How smart was T. rex?

How brainy was Tyrannosaurus rex? It depends on who you ask.

Last year, the iconic Cretaceous dinosaur received the glow-up of a lifetime when Suzana Herculano-Houzel, a neuroscientist at Vanderbilt University in Nashville, calculated that the predator had 3.3 billion neurons in one part of the forebrain alone. The discovery put T. rex’s forebrain on par with modern baboons’.

The finding raised eyebrows — and doubts. Any suggestion that these dinosaurs were as smart as primates “seems like a large leap,” says Cristián­ Gutiérrez-Ibáñez, a comparative neuroscientist at the U­niversity of Alberta in Edmonton, Canada. “Having the same number of neurons as a primate does not make you a primate.”

Now, Gutiérrez-Ibáñez and colleagues have come up with a much more conservative neuron count. The T. rex telencephalon, a part of the forebrain involved in sensory, cognitive and motor functions, had closer to 360 million neurons, researchers report April 26 in the Anatomical Record. The new estimate suggests that T. rex’s forebrain is more similar to that of modern crocodiles than of primates, the researchers say.

Calculating how many neurons an animal has requires knowing how densely packed the brain cells are. That can be tough not only because neuron density varies widely between animals, but also because these brain cells aren’t well preserved in the fossil record (SN: 11/14/13). When scientists look at extinct animals, they must use the neuron densities of modern relatives as proxies.

Here lies the issue. T. rex is related to both reptiles and birds. But these two groups have vastly different neuron densities — reptiles have fewer neurons per square centimeter of brain than birds. When calculating the number of neurons in extinct theropods, the dinosaur group that includes T. rex, researchers must decide whether to use the neuron densities of birds, reptiles or some combination of the two.

In the 2023 study, Herculano-Houzel calculated the ratio between brain size and body mass of around 30 dinosaurs and then looked at how they stacked up against modern birds and reptiles. Her calculations using ancient bird families (such as the group that includes pigeons) hinted that theropod brains were more like bird brains than that of other dinosaurs. Thus, she used the neuron densities of m­odern birds that are most closely related to theropods to calculate how many neurons T. rex had.

Even if birds are living dinosaurs, that assumption is flawed, Gutiérrez-Ibáñez says. Adding a broader range of living birds to the comparison of brain-to-body ratios brought T. rex’s more in line with that of scaled reptiles, he and colleagues argue in the new study. 

Herculano-Houzel’s study also inflated the numbers of neurons by assuming that dinosaur brains occupied the whole braincase, like modern bird brains do, Gutiérrez-Ibáñez says. The brain of T. rex and many other dinosaurs floated in fluid — a trait found in modern crocodiles.

Gutiérrez-Ibáñez’s team recalculated T. rex’s brain size using a smaller brain volume, which cut down the amount of neurons in the telencephalon from 3.3 billion to 1.2 billion. Using reptile neuron density reduced the amount even further to between 245 million and 360 million.

Herculano-Houzel says that her study did account for dinosaur brains not completely filling the braincase. She is also not convinced by the new findings.

Gutiérrez-Ibáñez and colleagues made a “fatal mistake” in their assumptions about the body-to-brain ratio that led to the lower cell count, she says. By throwing in distantly related birds like pelicans and penguins — which have fundamentally different body-to-brain ratios — Gutiérrez-Ibáñez’s team arrived at the incorrect conclusion, she says.

Whether T. rex had a lot of neurons to spare might not be a good indicator of the dinosaur’s overall brainpower anyway, says Amy Balanoff, an evolutionary biologist at Johns Hopkins School of Medicine who was not involved in either study.

Even if T. rex had a neuron density on par with primates, the cells probably helped the dinosaur move its large body or gather sensory information. For instance, the smell centers of the T. rex forebrain were huge, Balanoff says. “If there were a large number of neurons in the telencephalon, many of these are going to be devoted to processing olfactory information.”

Still, Balanoff says, “I do really appreciate the original study for starting the conversation. You can’t move science forward if you’re unwilling to put the data out there.”