were dinosaurs bird brains? rebekah a. wright department of geosciences, university of arizona...
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Were Dinosaurs Bird Brains?
Rebekah A. Wright
Department of Geosciences, University of ArizonaTucson, AZ 85721
Abstract
How smart were dinosaurs? If brain size is proportional to intelligence, the sizeof the braincase can be used to compare dinosaur intelligence among dinosaurs and to theintelligence of living reptiles and birds.
I used the Encephalization Quotient (EQ) to compare dinosaur intelligence toliving birds and reptiles. The EQ is the ratio of the measured brain size and the expectedbrain size of an organism. The measured brain size is the volume of the braincase, andthe predicted brain size comes from a log-log plot of the brain size and body size ofliving birds and reptiles. The equation, introduced by Jerison (1969), gives the predictedbrain size: E=kPz, where E is the expected brain size, k is the y-intercept, P is the bodysize of the organism, and z is the slope of the trendline.
Hopson (1980) compared dinosaur brain sizes with those of living reptiles. Hecalculated the EQs assuming that dinosaurs were more like reptiles and that their brain, asin living reptiles, occupied only half of the brain case. Hopson used the brain size tobody size relationship in living reptiles, E=0.005P0.66 and found that most dinosaurs werenot as intelligent as the average crocodile. I recalculated the dinosaur EQs assuming thatthe brain occupied the entire brain case and found that only the sauropods Brachiosaurusand Diplodocus were far less intelligent than the average crocodile.
I also compared the dinosaur EQs with those of birds. I assumed that dinosaurswere more similar to birds and that their brain would, like living birds, occupy the entirebrain case. I used recent dinosaur body size estimates and the bird brain size to body sizerelationship, E=0.12P0.55 (Nealen and Ricklefs, 2001) to calculate the EQ. Using theseassumptions, I found that the EQs of theropods such as Allosaurus, Tyrannosaurus andthe coelurosaur Troodon were within the range of most ground birds such as the ostrichand 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 thesauropods remain the least intelligent.
Because it is now accepted that birds are the closest relatives to dinosaurs, it isreasonable to believe that dinosaur intelligence should be modeled on that of birds ratherthan 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 werenot nearly as intellectually challenged as once thought.
Introduction
How smart were dinosaurs? Throughout historydinosaurs have been characterized as being dim-wittedcreatures. These great giants are often stereotyped as beingdumb lumbering reptiles.
Was a crocodile really smarter than a brachiosaur?
Vs.
Could a tyrannosaur have outsmarted an ostrich?
Hopson (1980) used the encephalization quotient (EQ) to compare dinosaur brainsize to that of reptiles. He found most dinosaurs to be less intelligent than the averagecrocodile. Reptiles’ brains only occupy about 50% of their braincase, so Hopsonassumed that dinosaur brains also would occupy only 50% of the braincase. He assumedthis for all dinosaurs except Troodon and the sauropods. In these dinosaurs, the cast ofthe braincase was very ‘brain like’, so he used the whole endocast size.
However, it may not be accurate to assume that all dinosaurs’ brains occupiedonly half the braincase. Birds are now thought to be dinosaurs’ closest living relatives.Therefore, it is reasonable to believe that, like birds, the dinosaur brain occupied thewhole braincase. When the size of the whole endocast is used, dinosaur EQs are equal toor higher than most reptiles and are lower than those of most modern birds. The highestEQs, belonging to the theropod and the ornithopod dinosaurs, are similar to those of largeground birds.
Methods
The Encephalization Quotient (EQ) was calculated for dinosaurs, birds andreptiles. The EQ is a ratio of the measured brain weight of an organism and the expectedbrain weight of the organism based on body size. The effect of body size is corrected forby using the equation from Bauchot (1978). The brain size of a fossil organism ismeasured from its endocast. The endocast is a cast of the brain cavity. See Figure 1.
The expected brain size is calculated using an equation introduced by Jerison(1969): E=kPz, where E is the expected brain size, k is the y-intercept obtained from alog-log plot of brain size to body size, p is the weight of the organism, and z is the slopeof the line. The equation for reptiles is E=0.005P0.66 (Hopson, 1980, figure 2). Theequation used for birds is E=0.12P0.55 (derived from Nealen and Ricklefs, 2001, figure 3).
Brain size estimates for dinosaurs were obtained from Hopson, 1980. The average bodyweights for dinosaurs were used to calculate dinosaur EQs. These are from Peczkis,1994. Brain and body weights for birds are from Armstrong and Bergeron, 1985. Reptilebrain and body weights are from Crile and Quiring, 1940.
Figure 2 . Modified from Hopson, 1980. This graph yields the equation E=0.005P0.66.
Reptile Brain vs. Body Size
Figure 3. Modified from Nealen and Ricklefs, 2001.This graph yields the equation E=0.12P0.55.
Bird Brain vs. Body Size
Encephalization Quotient
The encephalization quotient is the ratio of the measured brain size of anorganism to its expected brain size. This ratio indicates how an organism’s brain sizemeasures up to what it should be after correction for body size.
An animal that has an EQ of one has a measured brain size the same as itsexpected brain size. An organism with an EQ of five indicates that its brain is five timesgreater than predicted by its body weight. Organisms with EQs less than one have abrain that is smaller than predicted by its body size. The EQ is used because it showsrelative brain size among different organisms. Thus, if intelligence is proportional tobrain size, this method can be used to determine relative intelligence among dinosaurs,reptiles and birds.
Bird EQs vs Dinosaur EQs
0
0.5
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Species
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Figure 4. Dinosaur EQ values vs bird EQ values. The most intelligent bird is the great raven and the least intelligent is the hummingbird. The most intelligent dinosaur is Troodon and the least intelligent is Diplodocus.The most intelligent dinosaurs have EQs that are higher
or equal to that of the large ground birds.
Reptile EQ vs Dinosaur EQ
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Figure 5. Dinosaur EQ values vs reptile EQ values.
This graph shows that most dinosaurs have EQs that are higher or equal to those of reptiles.
Discussion
Dinosaurs, like other organisms, varied in intelligence. They ranged from thefairly intelligent theropods to the least intelligent sauropods (Figure 6). When dinosaurEQs are calculated using the reptile equation, theropod EQs are nearly 4 times those ofsauropods. When calculated as birds, the EQ difference is nearly six times the sauropods.There is even a wide range of intelligence levels within dinosaur orders.
Theropods, ornithopods, and Archaeopteryx (a primitive bird) have higher EQsthan those of reptiles. Reptile EQs range from around two to below one. Theornithopods, ceratopsians and ankylosaurs all fall within the range of modern reptiles.The stegosaurs and sauropods are below modern reptiles.
The most intelligent of the sampled dinosaurs is the coelurosaur, Troodon, with anEQ (when calculated as a reptile) of 6.28. The next most intelligent are the tyrannosaurs,allosaurs and anatosaurs. These dinosaurs have EQs equal to or just below that of largeground birds. The rest of the dinosaurs fall below the range for the large ground birds.This could imply that theropods and possibly ornithopods could have had behaviors ascomplex as those of the large ground birds.
Dinosaur EQ Values
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Order
EQ
Val
ues
Carnivores Herbivores
Figure 6. Dinosaur EQ values by order. This shows that Theropoda, carnivores, had
the highest EQ values when compared to herbiverous orders.
Conclusions
Dinosaur intelligence was lower than that of most modern birds, but was higher orequal to that of modern reptiles. Dinosaur intelligence was between that of modern birdsand reptiles. The highest dinosaur EQs are similar to those of large ground birds, butother dinosaurs fall well below modern birds in intelligence. However, dinosaurintelligence is not limited to a small range. Dinosaur EQs range from 0.0334 to 0.8607when calculated as birds and from 0.1231 to 6.28 when calculated as reptiles. Dinosaurswere not a population doomed to extinction because of a lack of intelligence. Most couldmatch wits with the average crocodile and even a bird or two. Dinosaurs were definitelynot as intellectually challenged as once thought
References
Anderson, J.F., Hall-Martin, A., and Russel, D.A., Long-bone circumferance andweight in mammals, birds and dinosaurs, Journal of Zoology, vol 207, Pp 53-61Armstrong, E., and Bergeron, R., 1985, Relative brain size and metabolism in birds,Brain Behav. Evol. Vol 26, Pp 141-153Bauchot, R., 1978, Encephalization in vertebrates, Brain Behav. Evol., vol 15, Pp 1-18Clos, L.M., 1998, Brain/body allometry in dinosaurs: Implications of endothermy, FossilNews Journal of Amateur Paleontology, vol 4(1), Pp 4-9.Hopson, J.A., 1980, Relative brain size in dinosaurs implications for dinosaurianendothermy in Thomas, R.D.K., and Olsen, E.C., A cold look at the Warm BloodedDinosaurs, American Association for the advancement of Science Selected SymposiaSeries. Pp 287-310.Hopson, J.A., 1977, Relative brain size and behavior in Archosaurian reptiles, AnnualReview of Ecology and Systematics, Vol 8, Pp 429-448.Jerison, H.J., 1973, Evolution of the brain and intelligence, Academic Press New Yorkand London.Nealen, P. M. and Ricklefs, R.E., 2001, Early Diversification of the avian brain:bodyrelationship, Journal of Zoology, London, vol 253, Pp 391-404Peczkis, J., 1994, Implications of body-mass estimates for dinosaurs, Journal ofVertebrate Paleontology, vol 14(4), Pp 520-533.