Australian species are older, study says

By Emma Young 21 July 2010
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Australian reptiles and amphibians are older than their northern counterparts and may be more vulnerable to extinction.

AUSTRALIA’S REPTILES AND AMPHIBIANS are much older than their Northern Hemisphere cousins largely due to past climate change, new research has revealed. Their age may make make them more vulnerable to extinction than more youthful species, say experts.

Researchers Rick Shine and Sylvain Dubey from the University of Sydney found that, on average, species from the Southern Hemisphere are 4.6 million years old, compared to 2.9 million years for the Northern Hemisphere.

The age difference is likely on account of the history of extremely cold weather in the Northern Hemisphere, say the researchers.

“Climatic conditions during the height of the last [ice age] were very severe in the temperate regions of the Northern Hemisphere, and most reptiles and amphibians probably were wiped out over the heavily glaciated areas,” says Rick.


This means the populations of reptiles and amphibians in the Northern Hemisphere today are mostly the descendants of relatively recent species that moved into these regions once the ice was gone.

Because Southern Hemisphere countries, such as Australia, haven’t experienced such extremes in climate, many species have managed to survive in their current ranges.

Work by other scientists suggests that – for reasons which aren’t clear – the older a species is, the more vulnerable it is to human activity. The age of a species seems to predict how resilient it is to changes to its habitat, says Rick. “Perhaps species lose some of their evolutionary flexibility if they…stay in the same place for a very long time.”

Other groups of species may also display similar trends between the hemispheres, says Rick, but some would be less sensitive to glacial extremes than reptiles and amphibians, which are cold-blooded.

Old and unique

Jane Melville, senior curator of terrestrial vertebrates at Museum Victoria in Melbourne, says, “This is an interesting concept, which has been gaining supporting evidence as more genetic studies are completed. If it is shown to be a general trend across vertebrates, it will be of considerable importance to Australia, which has some old and unique lineages of animals.”  

But this new work is very useful for understanding the origins of reptiles and amphibians, she says. “We really know very little about the majority of our unique species,” she adds, “and this study highlights the difference and significance of species in the southern hemisphere.”

To make the prediction, Rick and Sylvain looked at DNA sequences to work out the family trees of 157 modern species of reptile and amphibian from both hemispheres.  They then used these trees to work out roughly when each of these species evolved. The estimated age of each species varied hugely, from about 200,000 years for one Northern Hemisphere frog to 10 million years for a Southern Hemisphere gecko. 

The research is published this week in the journal Biology Letters.