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First published online December 14, 2006
Journal of Experimental Biology 210, vi (2007)
Copyright © 2007 The Company of Biologists Limited
doi: 10.1242/jeb.02638
Outside JEB |
BIG DINOSAURS: HOT OR NOT?
Stellenbosch University
jst{at}sun.ac.za
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Cold blooded animals like lizards can't generate their own heat. If it's a particularly cold day, their body temperature will fall, forcing them to take up basking in the sun in an effort to warm up. By contrast, warm blooded animals use their metabolism to stay warm, and maintain a constant body temperature. While it is relatively easy to study the cold blooded creatures alive today, evolutionary biologists have long puzzled over how the dinosaurs maintained body temperature: were they warm or cold blooded? After years of puzzling over the problem, James Gillooly and colleagues may not have definitively answered this question, but they have come up with a way of estimating a dinosaur's body temperature in a recent PLoS publication.
In the early 1970s, thermal biologists Jim Spotila, Paul Lommen, George Bakken and David Gates suggested that some dinosaurs may have taken advantage of their enormous body size to maintain their temperatures as the outside temperature fluctuated. They reasoned that animals with large bodies heat up and cool down slowly, like a large lake retains its summer heat through autumn, while smaller animals heat up and cool down more quickly. Using biophysical models, Spotila and his colleagues showed that how quickly an animal's body reacts to changes in external temperature could have been important in helping large dinosaurs stay warm when the temperature dropped and maintain relatively constant body temperatures despite fluctuation in the environment. However, Spotila's temperature regulation model has remained relatively untested due to a lack of direct evidence.
While sneaking up behind a critter with a thermometer clutched in a sweaty palm may appeal to some experimental biologists, this approach clearly does not work on extinct animals. So just how can you measure an extinct animal's body temperature? Gillooly and his team Andrew Allen and Eric Charnov solved this problem by developing a model based on modern warm and cold blooded animals that predicts their maximum growth rate according to the creature's body temperature and size. The team then ingeniously applied this model to dinosaur growth rates and body sizes to estimate the extinct creatures' body temperatures.
Gillooly and colleagues predict that larger dinosaurs had higher body temperatures than smaller dinosaurs, supporting Spotila's 1970s ideas. When they put dinosaur body size data into their model, the results supported this relationship: larger dinosaurs would have been warmer than smaller dinosaurs. At 13,000 kg Gillooly estimates that Apatosaurus excelsus had a body temperature of approximately 41°C, while at the opposite end of the size range the 12 kg Psittacosaurus mongoliensis had a body temperature of about 26°C.
Finally, Gillooly's team needed to test the model in a living cold blooded animal to see if their predictions were on the right track. They used temperature and body mass data for crocodiles, the dinosaur's closest living relatives, to test temperature variations with body size. The body temperature changes estimated by the model closely agreed with the temperature fluctuations measured in crocodiles over a range of sizes. Their results provide some of the first evidence that dinosaur body temperatures increased with body size, supporting the idea that larger dinosaurs could keep their heat when the weather got a bit chilly.
References
Gillooly, J. F., Allen, A. P. and Charnov, E. L. (2006). Dinosaur fossils predict body temperatures. PLoS Biology 4,e248 . DOI:10.1371/journal.pbio.0040248 .[CrossRef][Medline]
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