In December 2022, Vanderbilt University neuroscientist Suzana Herculano-Houzel published a paper that caused an uproar in the dinosaur world.
After analyzing previous research on fossilized dinosaur brain cavities and the neuron counts of birds and other related living animals, Herculano-Houzel extrapolated that the fearsome Tyrannosaurus rex may have had more than 3 billion neurons — more than a baboon.
As a result, she argued, the predators could have been smart enough to make and use tools and to form social cultures akin to those seen in present-day primates.
The original “Jurassic Park” film spooked audiences by imagining velociraptors smart enough to open doors. Herculano-Houzel’s paper described T. rex as essentially wily enough to sharpen their own shivs. The bold claims made headlines, and almost immediately attracted scrutiny and skepticism from paleontologists.
In a paper published Monday in “The Anatomical Record,” an international team of paleontologists, neuroscientists and behavioral scientists argue that Herculano-Houzel’s assumptions about brain cavity size and corresponding neuron counts were off-base.
True T. rex intelligence, the scientists say, was probably much closer to that of modern-day crocodiles than primates — a perfectly respectable amount of smarts for a therapod to have.
“What needs to be emphasized is that reptiles are certainly not as dim-witted as is commonly believed,” said Kai Caspar, a biologist at Heinrich Heine University Düsseldorf and co-author of the paper. “So whereas there is no reason to assume that T. rex had primate-like habits, it was certainly a behaviorally sophisticated animal.”
Brain tissue doesn’t fossilize, and so researchers examine the shape and size of the brain cavity in fossilized dinosaur skulls to deduce what their brains may have been like.
In their analysis, the authors took issue with Herculano-Houzel’s assumption that dinosaur brains filled their skull cavities in a proportion similar to bird brains. Herculano-Houzel’s analysis posited that T. rex brains occupied most of their brain cavity, analogous to that of the modern-day ostrich.
But dinosaur brain cases more closely resemble those of modern-day reptiles like crocodiles, Caspar said. For animals like crocodiles, brain matter occupies only 30% to 50% of the brain cavity. Though brain size isn’t a perfect predictor of neuron numbers, a much smaller organ would have far fewer than the 3 billion neurons Herculano-Houzel projected.
“T. rex does come out as the biggest-brained big dinosaur we studied, and the biggest one not closely related to modern birds, but we couldn’t find the 2 to 3 billion neurons she found, even under our most generous estimates,” said co-author Thomas R. Holtz, Jr., a vertebrate paleontologist at University of Maryland, College Park.
What’s more, the research team argued, neuron counts aren’t an ideal indicator of an animal’s intelligence.
Giraffes have roughly the same number of neurons that crows and baboons have, Holtz pointed out, but they don’t use tools or display complex social behavior in the way those species do.
“Obviously in broad strokes you need more neurons to create more thoughts and memories and to solve problems,” Holtz said, but the sheer number of neurons an animal has can’t tell us how the animal will use them.
“Neuronal counts really are comparable to the storage capacity and active memory on your laptop, but cognition and behavior is more like the operating system,” he said. “Not all animal brains are running the same software.”
Based on CT scan reconstructions, the T. rex brain was probably “ a long tube that has very little in terms of the cortical expansion that you see in a primate or a modern bird,” said paleontologist Luis Chiappe, director of the Dinosaur Institute at the Natural History Museum of Los Angeles County.
“The argument that a T. Rex would have been as intelligent as a primate — no. That makes no sense to me,” said Chiappe, who was not involved in the study.
Like many paleontologists, Chiappe and his colleagues at the Dinosaur Institute were skeptical of Herculano-Houzel’s original conclusions. The new paper is more consistent with previous understandings of dinosaur anatomy and intelligence, he said.
“I am delighted to see that my simple study using solid data published by paleontologists opened the way for new studies,” Herculano-Houzel said in an email. “Readers should analyze the evidence and draw their own conclusions. That’s what science is about!”
When thinking about the inner life of T. rex, the most important takeaway is that reptilian intelligence is in fact more sophisticated than our species often assumes, scientists said.
“These animals engage in play, are capable of being trained, and even show excitement when they see their owners,” Holtz said. “What we found doesn’t mean that T. rex was a mindless automaton; but neither was it going to organize a Triceratops rodeo or pass down stories of the duckbill that was THAT BIG but got away.”
