Annual growth lines etched in the femurs of 115 wild warm-blooded mammals such as giraffes, reindeer and gazelles are similar to those previously seen in the bones of reptiles and dinosaurs, researchers report online June 27 in Nature.

“People always said that mammals do not show these lines,” says study lead author Meike Köhler, a paleontologist at the University of Barcelona in Spain. This assumption is “like a myth that’s going around; you read it everywhere,” she says. “But people haven’t really studied mammals.”

In dinosaurs and reptiles, yearly cycles of growth and nutrition are stamped in the bones like the rings of a tree. In fat months, animals pack on blood vessel-rich bone tissue, and in lean months they skimp, laying down only thin sheets. Under a microscope, the slender sheets of bone look like dark lines. Because these “lines of arrested growth” or “rest lines” stripe the bones of both dinosaurs and reptiles, some scientists assumed that dinosaurs, like reptiles, were cold-blooded. But the new work shows that warm-blooded mammals have banded bones, too.