T. rex Brainpower: How to Debunk Myths?

How intelligent was the T. rex? Opinions vary. Last year, neuroscientist Suzana Herculano-Houzel from Vanderbilt University in Nashville proposed a groundbreaking idea, estimating that this Cretaceous predator possessed 3.3 billion neurons in a section of its forebrain. This impressive number suggested that the T. rex‘s forebrain could be comparable to that of modern baboons.

However, this claim was met with skepticism. Comparative neuroscientist Cristián Gutiérrez-Ibáñez from the University of Alberta in Edmonton, Canada, expressed doubts, pointing out that having a similar number of neurons to primates doesn’t necessarily equate to similar intelligence or characteristics.

In response, Gutiérrez-Ibáñez and his team conducted their own research, presenting a more conservative estimate. They calculated that the T. rex telencephalon, a part of the forebrain responsible for sensory, cognitive, and motor functions, contained around 360 million neurons. This new estimate aligns the T. rex‘s forebrain more closely with that of modern crocodiles rather than primates, as reported in the April 26 issue of the Anatomical Record.

Calculating the number of neurons in an animal’s brain requires understanding the density of brain cells, a challenging task due to significant variations in neuron density across different species. This complexity is compounded by the poor preservation of brain cells in the fossil record. As a result, scientists studying extinct animals like the T. rex must rely on the neuron densities of their modern relatives for estimation.

This reliance presents a problem because the T. rex is related to both reptiles and birds, which have vastly different neuron densities. Reptiles possess fewer neurons per square centimeter of brain compared to birds. Therefore, when estimating the neuron count in extinct theropods, the dinosaur group that includes T. rex, researchers face the dilemma of whether to use the neuron densities of birds, reptiles, or a combination of both.

In a 2023 study, Suzana Herculano-Houzel tackled this issue by calculating the ratio between brain size and body mass for about 30 dinosaurs and comparing these ratios with those of modern birds and reptiles. Her findings suggested that theropod brains, including those of the T. rex, resembled bird brains more closely than those of other dinosaurs. Consequently, she used the neuron densities of modern birds closely related to theropods to estimate the number of neurons in the T. rex brain.

However, this approach is not without controversy. Cristián Gutiérrez-Ibáñez argues that even if birds are living dinosaurs, using bird neuron densities exclusively may be flawed. By incorporating a broader range of living birds in the comparison of brain-to-body ratios, Gutiérrez-Ibáñez and his colleagues found that the T. rex‘s brain aligns more closely with that of scaled reptiles. This perspective offers a more conservative and arguably more accurate neuron count for the T. rex brain.

Herculano-Houzel’s study may have overestimated the number of neurons in the T. rex brain by assuming that dinosaur brains occupied the entire braincase, similar to modern birds. However, Gutiérrez-Ibáñez points out that the T. rex brain, like those of many other dinosaurs, likely floated in fluid, a characteristic seen in modern crocodiles.

To address this, Gutiérrez-Ibáñez’s team recalculated the T. rex‘s brain size using a smaller brain volume. This adjustment reduced the estimated neurons in the telencephalon from 3.3 billion to 1.2 billion. Further refinement using reptile neuron density brought the count down even more, to between 245 million and 360 million.

Despite these findings, Herculano-Houzel maintains that her original study did consider the incomplete filling of the braincase by dinosaur brains. She remains skeptical of the new estimates, suggesting that Gutiérrez-Ibáñez’s team made critical errors in their assumptions about the body-to-brain ratio. By including distantly related birds like pelicans and penguins, which have different body-to-brain ratios, the new study may have arrived at an inaccurate neuron count.

Additionally, Amy Balanoff, an evolutionary biologist at Johns Hopkins School of Medicine, notes that the sheer number of neurons may not necessarily reflect the overall cognitive capabilities of the T. rex. This perspective implies that neuron count alone might not be the best measure of the dinosaur’s brain power.

Even if the T. rex had a neuron density comparable to that of primates, these neurons likely served functions related to the dinosaur’s physicality and sensory processing. For example, the smell centers in the T. rex forebrain were notably large, suggesting that a significant number of neurons in the telencephalon were dedicated to processing olfactory information, according to Balanoff.

Despite the debate, Balanoff appreciates the original study for initiating the discussion. She believes that sharing such data is essential for scientific progress, as it fosters dialogue and advances understanding.

1. ^{ONLINE NEWS} Kreier, F. (2024, May 16). How smart was T. rex? Science News. [Science News]
2. ^JOURNAL Caspar, K. R., Gutiérrez‐Ibáñez, C., Bertrand, O. C., Carr, T., Colbourne, J. a. D., Erb, A., George, H., Holtz, T. R., Naish, D., Wylie, D. R., & Hurlburt, G. R. (2024). How smart was T. rex? Testing claims of exceptional cognition in dinosaurs and the application of neuron count estimates in palaeontological research. The Anatomical Record. [The Anatomical Record]
3. ^JOURNAL Herculano‐Houzel, S. (2023). Theropod dinosaurs had primate‐like numbers of telencephalic neurons. Journal of Comparative Neurology, 531(9), 962–974. [Journal of Comparative Neurology]