News,
An interview with palaeontologist Dr Amy Balanoff, Assistant Professor at the Center for Functional Anatomy and Evolution, Johns Hopkins University School of Medicine. In Part II, Dr Kai Caspar showed us how far an endocast will not take us, the cavity is not the brain, and neuron counts cannot be conjured from stone. This time: where in the dinosaur family tree can the stones speak most clearly?
The transition from dinosaurs to birds is a fascinating topic. However, we rarely discuss the brain aspect, which actually has some documented evidence. The behaviour, flight, is observable in living animals and if we can investigate the evolutionary change in nervous system that co-developed with flight, we might get some indications of what drives nervous system change across time and species. Having discussed in Part I how brains are investigated in the fossil record, and in Part II whether we can tell the cognitive abilities of theropods from that evidence, the question now is: what do we know about the shift in brain anatomy from dinosaurs to birds?
We are tempted to envision dinosaur nervous systems as homogeneous, yet the reality is far more diverse.
“Non-avian dinosaurs are a hugely diverse group and encompass over 100 million years of evolutionary history. We see all sorts of body plans originating within this history, so as you might imagine, their brains were probably very diverse as well. People sometimes lose sight of the fact that dinosaurs aren’t all the same but were at least as diverse as what we see in the mammals and birds of today.” Dr Amy Balanoff explains
As we discussed in previous articles (Parts I and II) it is extremely difficult to draw conclusions about the behaviours and cognitive abilities of dinosaurs based on the endocast data. Here, Dr Balanoff adds complexity in stating the probable diversity of nervous system anatomy across dinosaur species. But when do we see an expansion of the brain according to the fossil records?
“Birds are the one lineage of dinosaurs that survived the mass extinction at the end of the Cretaceous. It is in this surviving lineage where most of the behaviors that we associate with living birds first evolve, including flight. It is also along this lineage that we see the expansion of the brain. My colleagues and I published a paper in 2020 showing that the size of the brain relative to body size increases early in the history of paravian dinosaurs, which is a group that includes species such as Velociraptor and Troodon, but also Archaeopteryx and living birds.”
In other words, the brain expanded along the lineage leading to modern birds. Velociraptor and Archaeopteryx are examples of early paravians, and Archaeopteryx, as Dr Balanoff notes, already looks recognisably like a bird
“There isn’t another significant increase in relative brain size until you get up into the modern radiation of birds, which means that Velociraptor and a pigeon would have a similar relative brain size.”
Does the ‘cinema-certified genius’ Velociraptor possess a relative brain size comparable to that of a pigeon?
Now we have to be clear that the cinematic depiction of Velociraptor is not accurate, and they likely looked more like a toothed, dog-sized, flightless eagle. However, the inference we are interested in is the behavioural one. We might be tempted to assume the behavioural and cognitive abilities of these raptors based only on brain size. We need to tread carefully as overall size does not necessarily say what you think, what matters may be which regions expanded, and by how much, as Dr Balanoff explains.
“How different regions of the brain are contributing to this increase in brain size is the more interesting question and can provide us insights into their behavior. For example, the cerebrum expands dramatically within early paravians as well. The cerebrum is a large region that is doing many things, so parsing out what its expansion means is difficult, but can be related to things like sociality, parental care, or other higher cognitive functions.”
A vague hypothesis like “flight needs a big brain” is untestable. Flight should not enlarge the whole brain, it should enlarge specific, predictable regions.
“We recently published a paper using neuroimaging that shows flying birds have increased activity in their cerebellum relative to when they’re just sitting on a perch. This makes sense because the cerebellum is integrating sensory input, like touch and visual info, to help the bird plan and coordinate its movements.”
In this study, pigeons were imaged with PET scans (the same technology used in human hospitals), comparing their brains at rest versus immediately after flying (Balanoff et al., 2024). The cerebellum has many functions, several involving complex motor movements, balance and proprioception. It makes a lot of sense that this particular region is involved in flight. So did flight-related regions actually enlarge in these early paravians, or only once true birds arrived on the scene?
“We would expect that if we were going to see an increase in the size of the brain related to flight, it should be within the cerebellum. We would expect that increase to occur either prior to the evolution of flight or concurrent with it. If it happens before the evolution of flight, that increased size might be driven by some other pressures.”
“In fact, it looks like there is an increase in cerebellar size relative to total brain size somewhere around the point that oviraptorosaurs branch off of the theropod tree, at the origin of a group called Pennaraptora.”
That means the enlargement of a flight-related region evolved not with flight itself, but with life in three-dimensional space, climbing, leaping, gliding through trees, indicating the function of the cerebellum in birds.
“It is clear that most oviraptorosaurs probably weren’t flying. Their postcranial skeletons aren’t adapted for flight, but there is evidence to show that they are most closely related to scansoriopterygid dinosaurs. Scansoriopterygids have wing-like membranes and were most likely living in and gliding through trees. These dinosaurs essentially were living in three-dimensional environments. Even if they were not powered flyers (animals that are able to take off and fly under their own power), they were experiencing very similar sensory environments to birds. So, it would not be surprising to see an expansion in those regions of the brain that are highly active during flight at this point where oviraptorosaurs and scansoriopterygids branch off from the rest of the theropods.”
Scansoriopterygids, imagine a mix between a bat, dinosaur and a bird. Their brain reshaped by the demands of gliding through trees and snatching insects in the air, not by flight itself, which arrived later and inherited the equipment.
But how smart can a pigeon be? We asked this question in one of our previous articles (Are Humans Uniquely Intelligent?). From our human-centric view, they may not look very intelligent.
“Well, I can say that birds are not dumb. They are capable of complex behaviors similar to what we see in mammals. Birds can use tools, they are very social, and they take care of their young, just to name a few. They also communicate with each other. Some groups of birds have evolved vocal learning, which is the ability to use auditory input to learn and imitate vocalizations. Parrots are obvious examples of this behavior, but songbirds do it as well. Even hummingbirds are capable of learning their songs.”
“The brain of a bird is small, but so is its body. So relatively speaking they have fairly large brains. At least as large as those of some mammals.”
As we established in “Are Humans Uniquely Intelligent?”, how intelligent an animal is cannot be determined by using a human lens. They have cognitive strategies that match their specific environmental conditions. The cerebellum grew to deal with navigating through air, something humans cannot do solely by the use of our nervous system (without the use of tools).
“Additionally, the neuron density in some bird brains, like parrots, is really high. Parrots have a similar number of neurons in their cerebrum to a baboon (and that is absolute numbers, not relative).”
In Part II, Dr Caspar argued against the claim that T. rex had baboon-like forebrain neuron counts. However, parrots really do have baboon-level neuron counts, countable in actual tissue from actual brains. The same impressive number is solid when you can open the skull and count. However, it is not as strong when inferred from stone. We cannot assume neuron density from fossils. How can we infer behavioural and cognitive traits in extinct species though?
“We often think about the size of the brain as being an indicator of intelligence, but it’s important to understand that the brain is doing a lot of different things, including just maintaining the basic functions that we need to live. To get at the evolutionary history of things like intelligence or locomotory behaviors, we need to understand what parts of the brain underlie them in living animals. We can then look for these larger patterns across the evolutionary tree.”
In other words, if we know more about living brains and how they produce behavior, and we acquire more information about brain shapes across extinct species, we might actually be able to start making inferences with restraint.
Part I gave us the tool, Part II the humility, Part III the reward, a rich application, where endocasts meet a behaviour still alive today. We learned to measure first, and interpret later. Know which of the two you are doing. Here Dr Balanoff showed us how multiple lines of evidence, together with well-informed interpretations, can help us discover wondrous things about the evolutionary history of brains, like how cerebellum volume increased during the evolution of flight. The temptation of premature or inflated interpretation is easy. Restraint, however, is difficult as we are passionate about the pursuit of knowledge.
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
Balanoff, A., Ferrer, E., Saleh, L., Gignac, P.M., Gold, M.E.L., Marugán-Lobón, J., Norell, M., Ouellette, D., Salerno, M., Watanabe, A., Wei, S., Bever, G. and Vaska, P., 2024. Quantitative functional imaging of the pigeon brain: implications for the evolution of avian powered flight. Proceedings of the Royal Society B: Biological Sciences, 291(2015), 20232172. https://doi.org/10.1098/rspb.2023.2172
Ksepka, D.T., Balanoff, A.M., Smith, N.A., Bever, G.S., Bhullar, B.-A.S., Bourdon, E., Braun, E.L., Burleigh, J.G., Clarke, J.A., Colbert, M.W., Corfield, J.R., Degrange, F.J., De Pietri, V.L., Early, C.M., Field, D.J., Gignac, P.M., Gold, M.E.L., Kimball, R.T., Kawabe, S., […] Witmer, L.M., Wright, A.K., Zanno, L.E., Jarvis, E.D. and Smaers, J.B., 2020. Tempo and pattern of avian brain size evolution. Current Biology, 30(11), pp. 2026-2036.e3. https://doi.org/10.1016/j.cub.2020.03.060
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