Dinosaurs may have been neither warm- nor cold-blooded

BBC : Mammals and birds are warm-blooded, or endothermic, and reptiles and amphibians are cold-blooded, or ectothermic, but dinosaurs may have been something in between, according to a new study by John Grady of the University of New Mexico and colleagues. The researchers studied the growth rates of 381 different species both living and extinct, including 21 dinosaurs; rates for dinosaurs were calculated from their bones’ growth rings. Because warm-blooded animals eat more, they tend to grow faster. Assuming that growth rate and metabolic rate are linked, the researchers found that dinosaurs fall somewhere between the endotherms and the ectotherms—in a new middle category, called mesotherms. They point out that even today, a handful of species, such as the leatherback turtle and some sharks, could be classified as mesotherms because of their unusual energy habits. While some scientists support the addition of a new mesothermal category because they believe the cold-blooded–warm-blooded system to be too simplistic, others say the comparative lack of mesothermal species today shows that from an evolutionary standpoint, there are advantages to being either ectothermal or endothermal.