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Island gigantism

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Title: Island gigantism  
Author: World Heritage Encyclopedia
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Subject: Giant tortoise, Haast's eagle, Megafauna, Sphaerotheriida, Thaumatogryllus
Collection: Animal Size, Evolutionary Biology, Insular Ecology
Publisher: World Heritage Encyclopedia

Island gigantism

The large Haast's eagle and moa from New Zealand (both extinct)
The extinct Tenerife giant rat

Island gigantism or insular gigantism is a biological phenomenon in which the size of animals isolated on an island increases dramatically in comparison to their 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 1
  • Examples 2
    • Mammals 2.1
    • Birds 2.2
    • Reptiles 2.3
    • Arthropods 2.4
    • Flora 2.5
  • See also 3
  • Notes 4
  • References 5
  • External links 6

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 or reptiles, 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.[1][1] Small herbivores may also benefit from the absence of competition from missing types of large herbivores.

Diagram displaying the change in size of weta species in two ecosystems. The size and population of wetas are affected by predation. Rats introduced on the mainland began to prey on wetas, reducing their population; wetas shrank in response. On an island isolated from predation, such as Little Barrier Island, wetas have a dense population and have grown to a massive size. Insular species of giant wetas are the only ones not facing extinction. As wetas grow over time, bird predation declines.

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.[4]

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.[5] 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.[6] In contrast, insular dwarfism among predators more commonly results from the imposition of constraints associated with the limited prey resources available on islands.[6] 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.[7]

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.[8]

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.[9]


Examples of island gigantism include:


Archaeoindris, an extinct gorilla-sized sloth lemur of Madagascar.

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


The kakapo of New Zealand is the heaviest, and only flightless, parrot.
The dodo, an extinct flightless pigeon of Mauritius.
The largest tortoises in the world are found on the Seychelles and Galápagos Islands (example of the latter shown).


  • Turtles
    • Giant tortoises in the Galápagos Islands, Seychelles, and formerly the Mascarenes 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[12] and South America,[13] as well as on a number of other, more accessible islands.[12] In the late Pliocene they were also present in Africa.[14] 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.
The Komodo dragon of Flores and nearby islands is the largest living lizard.



Island plants often also exhibit "insular woodiness".[22]

See also


  1. ^ The reduction in predation on islands often also leads to tamer behavior of island prey species, a trend that has been analyzed in lizards.[2][3]


  1. ^ Herczeg, G. B.; Gonda, A. L.; Merilä, J. (2009-07-16). "Evolution of Gigantism in Nine-Spined Sticklebacks".  
  2. ^ Cooper, W. E.; Pyron, R. A.; Garland, T. (2014-01-08). "Island tameness: Living on islands reduces flight initiation distance". Proceedings of the Royal Society B: Biological Sciences 281 (1777): 20133019.  
  3. ^ Yong, E. (2014-01-08). "Islands make animals tamer". Nature.  
  4. ^ Jaffe, A. L.; Slater, G. J.; Alfaro, M. E. (2011-01-26). "The evolution of island gigantism and body size variation in tortoises and turtles". Biology Letters 7 (4): 558–561.  
  5. ^ Barahona, F.; Evans, S.E.; Mateo, J.A.; Garcia-Marquez, M.; Lopez-Jurado, L.F. (March 2000). on the Canary Islands"Gallotia"Endemism, Gigantism and Extinction in Island Lizards: The Genus . Journal of Zoology 250 (3): 373–388.  
  6. ^ a b Raia, P.; Meiri, S. (August 2006). "The island rule in large mammals: paleontology meets ecology".  
  7. ^ Keehn, J. E.; Nieto, N. C.; Tracy, C. R.; Gienger, C. M.; Feldman, C. R. (2013-08-27). "Evolution on a desert island: Body size divergence between the reptiles of Nevada's Anaho Island and the mainland around Pyramid Lake". Journal of Zoology 291 (4): 269–278.  
  8. ^ Lomolino, M. V. (2005-09-05). "Body size evolution in insular vertebrates: generality of the island rule".  
  9. ^ Filin, I.; Ziv, Y. (2004). "New Theory of Insular Evolution: Unifying the Loss of Dispersability and Body-mass Change." (PDF). Evolutionary Ecology Research 6: 115–124. 
  10. ^ a b  
  11. ^ Meijer H. J. M.; Due, R. A. (2010-11-04). "A new species of giant marabou stork (Aves: Ciconiiformes) from the Pleistocene of Liang Bua, Flores (Indonesia)".  
  12. ^ a b Hansen, D. M.; Donlan, C. J.; Griffiths, C. J.; Campbell, K. J. (April 2010). "Ecological history and latent conservation potential: large and giant tortoises as a model for taxon substitutions" (PDF).  
  13. ^ Cione, A. L.; Tonni, E. P.; Soibelzon, L. (2003). "The Broken Zig-Zag: Late Cenozoic large mammal and tortoise extinction in South America" (PDF). Rev. Mus. Argentino Cienc. Nat., n.s. 5 (1): 1–19.  
  14. ^ Harrison, T. (2011). "Tortoises (Chelonii, Testudinidae)". Paleontology and Geology of Laetoli: Human Evolution in Context, Vol. 2: Fossil Hominins and the Associated Fauna. Vertebrate Paleobiology and Paleoanthropology.  
  15. ^ Hocknull, S. A.; Piper, P. J.; van den Bergh, G. D.; Due, R. A.; Morwood, M. J.; Kurniawan, I. (2009-09-30). Turvey, Samuel T, ed. "Dragon's Paradise Lost: Palaeobiogeography, Evolution and Extinction of the Largest-Ever Terrestrial Lizards (Varanidae)".  
  16. ^ Pregill, G. K.; Worthy, T. H. (March 2003). "A New Iguanid Lizard (Squamata, Iguanidae) from the Lare Quaternary of Fiji, Southwest Pacific". Herpetologica (The Herpetologists' League) 59 (1): 57–67.  
  17. ^ Maca-Meyer, N.; Carranza, S.; Rando, J. C.; Arnold, E. N.; Cabrera, V. M. (2003-12-01). (Reptilia: Lacertidae), assessed using ancient mtDNA from its mummified remains"Gallotia goliath"Status and relationships of the extinct giant Canary Island lizard (PDF). Biological Journal of the Linnean Society ( 
  18. ^ a b Keogh, J. S.; Scott, I. A. W.; Hayes, C. (January 2005). "Rapid and repeated origin of insular gigantism and dwarfism in Australian tiger snakes".  
  19. ^ Whitman, D. W.; Vincent, S. (December 2008). "Large size as an antipredator defense in an insect". Journal of Orthoptera Research 17 (2): 353–371.  
  20. ^ Kleinhenz, P. (2011-06-14). "The Endangered Unknown: Lord Howe Island Stick Insect". blog post. Ecomii. Archived from the original on 2011-06-20. Retrieved 2015-08-01. 
  21. ^ Keppel, Gunnar; Lowe, Andrew J.; Possingham, Hugh P. (2009). "Changing perspectives on the biogeography of the tropical South Pacific: influences of dispersal, vicariance and extinction". Journal of Biogeography 36 (6): 1035–1054.  
  22. ^ Bowen, Lizabeth; Vuren, Dirk Van (1997). "Insular Endemic Plants Lack Defenses Against Herbivores". Conservation Biology 11 (5): 1249–1254.  

External links

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