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 weighed about 305 kg. Females were considerably smaller.

Insular dwarfism, a form of phyletic dwarfism,[1] is the process and condition of large animals evolving or having a reduced body size[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 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.

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		 E. holgeri Lower Saxony Late Jurassic / Middle Kimmeridgian Brachiosaurs

Magyarosaurus		 M. dacus Hateg Island Late Cretaceous / Maastrichtian Rapetosaurus

Paludititan		 P. nalatzensis Hateg Island Late Cretaceous / Maastrichtian Epachthosaurus

Telmatosaurus		 T. transsylvanicus Hateg Island Late Cretaceous Hadrosaurids

Tethyshadros		 T. insularis Trieste province Late Cretaceous Jintasaurus

Thecodontosaurus[9]		 T. antiquus Southern England Late Triassic / Rhaetian Plateosaurus

Lufengosaurus

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

Kangaroo Island emu[12]		 Dromaius novaehollandiae baudinianus Kangaroo Island, South Australia Extinct (c. AD 1827) Emu

King Island emu[13]		 Dromaius novaehollandiae minor King Island, Tasmania Extinct (AD 1822)
Cozumel thrasher[11] Toxostoma gluttatum Cozumel Critically endangered Typical thrashers

Squamates

Example Binomial Name Native Range Status Continental relatives

Madagascar dwarf chameleon		 Brookesia minima Nosy Be Island, Madagascar Endangered Madagascar leaf chameleons

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

Pygmy three-toed sloth		 Bradypus pygmaeus Isla Escudo de Veraguas, Panama Critically endangered Brown-throated sloth

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		 M. rodens

M. zile		 Cuba and Hispaniola Extinct (c. 2700 BC)

Neocnus		 Neocnus spp. Cuba and Hispaniola Extinct (c. 3000 BC)

Proboscideans

Example Binomial Name Native Range Status Continental relatives

Cretan mammoth		 Mammuthus creticus Crete Extinct Mammuthus

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

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
Flores Man[24] Homo floresiensis Flores Extinct (Late Pleistocene) Homo erectus
Modern pygmies of Flores[25] Homo sapiens Flores Extant Humans
Early Palau modern humans (disputed)[26] Homo sapiens Palau Extinct (?) Humans
Andamanese[27][28] Homo sapiens Andaman Islands Endangered Humans

Carnivorans

Example Binomial Name Native Range Status Continental relatives

Honshū wolf		 Canis lupus hodophilax Japan (excluding Hokkaido) Extinct (AD 1905) 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		 Panthera pardus adersi Unguja Island, Zanzibar Critically endangered or Extinct African leopard

Bali tiger		 Panthera tigris balica Bali Extinct (c. AD 1940) Tiger

Javan tiger		 Panthera tigris sondaica Java Extinct (c. AD 1975)

Cozumel raccoon		 Procyon pygmaeus Cozumel Critically endangered Common raccoon

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 hippopotamuses		 Choeropsis madagascariensis

Hippopotamus lalouema

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

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

Cretan dwarf hippopotamus		 Hippopotamus creutzburgi Crete Extinct (Middle Pleistocene) European hippopotamus

Maltese dwarf hippopotamus		 Hippopotamus melitensis Malta Extinct (Pleistocene) European hippopotamus

Cyprus dwarf hippopotamus		 Hippopotamus minor Cyprus Extinct (c. 8000 BC)

Sicilian hippopotamus		 Hippopotamus pentlandi Sicily Extinct (Pleistocene)
Cozumel collared peccary[11] Pecari tajacu nanus Cozumel Unknown Collared peccary
Philippines rhinoceros[29] 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[30] Bos primigenius siciliae[18] Sicily Extinct (Late Pleistocene) Eurasian aurochs
Cebu tamaraw Bubalus cebuensis Cebu, Philippines Extinct Wild water buffalo

Lowland anoa		 Bubalus depressicornis Sulawesi and Buton, Indonesia Endangered

Tamaraw		 Bubalus mindorensis Mindoro, Philippines Critically endangered

Mountain anoa		 Bubalus quarlesi Sulawesi and Buton, Indonesia Endangered

Balearic Islands cave goat		 Myotragus balearicus Majorca and Menorca Extinct (after 3000 BC) Gallogoral
Dahlak Kebir gazelle[31] Nanger soemmerringi ssp. Dahlak Kebir island, Eritrea Vulnerable Soemmerring's gazelle
Nesogoral[32] Nesogoral spp. Sardinia Extinct Gallogoral

Cervids and relatives

Example Binomial Name Native Range Status Continental relatives

Cretan dwarf megacerine deer		 Candiacervus spp. Crete Extinct (Pleistocene) Praemegaceros verticornis[9]
Ryukyu dwarf deer[33] Cervus astylodon Ryukyu Islands Extinct Sika deer (?)

Cervus praenipponicus (?)
Jersey red deer population[34] Cervus elaphus Jersey Extinct (Pleistocene) Red deer

Corsican red deer		 Cervus elaphus corsicanus Corsica and Sardinia Near Threatened
Pleistocene Sicilian deer[18] Cervus siciliae Sicily Extinct (Late Pleistocene)

Hoplitomeryx		 Hoplitomeryx spp. Gargano Island Extinct (Early Pliocene) Pecorans
Sicilian megacerine deer[18] Megaloceros carburangelensis Sicily Extinct (Late Pleistocene) Irish elk

Key deer		 Odocoileus virginianus clavium Florida Keys Endangered Virginia deer
Sardinian megacerine deer[9] Praemegaceros cazioti Sardinia Extinct (c. 5500 BC) Praemegaceros verticornis

Svalbard reindeer		 Rangifer tarandus platyrhynchus Svalbard Unknown Reindeer

Philippine deer		 Rusa marianna Philippines Vulnerable Sambar deer

See also

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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. doi:10.1371/journal.pgen.1001342. PMC 3060065. PMID 21436896.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  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. PMC 3306709. 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. doi:10.1073/pnas.251548698. ISSN 0027-8424. JSTOR 3057309. PMC 64714. PMID 11724953. Retrieved 2012-01-28.
  8. ^ "Dwarf dinosaur island really did exist, scientists claim". Telegraph Media Group. 2010-02-22. Retrieved 2010-02-26.
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  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. doi:10.1371/journal.pone.0018728. PMC 3073985. PMID 21494561.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  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. doi:10.1371/journal.pone.0031314. PMC 3279364. PMID 22348069. Retrieved 2012-02-17.{{cite journal}}: CS1 maint: unflagged free DOI (link)
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  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
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  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. ^ "Evolutionary history and adaptation of a human pygmy population of Flores Island, Indonesia". Science. 361 (6401): 511–516. 2018-08-03. doi:10.1126/science.aar8486. {{cite journal}}: Cite uses deprecated parameter |authors= (help)
  26. ^ "Ancient Small People on Palau Not Dwarfs, Study Says". National Geographic News. August 27, 2008.
  27. ^ Gibbons, A. (2018). "Island living can shrink humans". Science. doi:10.1126/science.aau9750.
  28. ^ Mondal, M.; Casals, F.; Xu, T.; Dall'Olio, G. M.; Pybus, M.; Netea, M. G.; Comas, D.; Laayouni, H.; Li, Q.; Majumder, P. P.; Bertranpetit, J. (2016). "Genomic analysis of Andamanese provides insights into ancient human migration into Asia and adaptation". Nature Genetics. 48 (9): 1066–1070. doi:10.1038/ng.3621.
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  30. ^ 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.
  31. ^ 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.
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External links

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