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[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 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.

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

Possible causesEdit

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 vs. gigantismEdit

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.[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 dwarfingEdit

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]

ExamplesEdit

Non-avian dinosaursEdit

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]

SauropodsEdit

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[11]
L. astibiae Ibero-Armorican Island Late Cretaceous
 
Paludititan
P. nalatzensis Hateg Island Late Cretaceous / Maastrichtian  
Epachthosaurus

OtherEdit

Example Species Range Time frame Continental relative
 
Langenberg Quarry
torvosaur (blue)
Unnamed Lower Saxony Late Jurassic / Middle Kimmeridgian  
Torvosaurus
 
Struthiosaurus[12]
S. austriacus

S. transylvanicus

S. languedocensis
Ibero-Armorican, Australoalpine, and Hateg islands Late Cretaceous  
Edmontonia
 
Telmatosaurus
T. transsylvanicus Hateg Island Late Cretaceous  
Hadrosaurids
 
Tethyshadros
T. insularis Trieste province Late Cretaceous
 
Thecodontosaurus[9]
T. antiquus Southern England Late Triassic / Rhaetian  
Plateosaurs
 
Zalmoxes[9] (purple)
Z. robustus

Z. shqiperorum
Hateg Island Late Cretaceous  
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).

BirdsEdit

Example Binomial name Native range Status Continental relative Insular / mainland
length or mass ratio
 
Hawaiian flightless ibises
Apteribis glenos Molokai Extinct (Late Quaternary)  
American ibises
Apteribis brevis Maui
Cozumel curassow[13] Crax rubra griscomi Cozumel Unknown  
Great curassow
 
Kangaroo Island emu[14]
Dromaius novaehollandiae baudinianus Kangaroo Island, South Australia Extinct (c. AD 1827)  
Emu
 
King Island emu[15] (black)
Dromaius novaehollandiae minor King Island, Tasmania Extinct (AD 1822) LR ≈ 0.48 [b]
 
Cozumel thrasher[13]
Toxostoma gluttatum Cozumel Critically endangered  
Other thrashers

SquamatesEdit

Example Binomial name Native range Status Continental relative Insular / mainland
length or mass ratio
 
Madagascar dwarf chameleon
Brookesia minima Nosy Be island, Madagascar Endangered  
Madagascar leaf chameleons
 
Nosy Hara chameleon[16]
Brookesia micra Nosy Hara island, Madagascar Vulnerable
Roxby Island tiger snake[5] Notechis 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 [c]
Tanahjampea reticulated python[19] Python reticulatus jampeanus Tanahjampea, between Sulawesi and Flores Unknown  
Reticulated python
LR ≈ 0.41, males
LR ≈ 0.49, females [d]

MammalsEdit

PilosansEdit

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. antillensis

A. odontrigonus

A. ye
Cuba, Hispaniola and Puerto Rico Extinct (c. 3000 BC)  
Continental ground sloths
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)

ProboscideansEdit

Example Binomial name Native range Status Continental relative
Sulawesi dwarf elephant Elephas celebensis Sulawesi Extinct (Early Pleistocene)  
Asian elephant
 
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[22][23] 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
(left)
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
Bumiayu dwarf sinomastodont[24] Sinomastodon bumiajuensis Bumiayu Island (now part of Java) Extinct (Early Pleistocene)  
Sinomastodon
 
Japanese stegodont[25]
Stegodon aurorae Japan and Taiwan[26] Extinct (Early Pleistocene)  
Chinese Stegodon
Greater Flores dwarf stegodont[3] Stegodon florensis Flores Extinct (Late Pleistocene)  
Sundaland Stegodon
Javan dwarf stegodonts Stegodon hypsilophus[24]

S. semedoensis[27]

S. sp.[24]
Java Extinct (Quaternary)
Mindanao pygmy stegodont[28] Stegodon mindanensis Mindanao and Sulawesi Extinct (Middle Pleistocene)
Sulawesi dwarf stegodont[24] Stegodon sompoensis Sulawesi Extinct
Lesser Flores dwarf stegodont[3] Stegodon sondaari Flores Extinct (Middle Pleistocene)
Sumba dwarf stegodont[29] Stegodon sumbaensis Sumba, Indonesia Extinct (Middle Pleistocene)
Timor dwarf stegodont[24] Stegodon timorensis Timor Extinct
Dwarf stegolophodont[30] Stegolophodon pseudolatidens Japan Extinct (Miocene)  
Stegolophodon

PrimatesEdit

Example Binomial name Native range Status Continental relative
Nosy Hara dwarf lemur[31] Cheirogaleus sp. Nosy Hara island off Madagascar Unknown  
Dwarf lemurs
 
Flores Man[32]
Homo floresiensis Flores Extinct (Late Pleistocene)  
Homo erectus
 
Callao Man
Homo luzonensis[33][34] Luzon, Philippines Extinct (Late Pleistocene)
Modern pygmies of Flores[35] Homo sapiens Flores Extant  
Homo sapiens
Early Palau modern humans (disputed)[36] Homo sapiens Palau Extinct (?)
 
Andamanese[37][38]
Homo sapiens Andaman Islands Extant
 
Sardinian macaque[39]
Macaca majori Sardinia Extinct (Pleistocene)  
Barbary macaque
 
Zanzibar red colobus
Piliocolobus kirkii Unguja Endangered  
Udzungwa red colobus

CarnivoransEdit

Example Binomial name Native range Status Continental relative Insular / mainland
length or mass ratio
 
Japanese wolf
Canis lupus hodophilax Japan (excluding Hokkaido) Extinct (AD 1905)  
Gray wolf
 
Sardinian dhole
(forward)
Cynotherium sardous Corsica and Sardinia Extinct (c. 8300 BC)  
Xenocyon
Trinil dog Mececyon trinilensis Java Extinct (Pleistocene)
Cozumel Island coati[13] 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 [e]
LR ≈ 0.75 [f]
Cozumel fox Urocyon sp. Cozumel Critically endangered or Extinct

Non-ruminant ungulatesEdit

Example Binomial name Native range Status Continental relative
 
Malagasy dwarf hippopotamuses
Hippopotamus laloumena

H. lemerlei

H. madagascariensis
Madagascar Extinct (c. AD 1000)  
Common hippopotamus
Bumiayu dwarf hippopotamus[24] 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)
 
Cyprus dwarf hippopotamus
Hippopotamus minor Cyprus Extinct (c. 8000 BC)
 
Sicilian dwarf hippopotamus
Hippopotamus pentlandi Sicily Extinct (Pleistocene)
Cozumel collared peccary[13] Pecari tajacu nanus Cozumel Unknown  
Collared peccary
Philippine rhinoceros[42] Rhinoceros philippinensis Luzon Extinct (Middle Pleistocene)  
Javan rhinoceros

BovidsEdit

Example Binomial name Native range Status Continental relative
Sicilian bison[25] Bison priscus siciliae Sicily Extinct (Late Pleistocene)  
Steppe bison
Sicilian aurochs[43] Bos primigenius siciliae[25] 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
Nesogoral[44] Nesogoral spp. Sardinia Extinct
Dahlak Kebir gazelle[45] Nanger soemmerringi ssp. Dahlak Kebir island, Eritrea Vulnerable  
Soemmerring's gazelle

Cervids and relativesEdit

Example Binomial name Native range Status Continental relative
 
Cretan dwarf megacerines[g]
Candiacervus spp. Crete Extinct (Pleistocene)  
Praemegaceros verticornis[9]
 
Sardinian megacerine[9]
(second from left)
Praemegaceros cazioti Sardinia Extinct (c. 5500 BC)
Ryukyu dwarf deer[48] Cervus astylodon Ryukyu Islands Extinct  
Sika deer (?)

Cervus praenipponicus (?)
Jersey red deer population[49] Cervus elaphus jerseyensis Jersey Extinct (Pleistocene)  
Red deer
 
Corsican red deer
Cervus elaphus corsicanus Corsica and Sardinia Near Threatened
Pleistocene Sicilian deer[25] Cervus siciliae Sicily Extinct (Late Pleistocene)
 
Hoplitomeryx[h]
Hoplitomeryx spp. Gargano Island Extinct (Early Pliocene)  
Pecorans
Sicilian megacerine[25] Megaloceros carburangelensis Sicily Extinct (Late Pleistocene)  
Irish elk
 
Florida Key deer
Odocoileus virginianus clavium Florida Keys Endangered  
Virginia deer
 
Svalbard reindeer
Rangifer tarandus platyrhynchus Svalbard Unknown  
Reindeer
 
Philippine deer
Rusa marianna Philippines Vulnerable  
Sambar deer

PlantsEdit

Possible example Binomial name Native range Status Continental relative
 
Insular elephant cacti[50][51]
Pachycereus pringlei Remote islands in the Sea of Cortez
(e.g. Santa Cruz, San Pedro Mártir)
Not evaluated  
Mainland elephant cacti

See alsoEdit

NotesEdit

  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]
  2. ^ Based on the heights in Fig. 1 of Heupink et al., 2011[15]
  3. ^ Based on maximum lengths of 2.5 m for the dwarf form[17] and 5.74 m for the mainland form[18]
  4. ^ Based on maximum Tanahjampea python total lengths (TL) of 2.10 m for males and 3.35 m for females[19] and maximum southern Sumatra python snout to vent lengths (SVL) of 4.5 m for males and 6.1 m for females[20] with SVLs corrected to TLs by multiplying by a factor of 1.127, derived from the average relative tail length (0.113) of African and Indian rock pythons[21]
  5. ^ For nearby mainland gray foxes[40]
  6. ^ For mainland gray foxes in general[41]
  7. ^ Like Hoplitomeryx, Candiacervus appears to be an unusual case in that members of this genus evolved into insular species of a wide range of sizes, not only dwarf forms but also some that might be considered giants.[46][47]
  8. ^ Hoplitomeryx is evidently quite an unusual case, because members of this genus apparently evolved into both dwarf and giant insular forms on the same island(s).[46]

ReferencesEdit

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