Breaking news: T.rex had baboon-like numbers of neurons!
I never really had an interest in dinosaurs - until I started paying attention to the intersection of growth and life history, energy use, and numbers of brain neurons. How could those creatures possibly get so big?
Then, just some months ago a former collaborator published the database that was missing to complete what I had started (and that we were supposed to complete together, but that's another story): the numbers of neurons in the brains of plenty more bird species, and also the other non-avian sauropsids, a.k.a. "reptiles" - though we're not supposed to use that word anymore... and I noticed that I probably had EVERYTHING that I needed, right there, to estimate what dinosaur brains were made of. So I dropped everything else that I was doing to give myself a crash course in dinosaur biology and evolution. I like to think that my beer hour companions, Jon Kaas and Ken Catania, were amused by the regular updates on my discoveries over the months that ensued.
There was only one catch: first I needed to convince myself of whether dinosaur brains could have abided by the scaling rules of endothermic sauropsids (the birds) or, instead, by the scaling rules of ectothermic sauropsids (the aforementioned "reptiles"). Trouble was, maybe they were in-between - "mesotherms", as a study published in Science in 2014 suggested: warm-ish, neither here nor there, in an undefinable mathematical chasm. Crap.
Then, just some months ago a former collaborator published the database that was missing to complete what I had started (and that we were supposed to complete together, but that's another story): the numbers of neurons in the brains of plenty more bird species, and also the other non-avian sauropsids, a.k.a. "reptiles" - though we're not supposed to use that word anymore... and I noticed that I probably had EVERYTHING that I needed, right there, to estimate what dinosaur brains were made of. So I dropped everything else that I was doing to give myself a crash course in dinosaur biology and evolution. I like to think that my beer hour companions, Jon Kaas and Ken Catania, were amused by the regular updates on my discoveries over the months that ensued.
There was only one catch: first I needed to convince myself of whether dinosaur brains could have abided by the scaling rules of endothermic sauropsids (the birds) or, instead, by the scaling rules of ectothermic sauropsids (the aforementioned "reptiles"). Trouble was, maybe they were in-between - "mesotherms", as a study published in Science in 2014 suggested: warm-ish, neither here nor there, in an undefinable mathematical chasm. Crap.
Me standing next to a life-size cast of Sue The T. rex (the one in the Chicago Field Museum) during a visit in 2017 to the Stone Age Institute, Indiana. The thing in my hands is an also life-sized copy of Sue's endocast. It's laughed at for being tiny compared to her body - but it's actually baboon brain-sized, and so perfectly sufficient to hold baboon-like numbers of neurons in the telencephalon! Copyright of the author. Use authorized if accompanied by a link to this page.
But it turned out that respecting dinosaur diversity, that is, not treating them all as the same, quickly showed that the "mesotherms" denomination was a mathematical fantasy, akin to throwing apples and oranges in a blender and deducing, from the mixed juice, that there is such a thing in the world as the mixed fruit. Looking at the relationship between estimated brain and body size, theropod dinosaurs (which includes T. rex and all other bipedal carnivorans) scaled just like modern ostriches and emus and chickens still do, which confirms several inferences by other researchers that they were endotherms already. Conversely, most of the big quadrupeds with data available scaled instead like modern ectothermic sauropsids still do, so they were probably ectotherms. Apples and oranges. Not mesotherms.
Which was great, because that meant that I could use the scaling rules that apply to ostriches/emus/chickens and similar birds today (which arose from ancestors that were contemporary and thus true cousins to T. rex) and use the estimated brain size of T. rex and other dinosaurs to calculate their numbers of neurons. The verdict: they were the PRIMATES of their time. Alosaurus had as many neurons in the telencephalon as a monkey; T. rex had as many as a modern baboon.
Which is, frankly, terrifying. Plus, with that many neurons and what I've learned about how life history scales with those, not body mass, I could also predict that dinosaurs took about 5 years to reach sexual maturity, and had a maximum longevity predicted at 40 years, just like a baboon - enough to learn to use and craft tools and build and transmit a culture. Even worse: a dinosaur that had developed the cheat of predigesting food before putting it in their mouths COULD, in theory, have evolved to have as many neurons as... humans did, in the 2-3 million years since our ancestors started processing food, first with tools, then with fire.
I'll never look at dinosaurs the same way again. Also: Thank you, asteroid hit of 65 million years ago :)
Which was great, because that meant that I could use the scaling rules that apply to ostriches/emus/chickens and similar birds today (which arose from ancestors that were contemporary and thus true cousins to T. rex) and use the estimated brain size of T. rex and other dinosaurs to calculate their numbers of neurons. The verdict: they were the PRIMATES of their time. Alosaurus had as many neurons in the telencephalon as a monkey; T. rex had as many as a modern baboon.
Which is, frankly, terrifying. Plus, with that many neurons and what I've learned about how life history scales with those, not body mass, I could also predict that dinosaurs took about 5 years to reach sexual maturity, and had a maximum longevity predicted at 40 years, just like a baboon - enough to learn to use and craft tools and build and transmit a culture. Even worse: a dinosaur that had developed the cheat of predigesting food before putting it in their mouths COULD, in theory, have evolved to have as many neurons as... humans did, in the 2-3 million years since our ancestors started processing food, first with tools, then with fire.
I'll never look at dinosaurs the same way again. Also: Thank you, asteroid hit of 65 million years ago :)
The video above summarizes the paper, which you can find here, published in The Journal of Comparative Neurology (which - full disclosure - I now edit, BUT the submission was of course handled by a different editor).
Reference:
Herculano-Houzel S (2023) Theropod dinosaurs had primate-like numbers of neurons in their telencephalon. Journal of Comparative Neurology.