Insular dwarfism

Form of phyletic dwarfism occurring on islands

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

There are several proposed explanations for the mechanism which produces such dwarfism. 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. 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. 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).

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. 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.

Example Species Range Time frame Continental relative Ampelosaurus A. atacis Ibero Armorican Island Late Cretaceous / Maastrichtian Nemegtosaurids Europasaurus E. holgeri Lower Saxony Late Jurassic / Middle Kimmeridgian Brachiosaurs Magyarosaurus M. dacus Hateg Island Late Cretaceous / Maastrichtian Rapetosaurus Lirainosaurus L. astibiae Ibero Armorican Island Late Cretaceous Paludititan P. nalatzensis Hateg Island Late Cretaceous / Maastrichtian Epachthosaurus

Example Species Range Time frame Continental relative Langenberg Quarrytorvosaur (blue) Unnamed Lower Saxony Late Jurassic / Middle Kimmeridgian Torvosaurus Struthiosaurus S. austriacusS. transylvanicusS. languedocensis Ibero Armorican, Australoalpine, and Hateg islands Late Cretaceous Edmontonia Telmatosaurus T. transsylvanicus Hateg Island Late Cretaceous Hadrosaurids Thecodontosaurus Dromaius novaehollandiae baudinianus Kangaroo Island, South Australia Extinct (c. AD 1827) Emu King Island emu (black) Dromaius novaehollandiae minor King Island, Tasmania Extinct (AD 1822) Based on the heights in Fig. 1 of Heupink et al., 2011 Megadyptes antipodes richdalei Chatham Islands, New Zealand Extinct (after 1300 AD) Yellow eyed penguin Cozumel thrasher Brookesia micra Nosy Hara island, Madagascar Vulnerable Roxby Island tiger snakeNotechis scutatus Roxby Island, South Australia Unknown Tiger snake Dwarf Burmese python
Python bivittatus progschai Java, Bali, Sumbawa and Sulawesi, Indonesia Unknown Burmese python LR ≈ 0.44 Tanahjampea reticulated python
Python reticulatus jampeanus Tanahjampea, between Sulawesi and Flores Unknown Reticulated python LR ≈ 0.41, malesLR ≈ 0.49, females

Example Binomial name Native range Status Continental relative Pygmy three toed sloth Bradypus pygmaeus Isla Escudo de Veraguas, Panama Critically endangered Brown throated sloth Acratocnus A. antillensisA. odontrigonusA. ye Cuba, Hispaniola and Puerto Rico Extinct (c. 3000 BC) Continental ground sloths Imagocnus I. zazae Cuba Extinct (Early Miocene) Megalocnus M. rodensM. zile Cuba and Hispaniola Extinct (c. 2700 BC) Neocnus Neocnus spp. Cuba and Hispaniola Extinct (c. 3000 BC)

Example Binomial name Native range Status Continental relative Sulawesi dwarf elephant Elephas celebensis Sulawesi Extinct (Early Pleistocene) Asian elephant Cabarruyan dwarf elephant Elephas beyeri Luzon Extinct Cretan dwarf 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 Mammuthus primigenius Saint Paul Island, Alaska Extinct (c. 3750 BC) Woolly mammoth Siculo Maltese elephants Palaeoloxodon antiquus leonardiP. mnaidriensisP. melitensisP. falconeri Sicily and Malta Extinct Straight tusked elephant(left) Cretan elephants Palaeoloxodon chaniensisP. 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 Bumiayu dwarf sinomastodont Stegodon miensisStegodon protoauroraeStegodon aurorae Japan (Also Taiwan for S. aurorae) Extinct (Early Pleistocene) Chinese Stegodon Greater Flores dwarf stegodont Stegodon florensis Flores Extinct (Late Pleistocene) Sundaland Stegodon Javan dwarf stegodonts Stegodon hypsilophusS. semedoensisS. sp. Stegodon mindanensis Mindanao and Sulawesi Extinct (Middle Pleistocene) Sulawesi dwarf stegodont Stegodon sumbaensis Sumba, Indonesia Extinct (Middle Pleistocene) Timor dwarf stegodont Stegolophodon pseudolatidens Japan Extinct (Miocene) Stegolophodon

Example Binomial name Native range Status Continental relative Nosy Hara dwarf lemur Cheirogaleus sp. Nosy Hara island off Madagascar Unknown Dwarf lemurs Flores Man Homo floresiensis Flores Extinct (Late Pleistocene) Homo erectus Callao Man Homo luzonensis Luzon, Philippines Extinct (Late Pleistocene) Modern pygmies of Flores Homo sapiens Flores Extant other members of Homo sapiens Early Palau modern humans (disputed) Homo sapiens Palau Extinct (?) Andamanese Homo sapiens Andaman Islands Extant Sardinian macaque Macaca majori Sardinia Extinct (Pleistocene) Barbary macaque Zanzibar red colobus Piliocolobus kirkii Unguja Endangered Udzungwa red colobus

Example Binomial name Native range Status Continental relative Insular / mainlandlength or mass ratio Sicilian wolf Canis lupus cristaldii Sicily Extinct (AD 1970) Gray wolf Japanese wolf Canis lupus hodophilax Japan (excluding Hokkaido) Extinct (AD 1905) Sardinian dhole(forward) Cynotherium sardous Corsica and Sardinia Extinct (c. 8300 BC) Xenocyon Trinil dog Mececyon trinilensis Java Extinct (Pleistocene) Cozumel Island coati Nasua narica nelsoni Cozumel Critically endangered Yucatan white nosed coati Zanzibar leopard Panthera pardus pardus Unguja Critically endangered or Extinct African leopard Bali tiger Panthera tigris sondaica Bali Extinct (c. AD 1940) Sumatran tiger Javan tiger Java Extinct (c. AD 1975) Cozumel raccoon Procyon pygmaeus Cozumel Critically endangered Common raccoon Island fox Urocyon littoralis Six of the Channel Islands of California Near Threatened Gray fox LR ≈ 0.84 LR ≈ 0.75 Cozumel fox Urocyon sp. Cozumel Critically endangered or Extinct

Example Binomial name Native range Status Continental relative Eumaiochoerus Eumaiochoerus etruscus Baccinello, Montebamboli Extinct (Miocene) Microstonyx Malagasy dwarf hippopotamuses Hippopotamus laloumenaH. lemerleiH. madagascariensis Madagascar Extinct (c. AD 1000) Common hippopotamus Bumiayu dwarf hippopotamus Nesorhinus philippinensis Luzon Extinct (Middle Pleistocene) Javan rhinoceros

Example Binomial name Native range Status Continental relative Sicilian bison Bos primigenius siciliae Nesogoral spp. Sardinia Extinct Dahlak Kebir gazelle Nanger soemmerringi ssp. Dahlak Kebir island, Eritrea Vulnerable Soemmerring's gazelle Tyrrhenotragus Tyrrhenotragus gracillimus Baccinello Extinct Antilopinae sp.

Example Binomial name Native range Status Continental relative Cretan dwarf megacerines Candiacervus spp. Crete Extinct (Pleistocene) Praemegaceros verticornis Cervus astylodon Ryukyu Islands Extinct Sika deer (? )Cervus praenipponicus (?) Jersey red deer population Cervus elaphus jerseyensis Jersey Extinct (Pleistocene) Red deer Corsican red deer Cervus elaphus corsicanus Corsica and Sardinia Near Threatened group = lower alpha Hoplitomeryx spp. Gargano Island Extinct (Early Pliocene) Pecorans Sicilian megacerine Pachycereus pringlei Remote islands in the Sea of Cortez(e. g. Santa Cruz, San Pedro Mártir) Not evaluated Mainland elephant cacti