Robert T. Bakker
As of 2014, it is not clear if "Tyrannosaurus" was endothermic ("warm-blooded"). "Tyrannosaurus", like most dinosaurs, was long thought to have an ectothermic ("cold-blooded") reptilian metabolism. The idea of dinosaur ectothermy was challenged by scientists like Robert T. Bakker and John Ostrom in the early years of the "Dinosaur Renaissance", beginning in the late 1960s. "T. rex" itself was claimed to have been endothermic ("warm-blooded"), implying a very active lifestyle. Since then, several paleontologists have sought to determine the ability of "Tyrannosaurus" to regulate its body temperature. Histological evidence of high growth rates in young "T. rex", comparable to those of mammals and birds, may support the hypothesis of a high metabolism. Growth curves indicate that, as in mammals and birds, "T. rex" growth was limited mostly to immature animals, rather than the indeterminate growth seen in most other vertebrates.