Transcript No Slide Title

Were Dinosaurs Bird Brains?
Rebekah A. Wright
Department of Geosciences, University of Arizona
Tucson, AZ 85721
[email protected]
Abstract
How smart were dinosaurs? If brain size is proportional to intelligence, the size
of the braincase 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 were 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 far 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.
Introduction
How smart were dinosaurs? Throughout history
dinosaurs have been characterized as being dim-witted
creatures. These great giants are often stereotyped as being
dumb 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 brain
size to that of reptiles. He found most dinosaurs to be less intelligent than the average
crocodile. Reptiles’ brains only occupy about 50% of their braincase, so Hopson
assumed that dinosaur brains also would occupy only 50% of the braincase. He assumed
this for all dinosaurs except Troodon and the sauropods. In these dinosaurs, the cast of
the braincase was very ‘brain like’, so he used the whole endocast size.
However, it may not be accurate to assume that all dinosaurs’ brains occupied
only 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 the
whole braincase. When the size of the whole endocast is used, dinosaur EQs are equal to
or higher than most reptiles and are lower than those of most modern birds. The highest
EQs, belonging to the theropod and the ornithopod dinosaurs, are similar to those of large
ground birds.
Methods
The Encephalization Quotient (EQ) was calculated for dinosaurs, birds and
reptiles. The EQ is a ratio of the measured brain weight of an organism and the expected
brain weight of the organism based on body size. The effect of body size is corrected for
by using the equation from Bauchot (1978). The brain size of a fossil organism is
measured 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 a
log-log plot of brain size to body size, p is the weight of the organism, and z is the slope
of the line. The equation for reptiles is E=0.005P0.66 (Hopson, 1980, figure 2). The
equation 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 body
weights 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. Reptile
brain and body weights are from Crile and Quiring, 1940.
Reptile Brain vs. Body Size
Figure 2 . Modified from Hopson, 1980. This
graph yields the equation E=0.005P0.66.
Bird Brain vs. Body Size
Figure 3. Modified from Nealen and Ricklefs, 2001.
This graph yields the equation E=0.12P0.55.
Encephalization Quotient
The encephalization quotient is the ratio of the measured brain size of an
organism to its expected brain size. This ratio indicates how an organism’s brain size
measures 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 its
expected brain size. An organism with an EQ of five indicates that its brain is five times
greater than predicted by its body weight. Organisms with EQs less than one have a
brain that is smaller than predicted by its body size. The EQ is used because it shows
relative brain size among different organisms. Thus, if intelligence is proportional to
brain size, this method can be used to determine relative intelligence among dinosaurs,
reptiles and birds.
Bird EQs vs Dinosaur EQs
3
Bird EQ's
EQ Value
2.5
Dinosaur EQ's
2
1.5
1
0.5
0
Species
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
7
EQ Value
6
5
Dinosaur EQs
4
Reptile EQs
3
2
1
0
Species
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 the
fairly intelligent theropods to the least intelligent sauropods (Figure 6). When dinosaur
EQs are calculated using the reptile equation, theropod EQs are nearly 4 times those of
sauropods. 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 EQs
than those of reptiles. Reptile EQs range from around two to below one. The
ornithopods, 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 an
EQ (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 large
ground 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 as
complex as those of the large ground birds.
Dinosaur EQ Values
Herbivores
7
6
5
4
3
2
1
0
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EQ Values
Carnivores
Order
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 or
equal to that of modern reptiles. Dinosaur intelligence was between that of modern birds
and reptiles. The highest dinosaur EQs are similar to those of large ground birds, but
other dinosaurs fall well below modern birds in intelligence. However, dinosaur
intelligence is not limited to a small range. Dinosaur EQs range from 0.0334 to 0.8607
when calculated as birds and from 0.1231 to 6.28 when calculated as reptiles. Dinosaurs
were not a population doomed to extinction because of a lack of intelligence. Most could
match wits with the average crocodile and even a bird or two. Dinosaurs were definitely
not as intellectually challenged as once thought
References
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Armstrong, E., and Bergeron, R., 1985, Relative brain size and metabolism in birds,
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