Theropod dinosaurs had primate‐like numbers of telencephalic neurons

Abstract

Understanding the neuronal composition of the brains of dinosaurs and other fossil amniotes would offer fundamental insight into their behavioral and cognitive capabilities, but brain tissue is only rarely fossilized. However, when the bony brain case is preserved, the volume and therefore mass of the brain can be estimated with computer tomography; and if the scaling relationship between brain mass and numbers of neurons for the clade is known, that relationship can be applied to estimate the neuronal composition of the brain. Using a recently published database of numbers of neurons in the telencephalon of extant sauropsids (birds, squamates, and testudines), here I show that the neuronal scaling rules that apply to these animals can be used to infer the numbers of neurons that composed the telencephalon of dinosaur, pterosaur, and other fossil sauropsid species. The key to inferring numbers of telencephalic neurons in these species is first using the relationship between their estimated brain and body mass to determine whether bird-like (endothermic) or squamate-like (ectothermic) rules apply to each fossil sauropsid species. This procedure shows that the notion of "mesothermy" in dinosaurs is an artifact due to the mixing of animals with birdlike and squamate-like scaling, and indicates that theropods such as Tyrannosaurus and Allosaurus were endotherms with baboon-and monkey-like numbers of telencephalic neurons, respectively, which would make these animals not only giant but also longlived and endowed with flexible cognition, and thus even more magnificent predators than previously thought.

Could theropod dinosaurs have evolved to a human level of intelligence?

“…One dinosaur currently known as troodon ( Troodon formosus ), but once known as Stenonychosaurus inequalis , in particular, shows a brain–body size ratio falling within the lower edge of the largely superimposed avian and mammalian ranges (Figures 2 and 3) (Hopson, 1977; Jerison, 1973; Russell & Seguin, 1982). Based on the similar brain size–body size scaling exhibited by living basal birds and theropod dinosaurs, Herculano‐Houzel (2023) has, in fact, suggested that telencephalic neuron abundance may also scale similarly for these two groups, with larger theropods thus potentially having rivaled some extant nonsimian primate species such as baboons and capuchin monkeys in total telencephalic neuron abundance (Olkowicz et al., 2016). Thus, the notion that dinosaurs were unsophisticated in their behavioral repertoires and that they dominated the land for over 150 million years because evolution had not yet brought forth a better alternative is untenable.…”

Dinosaurs: Clever creatures or bird brains?

“…Two recent studies in The Journal of Comparative Neurology explore potential capacities and limitations in the intelligence of dinosaurs. The first study by Herculano-Houzel (2023) addresses the issue of capacity by estimating the total number of neurons in various dinosaurs basing these calculations on neuron counts in extant birds, the living descendants of the dinosaur lineage. In order to accomplish this, the author takes into account the relationship between the size of the animal and the size of the brain, as larger animals tend to have larger brains, but details of this relationship differ between cold-blooded and warm-blooded animals.…”

How smart dinosaurs?

“…My proposal, like that of Dr. Herculano‐Houzel (Herculano‐Houzel, 2023a), is based on assumptions, and can be evaluated based on the merits of those assumptions. As noted, chief among these is that the avian nuclear pallial design has a latent wiring inefficiency that is exposed when pallial size grows beyond a certain point.…”

“…Even if I am right and a dinosaur humanoid was unlikely to have evolved, we are still left with the suggestion of Dr. Herculano‐Houzel in her article that large theropod dinosaurs could have possessed a monkey‐like number of pallial neurons, suggesting an impressive intelligence level for these large theropods (Herculano‐Houzel, 2023a). But how intelligent?…”

“…Rather, as emphasized in my own article (Reiner, 2023), numerous published studies have presented various lines of evidence that many dinosaur species functioned at least at an avian level of behavioral and cognitive complexity. In her own article, Dr. Herculano-Houzel takes this a step further, and from what we know about the relationship between brain size and neuron numbers suggests that larger theropod dinosaurs had an abundance of pallial ("cortical") neurons comparable to that in some non-ape primates such as baboons (Herculano-Houzel, 2023a). If total pallial neuron number matters for cognitive power, and it certainly should, this at the very least further emphasizes that many dinosaur types were not dull-witted.…”