How brainy was Tyrannosaurus rex? It is dependent upon who you ask.
Final yr, the enduring Cretaceous dinosaur obtained the glow-up of a lifetime when Suzana Herculano-Houzel, a neuroscientist at Vanderbilt College in Nashville, calculated that the predator had 3.3 billion neurons in a single a part of the forebrain alone. The invention put T. rex’s forebrain on par with trendy baboons’.
The discovering raised eyebrows — and doubts. Any suggestion that these dinosaurs have been as sensible as primates “looks as if a big leap,” says Cristián Gutiérrez-Ibáñez, a comparative neuroscientist on the University of Alberta in Edmonton, Canada. “Having the identical variety of neurons as a primate doesn’t make you a primate.”
Now, Gutiérrez-Ibáñez and colleagues have provide you with a a lot extra conservative neuron depend. The T. rex telencephalon, part of the forebrain concerned in sensory, cognitive and motor capabilities, had nearer to 360 million neurons, researchers report April 26 within the Anatomical Document. The brand new estimate means that T. rex’s forebrain is extra much like that of recent crocodiles than of primates, the researchers say.
Calculating what number of neurons an animal has requires realizing how densely packed the mind cells are. That may be powerful not solely as a result of neuron density varies broadly between animals, but in addition as a result of these mind cells aren’t effectively preserved within the fossil file (SN: 11/14/13). When scientists take a look at extinct animals, they have to use the neuron densities of recent kin as proxies.
Right here lies the difficulty. T. rex is said to each reptiles and birds. However these two teams have vastly totally different neuron densities — reptiles have fewer neurons per sq. centimeter of mind than birds. When calculating the variety of neurons in extinct theropods, the dinosaur group that features T. rex, researchers should determine whether or not to make use of the neuron densities of birds, reptiles or some mixture of the 2.
Within the 2023 examine, Herculano-Houzel calculated the ratio between mind measurement and physique mass of round 30 dinosaurs after which checked out how they stacked up in opposition to trendy birds and reptiles. Her calculations utilizing historical hen households (such because the group that features pigeons) hinted that theropod brains have been extra like hen brains than that of different dinosaurs. Thus, she used the neuron densities of modern birds which are most carefully associated to theropods to calculate what number of neurons T. rex had.
Even when birds reside dinosaurs, that assumption is flawed, Gutiérrez-Ibáñez says. Including a broader vary of residing birds to the comparability of brain-to-body ratios introduced T. rex’s extra consistent with that of scaled reptiles, he and colleagues argue within the new examine.
Herculano-Houzel’s examine additionally inflated the numbers of neurons by assuming that dinosaur brains occupied the entire braincase, like trendy hen brains do, Gutiérrez-Ibáñez says. The mind of T. rex and plenty of different dinosaurs floated in fluid — a trait present in trendy crocodiles.
Gutiérrez-Ibáñez’s workforce recalculated T. rex’s mind measurement utilizing a smaller mind quantity, which reduce down the quantity of neurons within the telencephalon from 3.3 billion to 1.2 billion. Utilizing reptile neuron density lowered the quantity even additional to between 245 million and 360 million.
Herculano-Houzel says that her examine did account for dinosaur brains not fully filling the braincase. She can also be not satisfied by the brand new findings.
Gutiérrez-Ibáñez and colleagues made a “deadly mistake” of their assumptions concerning the body-to-brain ratio that led to the decrease cell depend, she says. By throwing in distantly associated birds like pelicans and penguins — which have essentially totally different body-to-brain ratios — Gutiérrez-Ibáñez’s workforce arrived on the incorrect conclusion, she says.
Whether or not T. rex had lots of neurons to spare may not be a superb indicator of the dinosaur’s total brainpower anyway, says Amy Balanoff, an evolutionary biologist at Johns Hopkins College of Medication who was not concerned in both examine.
Even when T. rex had a neuron density on par with primates, the cells in all probability helped the dinosaur transfer its massive physique or collect sensory data. As an example, the odor facilities of the T. rex forebrain have been big, Balanoff says. “If there have been numerous neurons within the telencephalon, many of those are going to be dedicated to processing olfactory data.”
Nonetheless, Balanoff says, “I do actually admire the unique examine for beginning the dialog. You may’t transfer science ahead in case you’re unwilling to place the info on the market.”
