Dinosaur bones, like modern reptile bones, often show signs of not growing—as if these animals went through periods of little or no growth. One of the reasons for a lack of growth could be hibernation during periods of seasonal cold, indicating that the animals used an ectothermic method of heat regulation. Mammals and birds, on the other hand, are endothermic, and show no lines of arrested growth. Thus, the presence or absence of these lines give paleontologists possible clues about the way dinosaurs regulated their internal temperature.