Tyrannosaurus is at the center of debate about dinosaur intelligence

Assume the physical appearance of a dinosaur — or any extinct animal — based on its fossils is already a complicated proposition, with so many uncertainties involved . Assessing a dinosaur's intelligence, considering the countless factors that contribute to this characteristic, is exponentially more difficult.

A study published last year by neuroscientist Suzana Herculano-Houzel, from Vanderbilt University which assessed the intelligence of Tyrannosaurus rex focusing on the estimated size of the brain and the number of brain neurons, compared to those of primates — specifically the baboon — caused an uproar in scientific circles.

Now, an interdisciplinary scientific team has published a study in response, questioning Herculano-Houzel's methodology and challenging his assessment of T. Rex intelligence and other large predatory dinosaurs from the theropod suborder.

They suggest a more holistic approach to assessing the brain capacity of Tyrannosaurus or any extinct animal, with brain size and neuron count alongside other factors such as the animal's anatomy and ecology, data from living relatives, and fossil evidence about how he moved and fed that offer a glimpse into his life.

“Our main findings are that the brains of most dinosaurs, including T. Rex, were comparable in relative size to those of living reptiles such as crocodiles and alligators. Furthermore, the number of neurons was probably not exceptional, especially for animals of their body mass,” said zoologist Kai Caspar of Heinrich Heine University in Germany, who studies the behavior of living animals and was the lead author of the study. published this week in The Anatomical Record magazine.

“What needs to be emphasized is that reptiles are certainly not as stupid as people believe,” Caspar added. “Their behaviors can be very complex and the experimental data we have points to many cognitive similarities between them, mammals and birds. So while there is no reason to assume that T. Rex had primate-like habits, it was certainly a behaviorally sophisticated animal.”

Herculano-Houzel said she continues to endorse her findings and said the new analysis is flawed. “The only thing that is in dispute is what already existed at the time of my study: how big dinosaur brains actually were. Even back then, we were talking about the difference between a T. Rex brain being the size of a baboon or a monkey,” said Herculano-Houzel.

“Their conclusion depends on a single, extremely important point: whether theropods like T. Rex shared their (brain-body size) relationship with their extant ostrich and warm-blooded chicken cousins, or with their more distant living relatives, the crocodiles. I said the first option because I compared theropods to ostriches and chickens; now, they say it’s the second option,” Herculano-Houzel added.

Caspar said the comparison with modern birds is also an integral part of the new study.

“It’s not a trivial matter”

There are problems with trying to assess intelligence by counting brain neurons, Caspar said.

“The first obstacle is estimating the actual size of the brain of the extinct animal in question. This is not a trivial question in dinosaurs. Although the brain fills most of the cranial cavity in birds and mammals, this is not the case in reptile species, whose brains only fill between 30% and 50% of the cranial cavity,” said Caspar.

“The 2023 study assumed 100% fill in dinosaurs like T. Rex and that was certainly not the case,” he added.

It's not known how dense the neurons were in dinosaur brains, Caspar said. “However, looking at living animals, we see that neuron count is not actually a good indicator of intelligence in the first place, although this may seem intuitive at first glance,” Caspar added.

Dinosaurs, with the exception of their bird descendants, disappeared 66 million years ago after an asteroid hit Earth. In two centuries of scientific study, dinosaurs are becoming better defined, although there are still many uncertainties about Tyrannosaurus and other species.