Wobbles or variations in the Earth's orbit and tilt are associated with extinctions of rodent and mammalian species, Dutch scientists said on Wednesday in a study published in the journal Nature. 

Dr. Jan van Dam from the Utrecht University and colleagues reached this conclusion after studying the fossil record of rodents from central Spain over a 22-million-year span, showing a link between rodent extinction events and the climate record.

"Extinctions in rodent species occur in pulses which are spaced by intervals controlled by astronomical variations and their effects on climate change," said van Dam.

They found two cycles corresponding to the disappearance of rodent species. One lasts 2.4 million years and is linked to variations in the Earth's orbit. The other is a 1.2 million year cycle relating to shifts in the tilt on the Earth on its axis.

The cycles are associated with lower temperatures, changes in precipitation, habitats, vegetation and food availability which are the main factors influencing the extinction peaks, they said in the study.

"We looked at rodents because small mammals have a higher density of fossils than large mammals, and we concentrated on Spain, because the sediment series are very complete, contain many mammal fossils, and because much work has been done already," said van Dam, "Rodents are very sensitive to seasonal changes because they have such a short lifespan."

At the moment, the Earth is at the beginning of a cycle but the planet's climate system has changed so much in the past 3 million years that it is difficult to predict what will happen in the future, scientists believe. 

"The environment is responsible to what happens to species," said van Dam, "Biological factors are secondary, according to our results."

(Xinhua News Agency October 12, 2006)