These animals at times rely on internally-generated metabolic heat to maintain body temperatures, while being subject to external temperatures in others.
"They generate enough heat to warm their blood above ambient temperature, but don't do anything to maintain it, such as shivering which humans do when they are cold," says Grady.
"Meanwhile, echidna body temperatures can fluctuate by up to 10 degrees when they are active."
Dinosaurs evolved around 200 million years ago, and competed for resources with ectothermic animals like lizards.
Their higher metabolic rate meant they could move faster making them a more dangerous predator, or more elusive prey, says Grady.
"A higher metabolic rate gave them other competitive advantages as well: they could grow faster and reproduce faster.
"But being completely warm-blooded like a mammal limits the maximum size an animal can reach — it is doubtful that a lion the size of T. rex would be able to eat enough wildebeasts (or elephants) without starving to death.
"With their lower food demands, however, the real T. rex was able to get really big while still maintaining their advantage over their competition."
As well as helping us understand how warm-blooded animals evolved, understanding dinosaurs' energy use challenges our understanding of how life operates, Grady explains.
"They were ecologically dominant for more than 100 million years, and understanding how they lived and what contributed to their dominance helps us understand why some animals win over others.
"Dinosaurs' intermediate lifestyle may have been the key to their evolutionary success. Against today's polarised landscape, dinosaurs stand out as a successful middle way."
This article originally appeared on ABC Science Online.