Dinosaurs: Clever creatures or bird brains?

Dinosaurs, once thought of as bumbling brutes, are now considered to have included some highly intelligent forms. Similarly, birds once considered to be instinct-driven automatons (hence the pejorative phrase “Bird brain”) are now known to exhibit many intelligent behaviors including use of complex tools (Bayern et al., 2018). Intelligence -the ability to acquire and retain data to be applied toward adaptive behaviors (definition adapted from Wikipedia: “Intelligence” 2023) -is a function of both information processing capacity and speed (Kail & Salthouse, 1994). Both factors need to be taken into account when comparing capabilities across platforms or species. 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. While cold-blooded snakes, lizards, and most extant reptiles show a gradual increase of brain size with body size, warm-blooded birds—and presumably the closely related dinosaurs— have a much steeper body-to-brain curve so that birds have many more neurons than similarly sized reptilian relatives. Extrapolating warm-blooded scaling rules to dinosaurs, the author concludes that the largest-brained dinosaurs, for example, Tyrannosaurus, could have had a large number of neurons similar to modern baboons. This leads to the suggestion that these late-evolving dinosaurs may have had capabilities similar to modern baboons including use of primitive tools as well as intricate social groups. Indeed, fossil evidence also supports this, indicating complex social interactions such as shared nesting grounds and age-related herds (Pol et al., 2021). A complementary contribution by Reiner (2023) asks whether dinosaurs could ever have gained the capability of being even smarter, approaching human levels of intelligence. This study approaches the question of dinosaur intelligence from another perspective, that of potential limitations on information flow and scalability of dinosaur brains, again basing the argument on the presumed similarity between dinosaurs and birds. This study argues that despite the large number of brain neurons in dinosaurs, fundamental differences in brain organization between dinosaurs and mammals would enforce an upper limit on the processing capabilities and therefore overall intelligence in the dinosaur lineage. The human brain, like that of other mammals, is crowned by the six-layered cerebral cortex, which is the main site of complex cognition.While all mammals have a cerebral cortex, the relative volume and number of neurons of the human cortex surpass that of other mammals including our close relatives such as gorillas and chimpanzees. This evolutionary trend to a larger cortex correlates with our greater intellectual capacity. So, if mammals can expand the cortex to accommodate higher levels of information processing, Reiner asks, could bird/dinosaurs do the same? The answer may lie in the fundamental difference in the organization of mammalian cerebral cortex and the bird/dinosaur equivalent areas. The mammalian cortex is a layered sheet, so expanding the cortex largely involves expanding the sheet; the thickness and intrinsic connectivity remains relatively constant (Figure 1). While the cerebrum of birds/dinosaurs contains circuitry similar to cerebral cortex (Stacho et al., 2020), the circuit elements in birds are not organized in layers, but rather in discrete collections of neurons (nuclei) linked by streams of axons. Enlargement of that system then involves not only increasing the number of neurons in these interconnected centers, but also expanding and lengthening the axonal interconnections. Because axons conduct impulses rather slowly compared to wiring in an electronic computer, the increase in axonal length will significantly slow the processing speed in the same way a slow bus speed will limit computational power of an electronic computer. Further, as shown in Figure 1, in contrast to expanding a layered structure wherein intrinsic connectivity is constant, enlarging pools of neurons necessitates connections of different lengths, thereby impacting time of arrival of signals in the target areas. In such a network, maintaining consistency of spike train timing—crucial to cortical information processing (Singer, 2013)—would be difficult. Further, as described in more detail in the accompanying commentary by Herculano-Houzel, energetics and heat