Not too long ago, Brazilian neuroscientist Suzana Herculano-Houzel published a controversial study that boldly claimed that T. rex had an intellect rivaling modern baboons. The assumption was that Tyrannosaurus rex was very smart and might have even used tools (with those tiny hands?). It might have passed down knowledge to offspring through cultural transmission, this study said.

All of this naturally attracted a lot of media attention. Suddenly, the king of the dinosaurs was not only physically menacing but also ultrasmart (for a reptile) — that’s double the trouble.

However, a new study out this week would beg to differ, tempering our enthusiasm. According to an international team of paleontologists who took a second good look at Herculano-Houzel’s study, the estimates for the number of T. rex neurons were off. They provide a more conservative assessment instead, concluding that T. rex and other dinosaurs like it were probably as smart (or dumb) as modern-day crocodiles and lizards.

“Upon the publication of the original paper arguing many dinosaurs including T. rex had tremendously large neuron counts that possibly facilitated complex behaviours such as tool use, we immediately were curious of the methods used to reach these conclusions and the reliability of using neuron counts to predict intelligence. There has been much research into estimating neuron counts in living animals, but this has not been translated to the field of palaeontology, and that fascinated us,” co-author Hady George of the University of Bristol told ZME Science.

“After carefully reading through the original paper, we identified several shortcomings in the original paper and a lack of consideration for wider research in the field of palaeontology. This motivated us to attempt another go at estimating neuron counts in dinosaurs, and in particular T. rex as it has become a ‘model organism’ for fossil species. Our results made sense with what we already knew about dinosaur biology and intelligence, but we still believe that they are important and exciting as they are, so far, the most reliable estimates we have for how many neurons were in dinosaur brains.”

The pitfalls of estimating intelligence in dinosaurs

The earlier methodology used for assessing dinosaur brain size primarily relied on endocasts — natural or artificial molds of the brain cavity of the skull. These endocasts were assumed to accurately represent the brain’s size and shape, enabling neuroscientists to estimate brain volume. The basic morphological features were then compared to living species to infer cognitive abilities and behavioral traits. For instance, higher brain volumes relative to body size (encephalization quotient or EQ) were often linked with higher intelligence. The initial study crunched numbers that showed the T. rex EQ was about the same as that of primates.

However, this method had significant limitations. For one, it assumed that the endocast perfectly matched the brain’s morphology. This is not always true, especially in non-avian dinosaurs where the brain did not completely fill the cranial cavity. Secondly, using modern animals as direct analogs for extinct species ignores the vast evolutionary changes and adaptations that have occurred over millions of years.

“In the previous study, how densely the brains of dinosaurs were packed with neurons was determined by an attempt at deducing metabolism using a relative brain size metric. Basically, if the metric was high enough, the neuron density was thought to be the same as many warm-blooded modern birds, and if the metric was low enough, the neuron density was thought to be the same as many cold-blooded modern reptiles,” George said.

“This method is deeply flawed as it disregards other lines of evidence for inferring metabolism in dinosaurs and the differences in brain shape that are directly related to how densely neurons are packed in brains. We show that all dinosaurs, with the exception of those very closely related to birds, have brains that are probably packed with the same density as those of modern reptiles and not birds. This was a major reason why our results recovered dramatically lower neuron counts.”

Not a genius

The researchers argue that predicting intelligence in species that have been extinct for tens of millions of years simply by looking endocasts or brain cavities in general is not good practice. Instead, such investigations need to be more holistic. You need to look at other things too, such as skeletal anatomy, the behavior of living relatives, trace fossils, and so on.

Overall, the new study found that the previous estimate for the neuron count of T. rex was greatly overestimated, especially that of the forebrain. Rather than 3 billion neurons as previously stated, T. rex‘s brain had no more than 1.7 billion neurons. And even this figure may be too generous.

Perhaps a more realistic neuron count hovers around 250 million, which is about as many neurons as the cat brain, a much smaller species. T. rex was probably more like a giant crocodile in intellect and behavior, rather than a baboon. But that’s not to say it was unsophisticated. T. rex was still cognitively capable of subduing dangerous prey, nesting, and courting mates.

“Our paper builds upon the previous paper arguing for greater neuron counts, by not only providing further research into estimating neuron numbers, but also providing an example of how to do so while avoiding critical pitfalls and in appreciation of the wider literature surrounding palaeobiology and animal behaviour and intelligence. We hope future studies explore neuron number estimates in other fossil groups as this has the potential to greatly inform on the evolution of brains and cognition.”

The findings appeared in the journal The Anatomical Record.