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Rebekah A. Wright
Department of Geosciences, University of Arizona, Tucson, AZ 85721
How smart were dinosaurs? If brain size is proportional to intelligence, the size of the brain case can be used to compare dinosaur intelligence among dinosaurs and to the intelligence of living reptiles and birds.
I used the Encephalization Quotient (EQ) to compare dinosaur intelligence to living birds and reptiles. The EQ is the ratio of the measured brain size and the expected brain size of an organism. The measured brain size is the volume of the braincase, and the predicted brain size comes from a log-log plot of the brain size and body size of living birds and reptiles. The equation, introduced by Jerison (1969), gives the predicted brain size: E=kPz, where E is the expected brain size, k is the y-intercept, P is the body size of the organism, and z is the slope of the trendline.
Hopson (1980) compared dinosaur brain sizes with those of living reptiles. He calculated the EQs assuming that dinosaurs are more like reptiles and that their brain, as in living reptiles, occupied only half of the brain case. Hopson used the brain size to body size relationship in living reptiles, E=0.005P0.66 and found that most dinosaurs were not as intelligent as the average crocodile. I recalculated the dinosaur EQs assuming that the brain occupied the entire brain case and found that only the sauropods Brachiosaurus and Diplodocus were less intelligent than the average crocodile.
I also compared the dinosaur EQs with those of birds. I assumed that dinosaurs were more similar to birds and that their brain would, like living birds, occupy the entire brain case. I used recent dinosaur body size estimates and the bird brain size to body size relationship, E=0.12P0.55 (Nealen and Ricklefs, 2001) to calculate the EQ. Using these assumptions, I found that the EQs of theropods such as Allosaurus, Tyrannosaurus and the coelurosaur Troodon were within the range of most ground birds such as the ostrich and the emu. Ornithopods were within and just below the ground bird intelligence range. Ankylosaurs, stegosaurs and ceratopsians all fall below the ground bird range and the sauropods remain the least intelligent.
Because it is now accepted that birds are the closest relatives to dinosaurs, it is reasonable to believe that dinosaur intelligence should be modeled on that of birds rather than reptiles. Theropods, which are believed to be the closest dinosaur group to birds, and some ornithopods, have at least the same intelligence as an ostrich. Dinosaurs were not nearly as intellectually challenged as once thought.
Hopson, J. A., 1980, Relative brain size in dinosaurs implications for dinosaurian endothermy in Thomas, R.D.K. and Olson, E.C., A Cold Look at the Warm Blooded Dinosaurs, American Association for the Advancement of Science Selected Symposia Series. Pp287-310.
Jerison, H. J., 1969, Brain evolution and dinosaur brains, The American Naturalist, Vol 103, pp 575-588.
Nealen, P. M., Ricklefs, R.E., 2001, Early diversification of the avian brain:body relationship Journal of Zoology, London, Vol. 253, pp 391-404.