Mammal species do not seem to last very long in the grand scheme of things, persisting for an average of 2.5 million years, according to the fossil record. By studying the fossilized teeth of rodents from over a span of 22 million years, Jan van Dam of Utrecht University and his colleagues found that rodent species rise and fall in cycles that closely match variations in Earth's orbit. The cycle between an elliptical and circular orbit and a change in the tilt of Earth's axis combined to create periods in which our planet did not tilt very much as it revolved around the sun, thereby eliminating seasons and resulting in less climatic variability. “These climate changes may destroy the habitat of rodents or give new rodent forms a chance,” van Dam says. The finding appears in the October 12 Nature.