News,
An interview with zoologist Dr Kai Caspar. In Part I, palaeontologist Hady George walked me through how endocasts, the imprints brains leave inside skulls, are made, tested, and trusted. The natural follow-up: what do we do with them?
Films keep portraying T. rex as either having ape-like intelligence or as a dumb monster devouring everything in its path. In these films, they are usually endlessly hungry for human prey, an appetite the real animal would likely have satisfied with other, non-human, menu items.
We all would love to know the cognitive capacities of extinct species. Unfortunately, time is an unforgiving dimension. We need a lot of evidence to elucidate this. However, it is very tempting to speculate.
In a 2023 publication, Herculano-Houzel argued that T. rex had telencephalic neuron counts comparable to those of baboons, and speculated that this might have enabled tool use and cultural transmission. Dr Kai Caspar, a lecturer in zoology at Heinrich-Heine-Universität Düsseldorf, co-authored with Hady George a critique of this hypothesis (Caspar et al., 2024).
“As with all fields of science, this topic is subject to a suite of opinions. Some are more and some are less substantiated by evidence.”
“Every once in a while, most recently through Herculano-Houzel’s 2023 article, it is claimed that extinct dinosaurs were exceptionally intelligent and behaved similarly to great apes or large-brained birds like parrots. Media outlets like to pick up these assertions so that they circulate widely. However, in the relevant scientific community those views are regarded as fringe opinions and not taken very seriously from what I can tell.”
The headline claim rests on a method, and the method has two main problems. One is the assumption that neuron counts predict how cognitively able an animal is. Neuroscientists know from hard experience how seductive this simplification is. Dr Caspar explains:
“The evidence for this is surprisingly limited, especially if we compare cognitive performance across distantly related groups of animals (like extinct dinosaurs and their closest living relatives, the birds and crocodilians). As with more traditional measurements that have been used as cognitive proxies, like absolute or relative brain size, it is challenging to make sense of the complex and often noisy behavioral data based on neuron counts. One could point to studies where neuron counts appear to be well aligned with cognitive performance, and others where they are not. A robust empirical basis to meaningfully extrapolate for most extinct species, including Mesozoic dinosaurs, simply is not there.”
The second problem is that, even granting this idea, the cavity is not the brain.
“Second, it is very difficult to reconstruct neuron counts in extinct lineages without close living relatives. For this you need reliable estimates of brain size/proportions and neuron density. Let’s take Tyrannosaurus rex as an example: There are several decently preserved braincases of this animal which allow us to estimate the volume of the cranial cavity. However, in the living dinosaur, this space was not just occupied by the brain but also by the tissue of the meninges and quite copious amounts of cerebrospinal fluid. This is also the case in living crocodilians, which share important traits relating to the brain and braincase with T. rex. Therefore, the actual size of the brain cannot be estimated with much precision, we only know that it was quite a bit smaller than the brain cavity itself. Very different estimates have been published based on diverse methods of inference. Furthermore, we also do not know the density of neurons in the T. rex brain. It may have been low, as in modern reptiles, high as in modern birds, or anything in between. Inferring this is simply impossible, unfortunately.”
Just as Hady George pointed out in Reading Stones Part I, we are mainly investigating the cavity as a stand-in for the brain. Dr Caspar adds that we do not even know how much of that cavity was occupied by CSF and other tissues.
“Even if neuron counts were precise predictors of cognitive performance (which they are not), we could not accurately reconstruct them for animals like T. rex to make any meaningful claims about its behavior.”
In other words, even the cavity volume is uncertain, and the neuron density within it is pure guesswork. So why do some researchers even believe that some dinosaurs, like T. rex for instance, might have had large brains and/or high neuron density?
Some of it is based on the Endothermic Brain Hypothesis (EBH). According to this hypothesis, big brains coevolved with warm-bloodedness (or more correctly, endothermy) to power complex mental representations needed for efficient foraging. This is because endothermic animals require higher nutritional resources than ectothermic animals, so, according to the hypothesis, animals became more intelligent so that they could forage efficiently enough to fuel an endothermic metabolism. Dr Caspar gave two reasons, one empirical and one evolutionary, why this might not be the entire picture.
“…we simply do not know whether endotherms actually have cognitive capacities that consistently surpass those of ectotherms in that regard. Furthermore it is similarly unknown to what extent the ability to run mental simulations is influenced by brain size/neuron counts. Convincing evidence for the key claims of the EBH is so far essentially non-existent and I do not think that this hypothesis is helpful when it comes to making sense of theropod brain evolution through the Mesozoic.”
“Available evidence suggests that even the earliest theropods were already fully endothermic animals, yet they had small brains. In fact, some lineages of theropods retained small brains resembling those of modern reptiles in many respects until their demise at the end of the Mesozoic. Yet birds and their closest relatives among dinosaurs (Maniraptoriformes) deviate from this pattern. Why they took a different evolutionary route is unclear and the EBH cannot help us answer this question.”
Caspar and colleagues argued that the EBH lacks robust support from the fossil record (Caspar et al., 2025). His position, though, is more nuanced than a flat rejection:
“I am currently not convinced by the specifics of the EBH but I do agree that endothermy and brain evolution are intertwined (which is anything but a new idea). For instance, the very large brains of some modern birds and mammals surely appear incompatible with an ectothermic physiology. But that does not mean that all endotherms need large brains and that those per se grant them more advanced cognitive abilities than are found in ectotherms. The story appears to be a lot more complex and needs to be approached with nuance and consideration for the substantial knowledge gaps that still persist when it comes to understanding brain evolution.”
Whatever the eventual fate of the EBH, it is far from the only thing happening in palaeoneurology.
“If we speak about palaeoneurology as a whole, I think the field is making a lot of progress. There are several recent studies I can point to that have elucidated important aspects of large-scale patterns in the evolution of bird and mammal brains. It is clear however, that behavioural inferences, especially about individual species, will remain challenging. Oftentimes they are non-falsifiable and thus elude proper scientific evaluation. Discussing palaeoneurologic data in that context thus requires a lot of nuance, but colleagues working in that field are obviously aware of that.”
As a neuroscientist, what strikes me most from these conversations is not how little we know about the brains of extinct animals, but how much of what we “know” about the brains of living ones rests on similar inference. The endocast problem is, at a smaller scale, the problem of neuroscience as well. We infer minds from measurements, and every measurement carries caveats. The experts here know this. It is why they push back. That restraint, more than any headline about baboon-brained dinosaurs, is what good science sounds like.
The stones can be read, but only partially, and only with care. An endocast is not a brain any more than a footprint is a foot: it captures the shape of something long gone, while telling us very little of the mind that resided within it. Still, those imprints are the only message most extinct brains will ever leave. Reading them honestly, resisting the temptation to inflate T. rex into a primate, or shrink it into a lizard, is the difference between palaeoneurology as science and as storytelling.
Disclaimer: The interviews were conducted by email, and quotes are reproduced verbatim as approved by the interviewees. Their views are their own and not necessarily those of their institutions. Summaries of scientific studies are based on the original publications cited in the reference list, and readers are encouraged to consult those sources directly. The author is a neuroscientist by training and, as noted in the text, palaeontology falls outside his area of expertise. This article is an honest attempt to report the field accurately, not a comprehensive review of it.
Declaration of AI use: This article was written with the assistance of Lumo AI for grammar, spell checking, and proofreading.
References
Caspar, K.R., Gutiérrez-Ibáñez, C., Bertrand, O.C., Carr, T.D., Colbourne, J.A.D., Erb, A., George, H., Holtz, T.R. Jr., Naish, D., Wylie, D.R. and Hurlburt, G.R., 2024. How smart was T. rex? Testing claims of exceptional cognition in dinosaurs and the application of neuron count estimates in palaeontological research. The Anatomical Record 307(12), 3685-3716. https://doi.org/10.1002/ar.25459
Caspar, K.R., Gutiérrez-Ibáñez, C., George, H., Holtz, T.R. Jr., Naish, D. and Hurlburt, G.R., 2025. Endothermy, neuron counts, and other issues: Further remarks on neurocognitive evolution in fossil vertebrates. The Anatomical Record, 1-14. https://doi.org/10.1002/ar.70113
Herculano-Houzel, S., 2023. Theropod dinosaurs had primate-like numbers of telencephalic neurons. Journal of Comparative Neurology 531(9), 962-974. https://doi.org/10.1002/cne.25453
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