Did the gigantic dinosaurs also have gigantic brains and a gigantic number of neurons?

Longer answer

When picturing dinosaurs, you likely imagine a massive, menacing, yet somewhat clumsy lizard. The Tyrannosaurus rex, with its tiny arms and enormous teeth, fits this image perfectly. But what do we truly know about the brains of dinosaurs – what they could perceive through sight and smell or, more intriguingly, what they might have thought? A recent study by scientist Suzana Herculano-Houzel suggests that the T. rex’s brain may have housed as many neurons (the critical type of brain cell) as a baboon’s. The idea is both fascinating and unnerving: could this towering predator also possess the intelligence of a clever and inquisitive monkey?

First, it’s important to clarify that the group we classify as ‘dinosaurs’ encompasses an extraordinary range of species. Just as a shrew is vastly different from an elephant, so too are different dinosaur species. The extinct animals we call ‘dinosaurs’ are divided into several subgroups, including ornithischians (like Triceratops and Stegosaurus), sauropods (like Diplodocus), and theropods (like T. rex).

Image 1: The phylogenetic tree (family tree) of dinosaurs and their living relatives, such as reptiles (e.g., crocodiles) and birds. This tree reveals that dinosaurs were not simply a minor branch of reptiles but a vast and diverse group of species that spanned a genetic spectrum between modern reptiles and birds. Image credit: Matt Wedel, University of California Museum of Paleontology.

This diversity in species likely reflects significant variations in brain size. For instance, theropods are thought to have had relatively large brains, suggesting behaviour more akin to modern birds like parrots or ravens (and possibly baboons). By contrast, sauropods are believed to have had smaller brains, with intelligence levels more comparable to modern reptiles like iguanas or geckos. However, studying the brains of extinct species presents a major obstacle: we don’t have their brains!

Dinosaur fossils are already rare; in most cases, only hard tissues – such as bones and teeth – are preserved. These are the parts we see displayed as striking skeletal reconstructions in museums. What is lost are the soft tissues: feathers (it’s likely many more dinosaurs were feathered than previously assumed), pigmentation (we know little about their actual colours), and, crucially, their brains. For modern animals, studying their brains allows scientists to estimate the number and types of neurons. Suzana Herculano-Houzel is well-known for her innovative ‘brain soup’ method, which involves liquefying the brains of various species (including humans) to count the neurons they contain.

Image 2: (Left) Researcher Suzana Herculano-Houzel in 2012, with her large suitcase of brain samples from various species. (Right) The brain of the African elephant. Suzana has studied hundreds of brains, investigating the differences in neuron counts across species. The images are credited to Suzana Herculano-Houzel from Scientific American.

Although dinosaur brains are not available for direct study, other methods allow us to investigate them. One older but still common technique is creating ‘endocasts’ by filling a skull with plaster to estimate the size of its interior. More recently, CT scans have become the preferred tool, allowing researchers to digitally examine the fossil without needing to open it to provide an estimate of brain size. The general idea is that a larger skull means more space for a brain. Dinosaur skulls were relatively small compared to their body size, though there were variations between species. For instance, theropods (like T. rex) had larger skulls in relation to their bodies, while sauropods (long-necked plant-eaters) had much smaller heads.

By combining skull volume with data from living relatives of dinosaurs (such as reptiles and birds), scientists can estimate the number of neurons. Using these data, along with the metabolic rates of living species (indicating whether dinosaurs were warm- or cold-blooded), Herculano-Houzel suggested that the brain of a T. rex may have been similar in neuron count to that of modern primates like baboons.

However, there are significant limitations to these neuropaleontological findings (the study of fossilised brains). For example, a skull’s volume includes not only brain tissue but also blood vessels and cavities, so the actual brain size might be much smaller. This is the case with the Nile crocodile, where only 30% of the skull’s contents are brain tissue. Furthermore, different brain regions vary in neuron density and not all contribute equally to intelligence. For example, the olfactory bulb, which processes smell, is especially large in reptiles. The cerebrum, which handles decision-making and higher cognition, is, instead, expanded in humans and other intelligent species like primates, elephants, and parrots. In dinosaurs, it’s more challenging to pinpoint the size of specific brain regions, but evidence suggests that the olfactory bulb was likely larger than the cerebrum in many species. Suffice it to say that the debate continues in the scientific community.

Now, moving on to neuron size – how does it factor into all this? Neuron length and thickness are more related to the transmission of electrical signals than to cognitive power. The largest known neurons belong to the giant squid (Architeuthis), with neurons that can reach up to 1 mm in diameter –large enough to be seen with the naked eye! These large neurons are essential for quick signal transmission, enabling the squid to dart away rapidly. In vertebrates (mammals, birds, and reptiles), neurons are coated with myelin, which speeds up signal transmission without requiring the neuron itself to become significantly thicker (see the explanation of ‘Why do brain tissues have different colours?‘).

Thanks to modern neuroscientific methods, we can study long-extinct dinosaurs, even without their physical brains. Neuropaleontology underscores the diversity of dinosaur species; a diverse range of species means a diverse range of brain types. Although dinosaurs probably didn’t have larger neurons than modern birds, reptiles, or mammals, their neurons’ exact number and distribution remain an ongoing scientific debate. Future research may reveal whether the formidable T. rex also had the cunning intelligence of a curious monkey!

Read more:
https://www.youtube.com/watch?v=a1tEnm53zDs&ab_channel=SuzanaHerculano-Houzel-NeuroscienceOfficeHour

https://www.scientificamerican.com/article/how-did-dinosaurs-see-smell-hear-and-move/

https://www.scientificamerican.com/article/lessons-from-making-brain-soup/

Herculano‐Houzel, S., 2023. Theropod dinosaurs had primate‐like numbers of telencephalic neurons. Journal of Comparative Neurology, 531, p.962-974.