From: Patricia Reaney, Reuters
Published September 26, 2006 12:00 AM

Island Mammal Dwarfism Not Just a Matter of Size

LONDON — Dwarf animals that lived on islands thousands of years ago evolved to their tiny size due to a lack of predators and competitors, not just because they were too big for their habitats, researchers said on Monday.


Remains of smaller versions of bigger animals such as elephants, hippopotamuses and deer that lived on large land masses have been found on islands -- a phenomenon known as 'Island Rule'.


Their diminutive size has been attributed to the limited space in which they lived, but scientists at Imperial College London believe the explanation is more complex.


"Our study has shown that large mammals do not simply 'shrink' in response to the small size of their island homes," said Dr Shai Meiri, a member of the research team.


The scientists at the Centre for Population Biology studied the fossilised remains of miniature animals that lived on Mediterranean islands between 1.8 million and 4,500 years ago.


By comparing the remains of various animals they discovered that the creatures' size was due to a combination of factors including competitors and food supplies.


Grass eaters like hippos were more affected by dwarfism than meat eaters.


"Carnivores and herbivores don't respond to the same evolutionary pressures as far as their body sizes are concerned," Meiri added in an interview.


"Carnivores are affected by food availability and prey size, whereas herbivores are affected by the presence of other herbivores and also of predators."


When there is little competition for food and very few predators, herbivores tend towards miniaturisation because they don't need their size to survive, according to the scientists.


Smaller animals also tend to have more offspring so their tinier size could increase the number of babies herbivores produce.


Miniaturisation is less dramatic for carnivores and depends mainly on the size and abundance of their prey. If the prey is scarce or small, carnivores will evolve to be smaller.


The accepted view has been that the larger the animal the further it will shrink, and the smaller the area occupied, the tinier the animal will evolve.


But the scientists, who reported their findings in the latest edition of the journal Evolution, think that is only part of the story.


"It is much more complex than large creatures get smaller and small creatures don't," said Meiri.


Source: Reuters


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