T. rex was smarter than a crocodile, not a monkey. Study Overturns Conflicting Intelligence Data.

Since the 1970s, most scientists have agreed that Tyrannosaurus rex was about as intelligent as modern reptiles.

In 2023, a study by a Brazilian neuroscientist claimed that the cognitive intelligence of T. rex is closer to that of primates. Could this iconic dinosaur be as intelligent as a baboon or even a human?

Probably not, says a new study published in The Anatomical Record. An international team of researchers confirmed the older notion that the intelligence of T. rex was probably similar to that of a crocodile.

"This traditional view is probably correct," first author Kai Caspar, a zoologist at Heinrich Heine University in Düsseldorf, Germany, told Live Science. Caspar studies the evolution of thinking and different cognitive abilities among animal groups.

To study brain biology in long-extinct animals such as dinosaurs, researchers use endocasts—forms of fossilized skulls—to estimate the relative size of the brain and the proportion of tissue it once contained. It's not a perfect proxy, but it's the best model right now.

The brain of modern alligators and crocodiles occupies only about 30% of the cranial cavity. Birds and mammals have it almost 100%.

According to Caspar and his colleagues, this was one of the biggest problems with the 2023 study: It overestimated the size of T. rex's brain by assuming that the brain filled the entire endocranial cavity. Given the shape and form of the dinosaur endocasts he studied, Caspar said the brain-to-brain ratio is probably closer to that of reptiles than modern birds.

Another problem was data inconsistency. The estimates of brain size from the Brazilian study included other structures in T. rex heads, which sit in the brain cavity but are not part of the brain itself, like the olfactory bulb. The data also used a combination of juvenile and adult T. rex specimens.

In the new study, Caspar and colleagues cleaned up the data set by standardizing body mass estimates and excluding structures within the T. rex skull that are not involved in neural activity, such as the olfactory bulb, pituitary gland and inner ear.

However, they were unable to replicate the results of the 2023 study.

"We got very different results," Caspar said.

They estimated the number of neurons, or neurons, in the T. rex brain using their new estimates of brain volume.

They found T. rex had between 250 and 1.7 billion neurons, similar to crocodiles. In contrast, a Brazilian study estimated T. rex to have 3.3 billion neurons, similar to a baboon.

However, the study points out that brain size and the number of neurons are not the best predictors of cognitive intelligence. The number of neurons and brain size varies greatly between animals, but this does not always correlate with higher competence.

Pigeons, for example, have small brains and high neuron density, and can count as well as monkeys with larger brains.

"Neuron count was not a very good predictor of intelligence in modern animals. So why should we assume they are extinct?" Caspar said.

While T. rex probably had the intelligence of a crocodile, Caspar said he doesn't see that as a decline from primate-level intelligence.

Many dinosaur behaviors have been lost over time, and understanding the brain biology of T. rex can help study modern reptiles, Caspar said.

"Just because dinosaurs cognitively resembled living reptiles doesn't mean they were dumb animals," he said. "Reptiles can be fascinating too."