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[lower-alpha 1] 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 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 vs. 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.[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]

Sauropods

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

Other

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).

Birds

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 [lower-alpha 2]
Cozumel thrasher[13]	Toxostoma gluttatum	Cozumel	Critically endangered	Other thrashers

Squamates

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 [lower-alpha 3]
Tanahjampea reticulated python[19]	Python reticulatus jampeanus	Tanahjampea, between Sulawesi and Flores	Unknown	Reticulated python	LR ≈ 0.41, males LR ≈ 0.49, females [lower-alpha 4]

Mammals

Pilosans

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)

Proboscideans

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

Primates

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

Carnivorans

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 [lower-alpha 5] LR ≈ 0.75 [lower-alpha 6]
Cozumel fox	Urocyon sp.	Cozumel	Critically endangered or Extinct

Non-ruminant ungulates

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

Bovids

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 relatives

Example	Binomial name	Native range	Status	Continental relative
Cretan dwarf megacerines[lower-alpha 7]	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[lower-alpha 8]	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

Plants

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 also

• Island gigantism
• Foster's rule
• Dwarfing
• Franz Nopcsa
• Island tameness
• Pleistocene extinctions

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]
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]

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External links

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