Island Gigantism - Very Large Animals

Island gigantism or insular gigantism is a biological phenomenon in which the size of an animal isolated on an island increases dramatically in comparison to its mainland relatives. Island gigantism is one aspect of the more general "island rule", which posits that when mainland animals colonize islands, small species tend to evolve larger bodies, and large species tend to evolve smaller bodies. With the arrival of humans and associated predators (dogs, cats, rats, pigs), many giant as well as other island endemics have become extinct.

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 takahÄ" 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

See also

• Deep-sea gigantism
• Foster's rule
• Insular dwarfism
• Island tameness
• Megafauna
• Pleistocene extinctions

Notes

References

External links

• PBS NOVA: Why Do Islands Breed Giants (And Sometimes Dwarfs)?