A 2023 study suggested that the Tyrannosaurus rex, or T. rex, may have had a brain size similar to that of a baboon, which could have allowed for complex behaviors such as problem-solving, tool use, and the transmission of knowledge across generations. The study, led by neuroscientist Suzana Herculano-Houzel from Vanderbilt University, used data from theropod dinosaurs—predatory dinosaurs that include the T. rex—and applied scaling rules observed in modern birds, which are theropod descendants. By analyzing the number of neurons in the telencephalon, the part of the brain responsible for sensory processing, cognition, and movement, Herculano-Houzel estimated the T. rex had around 3.3 billion neurons, a number similar to that of a baboon. However, this conclusion faced criticism in a 2024 study published in The Anatomical Record. Researchers Kai Caspar, Cristián Gutiérrez-Ibáñez, and Grant Hurlburt argued that the original study overestimated the T. rex’s brain size by assuming it filled nearly the entire cranial cavity, which is not typical of most dinosaurs. They pointed out that parts of the skull are occupied by other structures, such as blood vessels and membranes, and that some brain regions, like the olfactory bulbs, were incorrectly included in the original analysis. The team also questioned the use of avian neuronal density as a reference, suggesting that reptiles, like crocodiles, might be a more accurate comparison due to their lower neuronal density. According to the 2024 study, the T. rex’s telencephalon likely contained about 360 million neurons—nearly ten times fewer than the 2023 estimate. The researchers emphasized that the number of neurons alone does not necessarily reflect intelligence, as the organization of brain circuits varies significantly between reptiles and birds or mammals. They argued that attributing primate-like intelligence to the T. rex might be an overreach based on current data. Herculano-Houzel has defended her findings, stating that she considered the brain’s size relative to the skull and did not assume it occupied the entire space. The debate highlights the challenges of using neuronal counts to infer intelligence in extinct animals and raises questions about the reliability of such methods. While the study of dinosaur brains continues to evolve, it underscores the complexity of understanding ancient cognition through modern scientific techniques.