Size doesn't matter: island mammals 'dwarfed' by lack of competitors and predators

Sep 25, 2006
The world's smallest elephant, Elephas falconeri, from the middle-Pleistocene of Sicily
The world´s smallest elephant, Elephas falconeri, from the middle-Pleistocene of Sicily.

The evolution of miniature or 'dwarf' versions of animals like elephants and hippos on islands is caused by lack of competition for food and the absence of predators, and not just because they are too large for their habitats, new research claims today.

The study, published in this month's issue of Evolution journal, examines the phenomenon of the 'Island Rule' which states that large mammals on islands evolve to be significantly smaller than their counterparts living on large continental land masses.

The researchers examined the fossilised remains of small elephants and other miniaturised mammals living on various sized Mediterranean islands in the Pleistocene and Holocene epochs, between 1.8 million and 4,500 years ago. Their findings suggest that this evolutionary tendency is caused by the specific way that predators, prey, and competitors for food interact in island ecosystems. They also found evidence that miniaturisation occurs for distinctly different reasons in carnivores and herbivores, and that herbivores are more dramatically affected by the phenomenon.

The team's findings show that for miniature herbivores such as hippos to evolve, there needs to be little competition for food on the island, and very few predators. In these circumstances herbivores tend towards miniaturisation as they do not need their size advantage to survive. The researchers think this could mean that over time there will be an increase in the number of offspring each female has in a lifetime, as smaller mammals tend to have more babies. In this way, size evolution on islands can occur very quickly as the island-dwelling herbivores are selected to expend more energy on reproduction at the expense of growth.

In the case of carnivores, however, the research team discovered that miniaturisation occurs less dramatically, and believe it is primarily caused by the size and abundance of their prey. If prey is scarce or small, then carnivores too will evolve to be smaller, as they will not need such a size advantage to hunt and compete for food.

Dr Shai Meiri from Imperial College London's Centre for Population Biology who carried out the research said: "Our study has shown that large mammals do not simply 'shrink' in response to the small size of their island homes. By comparing fossils of elephants from a number of islands of various sizes, we saw clearly that species' miniaturisation did not occur relative to the size of the island they inhabited. The complex interaction between mammals, their food and their competitors drives the evolutionary process, allowing mammals, particularly herbivores, to minimise their size whilst maximising their reproductive effort.

"Now that we have a clearer idea about what drives size evolution of large mammals on islands, we hope to investigate whether similar patterns of size and shape change exist among mainland mammals, and what effect climate change has on size evolution."

The research was carried out jointly by Dr Shai Meiri from Imperial College London's Centre for Population Biology and Dr Pasquale Raia, from the Università degli Studi Del Molise in Italy.

Source: Imperial College London

Explore further: Stanford researchers rethink 'natural' habitat for wildlife

add to favorites email to friend print save as pdf

Related Stories

The Isthmus of Panama: Out of the Deep Earth

Apr 01, 2014

As dates in geologic history go, the formation of the slender land bridge that joins South America and North America is a red-letter one. More than once over the past 100 million years, the two great landmasses ...

Exxon Valdez Runs Aground in 1989

Mar 24, 2014

Early on March 24, 1989, Dean Fosdick, the Alaska bureau chief of The Associated Press, was awakened around 5:30 a.m. by a phone call. The caller had a tip that a tanker had run aground outside Valdez.

Boaters need to be mindful of dolphins

Jan 31, 2014

Boaties heading out into the Hauraki Gulf over the summer need to take greater care in sharing the water, Massey researchers say, as they uncover the impact of collisions between vessels and marine mammals ...

Reconstructing the New World monkey family tree

Jan 03, 2014

When monkeys landed in South America 37 or more million years ago, the long-isolated continent already teemed with a menagerie of 30-foot snakes, giant armadillos and strange, hoofed mammals. Over time, the ...

Recommended for you

Plants with dormant seeds give rise to more species

Apr 18, 2014

Seeds that sprout as soon as they're planted may be good news for a garden. But wild plants need to be more careful. In the wild, a plant whose seeds sprouted at the first warm spell or rainy day would risk disaster. More ...

Researchers successfully clone adult human stem cells

Apr 18, 2014

(Phys.org) —An international team of researchers, led by Robert Lanza, of Advanced Cell Technology, has announced that they have performed the first successful cloning of adult human skin cells into stem ...

User comments : 0

More news stories

Researchers successfully clone adult human stem cells

(Phys.org) —An international team of researchers, led by Robert Lanza, of Advanced Cell Technology, has announced that they have performed the first successful cloning of adult human skin cells into stem ...

NASA's space station Robonaut finally getting legs

Robonaut, the first out-of-this-world humanoid, is finally getting its space legs. For three years, Robonaut has had to manage from the waist up. This new pair of legs means the experimental robot—now stuck ...

Ex-Apple chief plans mobile phone for India

Former Apple chief executive John Sculley, whose marketing skills helped bring the personal computer to desktops worldwide, says he plans to launch a mobile phone in India to exploit its still largely untapped ...

Filipino tests negative for Middle East virus

A Filipino nurse who tested positive for the Middle East virus has been found free of infection in a subsequent examination after he returned home, Philippine health officials said Saturday.

Egypt archaeologists find ancient writer's tomb

Egypt's minister of antiquities says a team of Spanish archaeologists has discovered two tombs in the southern part of the country, one of them belonging to a writer and containing a trove of artifacts including reed pens ...