct (Early Pleistocene) Chinese Stegodon
Larger Flores dwarf stegodon[3] Stegodon florensis Flores Extinct (Late Pleistocene) Sundaland Stegodon
Javan dwarf stegodon[20] Stegodon hypsilophus Java Extinct
Mindanao pygmy stegodon[21] Stegodon mindanensis Mindanao and Sulawesi Extinct (Middle Pleistocene)
Sulawesi dwarf stegodon[20] Stegodon sompoensis Sulawesi Extinct
Lesser Flores dwarf stegodon[3] Stegodon sondaari Flores Extinct (Middle Pleistocene)
Sumba stegodon[22] Stegodon sumbaensis Sumba, Indonesia Extinct (Middle Pleistocene)
Timor dwarf stegodon[20] Stegodon timorensis Timor Extinct
Sambungmacan dwarf stegodon[20] Stegodon sp. Kalibeng Island (now part of Java) Extinct (Early Pleistocene)
Bumiayu tetralophodon[20] Tetralophodon bumiajuensis Bumiayu 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 Madagascar Unknown Dwarf lemurs
Specimen LB1.jpg
Flores Man[24]
Homo floresiensis Flores Extinct (Late Pleistocene) Homo erectus
Early Palau modern humans (disputed)[25] Homo sapiens Palau Extinct (?) Humans

Carnivorans

Example Binomial Name Native Range Status Continental relatives
Japanese Wolf.jpg
Honshū wolf
Canis lupus hodophilax Japan (excluding Hokkaido) Extinct (1905 AD) Gray wolf
Sardinian dhole Cynotherium sardous Corsica and Sardinia Extinct Xenocyon (?)
Cozumel Island coati[11] Nasua narica nelsoni Cozumel Critically endangered Yucatan white-nosed coati
Zanzibar Leopard 2.JPG
Zanzibar leopard
Panthera pardus adersi Unguja Island, Zanzibar Critically endangered or Extinct African leopard
Bali tiger zanveld.jpg
Bali tiger
Panthera tigris balica Bali Extinct (c. 1940 AD) Tiger
Java Tiger.jpg
Javan tiger
Panthera tigris sondaica Java Extinct (c. 1975 AD)
Cozumel Raccoon2.jpg
Cozumel raccoon
Procyon pygmaeus Cozumel Critically endangered Common raccoon
Urocyon littoralis pair.jpg
Channel Island fox
Urocyon littoralis Channel Islands of California Near Threatened Gray fox
Cozumel fox Urocyon sp. Cozumel Critically endangered or Extinct

Non-ruminant ungulates

Example Binomial Name Native Range Status Continental relatives
Malagasy Hippopotamus.jpg
Malagasy hippopotamuses
Choeropsis madagascariensis

Hippopotamus lalouema

H. lemerlei
Madagascar Extinct (c. 1000 AD) Pygmy hippopotamus

Common hippopotamus
Bumiayu dwarf hippopotamus[20] Hexaprotodon simplex Bumiayu Island (now Java) Extinct (Early Pleistocene) Asian hippopotamuses
Hippopotamus cruetzburgi.JPG
Cretan dwarf hippopotamus
Hippopotamus creutzburgi Crete Extinct (Middle Pleistocene) European hippopotamus
Hippopotamus amphibius Linn at Ghar Dalam, Malta.png
Maltese dwarf hippopotamus
Hippopotamus melitensis Malta Extinct (Pleistocene) European hippopotamus
Hippo-Cyprus.JPG
Cyprus dwarf hippopotamus
Hippopotamus minor Cyprus Extinct (c. 8000 BC)
Hippopotamus pentlandi 3.JPG
Sicilian hippopotamus
Hippopotamus pentlandi Sicily Extinct (Pleistocene)
Cozumel collared peccary[11] Pecari tajacu nanus Cozumel Unknown Collared peccary
Philippines rhinoceros[26] Rhinoceros philippinensis Luzon, Philippines Extinct (Middle Pleistocene) Javan rhinoceros

Bovids

Example Binomial Name Native Range Status Continental relatives
Sicilian bison[18] Bison priscus siciliae Sicily Extinct (Late Pleistocene) Steppe bison
Sicilian aurochs[27] Bos primigenius siciliae[18] Sicily Extinct (Late Pleistocene) Eurasian aurochs
Cebu tamaraw Bubalus cebuensis Cebu, Philippines Extinct Wild water buffalo
Lowland anoa.jpg
Lowland anoa
Bubalus depressicornis Sulawesi and Buton, Indonesia Endangered
Bubalus mindorensis by Gregg Yan 01.jpg
Tamaraw
Bubalus mindorensis Mindoro, Philippines Critically endangered
Buablus quarlesi2.jpg
Mountain anoa
Bubalus quarlesi Sulawesi and Buton, Indonesia Endangered
Myotragus balearicus.JPG
Balearic Islands cave goat
Myotragus balearicus Majorca and Menorca Extinct (after 3000 BC) Gallogoral
Dahlak Kebir gazelle[28] Nanger soemmerringi ssp. Dahlak Kebir island, Eritrea Vulnerable Soemmerring's gazelle
Nesogoral[29] Nesogoral spp. Sardinia Extinct Gallogoral

Cervids and relatives

Example Binomial Name Native Range Status Continental relatives
Candiacervus ropalophorus.jpg
Cretan dwarf megacerine deer
Candiacervus spp. Crete Extinct (Pleistocene) Praemegaceros verticornis[9]
Ryukyu dwarf deer[30] Cervus astylodon Ryukyu Islands Extinct Sika deer (?)

Cervus praenipponicus (?)
Jersey red deer population[31] Cervus elaphus Jersey Extinct (Pleistocene) Red deer
Daino a Porto Conte archivio Marco Busdraghi AHO.jpg
Corsican red deer
Cervus elaphus corsicanus Corsica and Sardinia Near Threatened
Pleistocene Sicilian deer[18] Cervus siciliae Sicily Extinct (Late Pleistocene)
Hoplitomeryx matthei.jpg
Hoplitomeryx
Hoplitomeryx spp. Gargano Island Extinct (Early Pliocene) Pecorans
Sicilian megacerine deer[18] Megaloceros carburangelensis Sicily Extinct (Late Pleistocene) Irish elk
Key deer male.jpg
Key deer
Odocoileus virginianus clavium Florida Keys Endangered Virginia deer
Sardinian megacerine deer[9] Praemegaceros cazioti Sardinia Extinct (c. 5500 BC) Praemegaceros verticornis
Spitsbergen reindeer01.jpg
Svalbard reindeer
Rangifer tarandus platyrhynchus Svalbard Unknown Reindeer
Rusa marianna by Gregg Yan.jpg
Philippine deer
Rusa marianna Philippines Vulnerable Sambar 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

  1. ^ Prothero, D. R.; Sereno, P. C. (Winter 1982). "Allometry and Paleoecology of Medial Miocene Dwarf Rhinoceroses from the Texas Gulf Coastal Plain". Paleobiology. 8 (1): 16–30. JSTOR 2400564. 
  2. ^ Perelman, P.; et al. (2011). "A Molecular Phylogeny of Living Primates". PLOS Genetics. 7 (3): 1–17. PMC 3060065Freely accessible. PMID 21436896. doi:10.1371/journal.pgen.1001342. 
  3. ^ a b c d e Van Den Bergh, G. D.; Rokhus Due Awe; Morwood, M. J.; Sutikna, T.; Jatmiko; Wahyu Saptomo, E. (May 2008). "The youngest Stegodon remains in Southeast Asia from the Late Pleistocene archaeological site Liang Bua, Flores, Indonesia". Quaternary International. 182 (1): 16–48. doi:10.1016/j.quaint.2007.02.001. Retrieved 2011-11-27. 
  4. ^ a b c d Raia, P.; Meiri, S. (August 2006). "The island rule in large mammals: paleontology meets ecology". Evolution. 60 (8): 1731–1742. doi:10.1111/j.0014-3820.2006.tb00516.x. Retrieved 2011-11-27. 
  5. ^ a b Keogh, J. S.; Scott, I. A. W.; Hayes, C. (January 2005). "Rapid and repeated origin of insular gigantism and dwarfism in Australian tiger snakes". Evolution. 59 (1): 226–233. doi:10.1111/j.0014-3820.2005.tb00909.x. 
  6. ^ a b Evans, A. R.; et al. (2012-01-30). "The maximum rate of mammal evolution". PNAS. 109. doi:10.1073/pnas.1120774109. Retrieved 2011-02-11. 
  7. ^ a b Burness, G. P.; Diamond, J.; Flannery, T. (2001-12-04). "Dinosaurs, dragons, and dwarfs: The evolution of maximal body size". Proceedings of the National Academy of Sciences. 98 (25): 14518–14523. ISSN 0027-8424. JSTOR 3057309. PMC 64714Freely accessible. PMID 11724953. doi:10.1073/pnas.251548698. Retrieved 2012-01-28. 
  8. ^ "Dwarf dinosaur island really did exist, scientists claim". Telegraph Media Group. 2010-02-22. Retrieved 2010-02-26. 
  9. ^ a b c d e Benton, M. J.; Csiki, Z.; Grigorescu, D.; Redelstorff, R.; Sander, P. M.; Stein, K.; Weishampel, D. B. (2010-01-28). "Dinosaurs and the island rule: The dwarfed dinosaurs from Haţeg Island" (PDF). Palaeogeography, Palaeoclimatology, Palaeoecology. Elsevier. 293 (3-4): 438–454. doi:10.1016/j.palaeo.2010.01.026. Retrieved 2017-07-30. 
  10. ^ Dyke, G. (2011-09-20). "The Dinosaur Baron of Transylvania". Scientific American. 305 (4): 80–83. PMID 22106812. doi:10.1038/scientificamerican1011-80. 
  11. ^ a b c d Cuarón, A. D.; Martínez-Morales, M. A.; McFadden, K. W.; Valenzuela, D.; Gompper, M. E. (2004). "The status of dwarf carnivores on Cozumel Island, Mexico" (PDF). Biodiversity and Conservation. Kluwer Academic Publishers. 13: 317–331. Retrieved 2017-07-31. 
  12. ^ Parker S (1984) The extinct Kangaroo Island Emu, a hitherto-unrecognised species. Bulletin of the British Ornithologists' Club 104: 19–22.
  13. ^ Heupink, T. H.; Huynen, L.; Lambert, D. M. (2011). "Ancient DNA Suggests Dwarf and ‘Giant’ Emu Are Conspecific". PLoS ONE. 6 (4): e18728. PMC 3073985Freely accessible. PMID 21494561. doi:10.1371/journal.pone.0018728. 
  14. ^ Glaw, F.; Köhler, J.; Townsend, T. M.; Vences, M. (2012-02-14). "Rivaling the World's Smallest Reptiles: Discovery of Miniaturized and Microendemic New Species of Leaf Chameleons (Brookesia) from Northern Madagascar". PLoS ONE. 7 (2): e31314. PMC 3279364Freely accessible. PMID 22348069. doi:10.1371/journal.pone.0031314. Retrieved 2012-02-17. 
  15. ^ Auliya, M.; Mausfeld, P.; Schmitz, A.; Böhme, W. (2002-04-09). "Review of the reticulated python (Python reticulatus Schneider, 1801) with the description of new subspecies from Indonesia". Naturwissenschaften. 89 (5): 201–213. doi:10.1007/s00114-002-0320-4. Retrieved 2012-04-08. 
  16. ^ Schirber, Michael. Surviving Extinction: Where Woolly Mammoths Endured. Live Science. Imaginova Corporation. Retrieved 2007-07-20.
  17. ^ 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
  18. ^ a b c d e Sondaar, P. Y.; A.A.E. van der Geer (2005). "Evolution and Extinction of Plio-Pleistocene Island Ungulates" (PDF). International Journal of the French Quaternary Association. 2: 241–256. Retrieved 2017-07-31. 
  19. ^ http://www.rhinoresourcecenter.com/pdf_files/129/1291330178.pdf
  20. ^ a b c d e f Aziz, F.; van den Bergh, G. D. (September 25, 1995). "A dwarf Stegodon from Sambungmacan (Central Java, Indonesia)" (PDF). Proc. Kon. Ned. Akad. v. Wetensch. 98 (3): 229–241. Retrieved 2017-07-31. 
  21. ^ Zaim, Y. (20 August 2010). "Geological Evidence for the Earliest Appearance of Hominins in Indonesia". In Fleagle, J. G; Shea, J. J.; Grine, F. E.; Baden, A. L.; Leakey, R. E. Out of Africa I: The First Hominin Colonization of Eurasia. Springer Science & Business Media. p. 106. ISBN 978-90-481-9036-2. OCLC 668096676. 
  22. ^ http://ro.uow.edu.au/cgi/viewcontent.cgi?article=3055&context=smhpapers
  23. ^ http://www.bbc.com/earth/story/20150812-tiny-lemur-may-be-worlds-rarest
  24. ^ Scientist to study Hobbit morphing, abc.net.au
  25. ^ "Ancient Small People on Palau Not Dwarfs, Study Says". National Geographic News. August 27, 2008.
  26. ^ Renema, Willem (2007). Biogeography, Time and Place: Distributions, Barriers and Islands. Springer Science & Business Media. p. 334. ISBN 978-1-4020-6374-9. OCLC 228153573. 
  27. ^ van Vuure, Cis (2005). Retracing the Aurochs: History, Morphology and Ecology of an Extinct Wild Ox. Coronet Books Incorporated. ISBN 978-954-642-235-4. OCLC 472741798. 
  28. ^ Chiozzi, G.; Bardelli, G.; Ricci, M.; De Marchi, G.; Cardini, A. (2014). "Just another island dwarf? Phenotypic distinctiveness in the poorly known Soemmerring's Gazelle, Nanger soemmerringii (Cetartiodactyla: Bovidae), of Dahlak Kebir Island". Biological Journal of the Linnean Society. 111 (3): 603–620. doi:10.1111/bij.12239. 
  29. ^ van der Geer, A.; Lyras, G; de Vos, J.; Dermitzakis, M. (14 February 2011). "Sardinia and Corsica". Evolution of Island Mammals: Adaptation and Extinction of Placental Mammals on Islands. John Wiley & Sons. ISBN 978-1-4443-9128-2. OCLC 894698082. 
  30. ^ Kaifu, Y.; Fujita, M.; Yoneda, M.; Yamasaki, S. (15 February 2015). "Pleistocene Seafaring and Colonization of the Ryuku Islands, Southwestern Japan". In Kaifu, Y.; Izuho, M.; Goebel, T.; Sato, H.; Ono, A. Emergence and Diversity of Modern Human Behavior in Paleolithic Asia. Texas A&M University Press. ISBN 978-1-62349-277-9. OCLC 985023261. 
  31. ^ Lister, A. M. (1989-11-30). "Rapid dwarfing of red deer on Jersey in the Last Interglacial". Nature. 342 (6249): 539–542. PMID 2685610. doi:10.1038/342539a0. Retrieved 2011-02-28. 

External links

  • Strange world of island species October 31, 2004 The Observer
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