Billions of the giant Tyrannosaurus rex dinosaur existed during the species' millions of years of existence, although not all at once. Scientists estimate that about 2.5 billion of the giant dinosaur king roamed North America throughout the species' 2.4 million years of existence. That means the population density of the T. rex was not great, but there were enough for humans to know they existed over 65 million years after dinosaurs went extinct.
Researchers at the University of California, Berkley came to the 2.5 billion number using calculations based on body size, sexual maturity, and the Tyrannosaurus rex's energy needs. They estimated that the dinosaurs lived over 127,000 generations and published their findings in the journal Science. "That’s a lot of jaws," Charles Marshall, the study's lead author and the director of the University of California Museum of Paleontology, told the Associated Press. "That's a lot of teeth. That's a lot of claws."
The T. rex species lived in North America for 1.2 million to 3.6 million years, so scientists believe that the population density was very low at any one time. Although the 2.5 billion number sounds enormous, Marshall explained that the estimate is a testament to how lucky we are to know about them today. Only about 100 T. rex fossils have ever been found, and only 32 of those have enough material left to determine they are adults. If there were only 2.5 million T. rex instead of 2.5 billion, Marshall believes we would never even know they existed.
Marshall and his team also thought about modern-day biology when they reached their estimate. Even today, the bigger the animal, the less dense its population. The population density might also have been small because it required more energy to live. A T. rex is also believed to have reached sexual maturity between 14 and 17 years old, and lived to 28 years old.
There is still a huge margin of error for their calculations. Marshall's team was uncertain about how long each generation would live or how large their ranges were. "We estimate that its abundance at any one time was ~20,000 individuals, that it persisted for ~127,000 generations, and that the total number of T. rex that ever lived was ~2.5 billion individuals, with a fossil recovery rate of 1 per ~80 million individuals or 1 per 16,000 individuals where its fossils are most abundant," reads the study's abstract. "The uncertainties in these values span more than two orders of magnitude, largely because of the variance in the density–body mass relationship rather than variance in the paleobiological input variables."
Despite all this uncertainty that comes with trying to understand animals that existed millions of years ago, a study like this is still interesting for researchers. "Previously researchers have tried to estimate things like the likely home range size of Tyrannosaurus, and its basic energetic needs, so this is a neat extension of previous work, and it includes lots of updated information on Tyrannosaurus," Nizar Ibrahim, a paleontologist at the University of Portsmouth (UK) and National Geographic Explorer, who did not participate in the study, told CNN. "We just have to keep in mind that all of these intriguing studies come with a certain dose of uncertainty -- there is just so much we still don't know about dinosaurs, even a Hollywood star like T. rex."