Island gigantism

Possible causes

Large mammalian carnivores are often absent on islands because of insufficient range or difficulties in over-water dispersal. In their absence, the ecological niches for large predators may be occupied by birds, reptiles or smaller carnivorans, which can then grow to larger-than-normal size. For example, on prehistoric Gargano Island in the Miocene-Pliocene Mediterranean, on islands in the Caribbean like Cuba, and on Madagascar and New Zealand, some or all apex predators were birds like eagles, falcons and owls, including some of the largest known examples of these groups. However, birds and reptiles generally make less efficient large predators than advanced carnivorans.

Since small size usually makes it easier for herbivores to escape or hide from predators, the decreased predation pressure on islands can allow them to grow larger. Small herbivores may also benefit from the absence of competition from missing types of large herbivores.

Benefits of large size that have been suggested for island tortoises include decreased vulnerability to scarcity of food and/or water, through ability to survive for longer intervals without them, or ability to travel longer distances to obtain them. Periods of such scarcity may be a greater threat on oceanic islands than on the mainland.

Thus, island gigantism is usually an evolutionary trend resulting from the removal of constraints on the size of small animals related to predation and/or competition. Such constraints can operate differently depending on the size of the animal, however; for example, while small herbivores may escape predation by hiding, large herbivores may deter predators by intimidation. As a result, the complementary phenomenon of island dwarfism can also result from the removal of constraints related to predation and/or competition on the size of large herbivores. In contrast, insular dwarfism among predators more commonly results from the imposition of constraints associated with the limited prey resources available on islands. As opposed to island dwarfism, island gigantism is found in most major vertebrate groups and in invertebrates.

Territorialism may favor the evolution of island gigantism. A study on Anaho Island in Nevada determined that reptile species that were territorial tended to be larger on the island compared to the mainland, particularly in the smaller species. In territorial species, larger size makes individuals better able to compete to defend their territory. This gives additional impetus to evolution toward larger size in an insular population.

A further means of establishing island gigantism may be a founder effect operative when larger members of a mainland population are superior in their ability to colonize islands.

Island size plays a role in determining the extent of gigantism. Smaller islands generally accelerate the rate of evolution of changes in organism size, and organisms there evolve greater extremes in size.

Examples

Examples of island gigantism include:

Mammals

Many rodents grow larger on islands, whereas carnivores, proboscideans and artiodactyls usually become smaller.

Soricomorphs

Mediterranean giant shrews - Asoriculus (extinct)

Corsican giant shrew - Asoriculus corsicanus, Corsica (extinct)

Sardinian shrew - Asoriculus similis, Sardinia (extinct)

Balearic giant shrew - Asoriculus hidalgo, Majorca and Minorca (extinct)

Solenodon

Giant solenodon - Solenodon arredondoi, Cuba (extinct)

Cuban solenodon - Solenodon cubanus, Cuba

Hispaniolan solenodon - Solenodon paradoxus, Hispaniola

Erinaceomorphs

Deinogalerix - Gargano Island (extinct)

Rodents

Flores giant rat - Papagomys armandvillei, Flores

Admiralty giant rat - Rattus detentus, Manus

Sulawesi giant rat - Paruromys dominator, Sulawesi

Canary Islands giant rats - Canariomys (extinct)

Gran Canaria giant rat - Canariomys tamarani; Gran Canaria, Canary Islands (extinct)

Tenerife giant rat - Canariomys bravoi; Tenerife, Canary Islands (extinct)

Giant hutias of the West Indies (extinct)

Balearic giant dormice - Hypnomys (extinct)

Majorcan giant dormouse - Hypnomys morpheus, Majorca (extinct)

Minorcan giant dormouse - Hypnomys mahonensis, Minorca (extinct)

Sicilian giant dormice - Leithia (extinct)

Sicilian giant dormouse - Leithia cartei, Sicily (extinct)

Maltese giant dormouse - Leithia melitensis, Malta (extinct)

Formentera black-tailed garden dormouse - Eliomys quercinus ophiusae, Formentera, Balearic islands (descendant of garden dormice introduced from the continent by Neolithic humans)

St Kilda field mouse - Apodemus sylvaticus hirtensis, St Kilda (descendant of wood mice introduced by humans)

Lagomorphs

Minorcan giant lagomorph - Nuralagus rex (extinct)

Different Prolagus Mediterranean species, including the extinct Sardinian pika (P. sardus) and P. imperialis from Gargano

Primates

Giant lemurs Archaeoindris, Palaeopropithecus and Megaladapis of Madagascar (all extinct)

Carnivorans

Sardinian giant otter - Megalenhydris barbaricina (extinct)

Madagascar's fossa, Cryptoprocta ferox and the extinct giant fossa Cryptoprocta spelaea, carnivorans related to herpestids (and convergent in appearance with cougars)

Chapalmalania, an extinct bear-sized procyonid that lived in South America before the formation of the isthmus of Panama, whose smaller ancestors dispersed there from Central America

Birds

Stem Birds

Gargantuavis, a flightless ostrich-sized bird from Late Cretaceous southern France (which was an island at the time). Largest known bird from the Mesozoic.

Balaur bondoc from Late Cretaceous Hateg Island (now in Romania). Initially described as a Velociraptor-sized dromaeosaurid, Balaur was later reclassified as a secondarily flightless primitive bird, closer to extant birds than Jeholornis. Its skeleton shows fused bones and increased robustness analogous to those of insular vegetarian mammals.

Ratites

The extinct elephant birds, among the largest birds ever, formerly living on Madagascar

The extinct moa of New Zealand

Waterfowl

Cygnus falconeri of Sicily and Malta, an extinct giant swan

Garganornis ballmanni of Gargano and Scontrone islands, an extinct flightless giant goose

Moa-nalo, extinct flightless giant ducks from Hawaii

Wildfowl

Sylviornis neocaledoniae a huge extinct flightless megapode-like bird from New Caledonia

Some extinct Polynesian megapodes

Rails

The flightless takahē from New Zealand and other Porphyrio as well as many Gallirallus species from Melanesia and Polynesia and a few other rallids

Seabirds

The extinct spectacled cormorant from Bering Island

Pigeons

Dodo and Rodrigues solitaire, both flightless and extinct, from the Mascarenes

The extinct flightless Viti Levu giant pigeon

Birds of prey

Haast's eagle and Eyles' harrier of New Zealand; the titan-hawk Titanohierax and giant-hawk Gigantohierax from the Caribbean; the giant buteonine hawk Buteogallus borrasi of Cuba (all now extinct)

Parrots

The extinct broad-billed parrot from Mauritius, an undescribed huge extinct parrot from Easter Island, and the critically endangered flightless kakapo of New Zealand

Owls

The Cuban giant flightless owl Ornimegalonyx, the possibly flightless Cretan owl, and several Tyto barn owls from the Mediterranean (Tyto robusta, Tyto gigantea), Caribbean (flightless Tyto pollens) and Melanesia (all of them extinct)

Storks

The Flores flightless stork, Leptoptilos robustus, a very large extinct stork from Flores

Reptiles

Pterosaurs

Hatzegopteryx from Hateg Island, an unusually large and robust azhdarchid pterosaur believed to be the island's top predator.

Turtles

Giant tortoises in the Galápagos Islands, Seychelles, and formerly the Mascarenes and Canary Islands are often considered examples of island gigantism. However, during the Pleistocene, comparably sized or larger tortoises were present in Australia (Meiolania), southern Asia (Colossochelys atlas), Madagascar (Dipsochelys) and North and South America, as well as on a number of other, more accessible islands. In the late Pliocene they were also present in Africa. The present situation of large tortoises being only found on remote islands may reflect that these islands were discovered by humans fairly recently and have not been heavily populated, making their tortoises less subject to overexploitation.

Lizards

The Komodo dragon and a similar (extinct) giant monitor lizard from Timor have been regarded as examples of giant insular carnivores. Since islands tend to offer limited food and territory, their mammalian carnivores (if present) are usually smaller than continental ones. These cases involve ectothermic carnivores on islands too small to support much mammalian competition. However, these lizards are not as large as their extinct Australian relative Megalania, and it has been proposed based on fossil evidence that the ancestors of these varanids first evolved their large size in Australia and then dispersed to Indonesia. If this is true, rather than being insular giants they would be viewed as examples of phyletic gigantism. Nevertheless, given that Australia is sometimes viewed as the world's largest island, the former view may still be valid.

The Angel Island chuckwalla (Sauromalus hispidus) and the San Esteban chuckwalla (Sauromalus varius) of islands off Baja California

The extinct iguanas Lapitiguana (1.5 m long) from Fiji and Brachylophus gibbonsi (1.2 m) from Tonga

Leiolopisma mauritiana and Macroscincus coctei, two extinct skinks from Mauritius and Cape Verde, the Solomon Islands skink and the rare New Caledonian skink Phoboscincus bocourti

The extinct Rodrigues giant day gecko and New Zealand giant gecko, and the extant New Caledonian giant gecko

Four extant and one extinct species of lacertid lizard of the genus Gallotia in the Canary Islands

Snakes

Tiger snake populations on Mount Chappell Island (Tasmania) and Williams Island, Hopkins Island and islands of the Nuyts Archipelago (South Australia). On these islands the available prey is restricted to larger sizes than commonly taken by mainland snakes; restricted seasonal availability of prey also appears to contribute to gigantism.

Arthropods

Conant's giant Nihoa tree cricket

Garypus titanius, a pseudoscorpion of Boatswain Bird Island (off Ascension Island)

Giant pill-millipedes of Madagascar

Giant wetas of New Zealand

Lord Howe Island stick insect

Saint Helena earwig (extinct)

Taveuni and giant Fijian long-horned beetles of Fiji

Flora

Island plants often also exhibit "insular woodiness".

Megaherbs of the New Zealand Subantarctic Islands