Insular gigantism and dwarfism Explained.

Insular gigantism or island gigantism is an unusual, but effective phenomenon in which the size of an animal will drastically increase in comparison to its mainland relatives. The opposite occurs with insular dwarfism, where members of a species get smaller depending on the resources available. This form of evolutionary biology, known as Foster’s rule (the island rule), was first stated in the journal Nature in 1964 by J. Bristol Foster.

Why become giants?

Being large in size does have its benefits. Hunting down larger predators becomes easier, allowing more resources to be collected. The larger the animal, the more water and energy it can store, allowing it to survive harsher climates.

Why shrink in size?

Well becoming smaller also has its perks. Larger animals need less food and water if they are physically smaller. They also need fewer resources to reproduce and therefore have a higher reproductive output. Being small allows animals to find better shelter and escape harsh environment conditions. Sizes can be reduced in response to the lack in competition with other large herbivores or carnivores on the island.


How accurate is Fosters rule or island rule?

Well the island rule suggests that small mammals evolve into larger sizes whilst larger animals dwarf. Reasoning for this biological evolution was mainly due to resources and competition. Ted J. Case wrote a paper, ‘A General Explanation for Insular Body Size Trends in Terrestrial Vertebrates’, outlining how the fifty year old rule was right in some aspects, but fell short in others. Whilst investigating Case studied a giant chuckwalla. He noted that the lizard would be relatively larger on one island, but not at others.

Another study by Marquet & Taper (1998), hypothesised that medium-sized species were able to sustain higher population densities. If animals changed their body sizes to reduce energy requirements, they could populate a smaller space more easily.  This in turn would be an evolutionary benefit, reducing the chances of extinction.

There are many theories as to why animals change their body sizes in response to the environmental change, however the community do agree that the physical changes are in response to food and energy limitations.

An example of insular gigantism and dwarfism

Flores giant rat (Papagomys armandvillei) (Gigantism)

Flores giant rat (Papagomys armandvillei) (Gigantism)

The Flores giant rat is a rodent of the family Muridae that occurs on the Indonesian island of Flores.  The FGR is nearly twice the size of the average brown rat, having a head and body length of 41-44cm and a tail length of 35-72cm. Over the years there has been a decline in the species due hunting and predation by dogs and cats.


Stegodon (roofed-tooth) (dwarfism)

Stegodon are an extinct species belonging to the Elephantidae family. They were present from 11.6mya to the present Pleistocene and fossils have been found throughout Asia, Africa and America. The mainland Stegodon was a large animal, reaching heights of 3.87m and weights of 12.7 tonnes. However, up until about 12,000 years ago, a dwarf population existed on the island of Flores, Indonesia. The smaller dwarf species on the island of Flores was the S. sondaari, which when an adult weighed around 300kg.  This small dwarf species lived on the island of Flores up until the Mid-Pleistocene when a new they were replaced by new migrating species called Stegodon florensis.


This article was first published on Flavible.

Saad Bhatty

Blogger, journalist, geologist and Tech-enthusiast. There is always something to write about!

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1 Response

  1. January 6, 2016

    […] Insular gigantism and dwarfism Explained. […]

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