 Tyrannosaurus , like most dinosaurs , was long thought to have an ectothermic ( " cold @-@ blooded " ) reptilian metabolism but was challenged by scientists like Robert T. Bakker and John Ostrom in the early years of the " Dinosaur Renaissance " , beginning in the late 1960s . Tyrannosaurus 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 . It has been indicated that the temperature difference may have been no more than 4 to 5 ° C ( 7 to 9 ° F ) between the vertebrae of the torso and the tibia of the lower leg . This small temperature range between the body core and the extremities was claimed by paleontologist Reese Barrick and geochemist William Showers to indicate that T. rex maintained a constant internal body temperature ( homeothermy ) and that it enjoyed a metabolism somewhere between ectothermic reptiles and endothermic mammals . Later they found similar results in Giganotosaurus specimens , who lived on a different continent and tens of millions of years earlier in time . Even if Tyrannosaurus rex does exhibit evidence of homeothermy , it does not necessarily mean that it was endothermic . Such thermoregulation may also be explained by gigantothermy , as in some living sea turtles . 
