Scientists have named a prime suspect in the mysterious case of the missing mammals. The reason history is littered with suspiciously regular extinctions is all down to the Earth's wobbly orbit, according to research published today.

Apart from mass extinctions caused by asteroids thumping into the planet and other cataclysmic natural events, records reveal that mammal species die out anyway, usually 2.5m years after they first emerge. Palaeontologists have struggled to explain it, with some blaming competition with neighbouring creatures and others suspecting dramatic swings in climate.

After examining the remains of 80,000 fossilised teeth from 132 different rodents that span a 22 million-year period, Jan van Dam at Utrecht University in the Netherlands believes he has the answer.

He conducted detailed analyses of the fossil fragments, which were excavated from four pristine sites throughout Spain, to work out when each species emerged and when they became extinct. The study, which focused on rodents because they are easily identified from their dental remains, included fossils of squirrels, mice, beavers, voles and hamster-like rodents, dating to between 24.5m and 2.5m years ago and only a few of which survive today.

Dr van Dam correlated the results with natural cycles in the Earth's orbit and found that every 2.4m years there was a flurry of both mammal extinctions and new species. The extinctions coincided with a cycle that sees the Earth's orbit vary from almost perfectly circular to elliptical. Another cycle of extinctions and emerging species overlapped with changes in the tilt of the Earth's axis, which occur every 1m years. The axis today stands at 23 degrees to the vertical.

Writing in the journal Nature today, Dr van Dam says that when the Earth is in a very circular orbit the climate is less changeable, and summer heat will be less extreme. The more mild weather encourages glaciers to grow down from the poles, and causes a drop in ocean levels and changes to rainfall. "The ice expansion affects the global climate via atmospheric and oceanic currents," Dr van Dam said.

The climatic upheaval put pressure on mammals by wiping out food sources and fragmenting their habitats. For some species, the change in climate is enough to wipe them out, while others, by being forced to live in smaller, isolated communities, are likely to become new species, Dr van Dam claims. The evidence that the planet's wobbly orbit is to blame for mammals' 2.5m-year lifespan is "a crucial missing piece in the puzzle", said Dr van Dam.