Were dinosaurs warm-blooded, cold-blooded, or what? This is one of the most persistent questions in paleontology, and one that we don’t yet have a good answer to. Aside from the flaws in the question itself – life isn’t neatly divided between the warm and the cool – reconstructing a dinosaur’s physiology requires a level of understanding of squishy anatomy and behavior that can be difficult to attain from bones and traces. Yet paleontologists have continued to strive to understand the physiology of Triceratops and its ilk, which has bubbled over into another exchange over whether dinosaurs ran hot.
Just about a year ago University of New Mexico biologist John Grady and colleagues published a big-picture analysis of dinosaur physiology by looking at growth rates. From comparisons to modern animals with known physiological profiles, they concluded that dinosaurs were likely mesotherms – able to maintain a body temperature higher than that of the surrounding environment, but not at a fixed point. Like some living animals like echidnas and tuna, dinosaurs may have had what Ed Yong called a “wobbly thermostat” in his report of the study.
Given that paleontologists have been debating the details of dinosaur physiology since the time the bones of the fantastic creatures became objects of scientific attention in the 19th century, though, it’s not surprising that other researchers aren’t sold on mesothermic dinosaurs. In the latest issue of Science, Stony Brook University paleontologist Mike D’Emic and patent troll Nathan Myhrvold independently criticize the idea.
Myhrvold’s criticisms center around the charge that Grady and colleagues “deviated from accepted statistical practice” in estimating growth rate, and these errors ended up creating an “illusion” of mesothermic dinosaurs. After running the numbers according to his preferred methods, Myhrvold concludes “growth rate does not predict metabolism.”
D’Emic points out a different flaw. Dinosaur growth estimates are based on evidence from bones. This is done by counting lines of arrested growth – or LAGs – which are thought to be deposited once each year and therefore provide the basis for estimating a dinosaur’s age at death. So far, so good. But, D’Emic notes, Grady and colleagues used the annual growth rate to estimate daily growth rates for dinosaurs without accounting for two confounding factors. The first is that the researchers didn’t adjust for differences in the length and number of days at different times in the Mesozoic, and, more importantly, the daily growth estimates assumed that dinosaurs grew at a constant rate throughout the year.
Tyrannosaurus and kin probably didn’t grow at a steady rate from day-to-day. Most dinosaurs lived in highly-seasonal environments, D’Emic points out, meaning that dinosaurs likely did most of their growing in the lush wet seasons and halted growth in the harsh dry seasons. There may have only been three to nine months out of each year when dinosaurs grew. In this case, D’Emic writes, the bone tissue between each LAG might represent only a few months of time – not an entire year – and led to underestimates of growth rates in the Grady et al. study.
Doubling the dinosaur growth rates to account for these shifts in growth and running them through the regression proposed by Grady and colleagues, D’Emic found that dinosaurs clustered around placental mammals – in other words, that their growth rates suggested an endothermic physiology with a high, constant body temperature. Does this mean that dinosaurs were hot-blooded? Not necessarily. More that the adjusted data comes out that way in the framework suggested by Grady and coauthors.
While Grady and coauthors admit to some minor errors in their data set, they reaffirm their previous conclusions. Adding in Myhrvold’s statistical recommendations, the researchers write, “has virtually no effect on our conclusions” and they dismiss D’Emic’s criticisms by arguing that basing maximum growth rate from growth spurts will lead to biased results without mathematical corrections. In short, Grady and coauthors still find dinosaurs of various shapes and sizes to be mesotherms.
Over four decades since questions of prehistoric physiology helped spark the “Dinosaur Renaissance”, paleontologists are still trying to comprehend the biological basics of these beloved creatures. That dinosaurs were highly-active, typically fast-growing animals isn’t in dispute. The question is what sort of physiological profile allowed them to live so vibrantly.
Part of the problem, as D’Emic notes, is that paleontologists are still working to refine how to accurately estimate dinosaur growth rates and figure out how such biological signals relate to phenomena like metabolism. This is more likely to lead to a picture of variable dinosaurs than a saurian standard. It would actually be surprising if all dinosaurs – which ranged from feathery, pigeon-sized critters to “living tanks” and long-necked giants over 100 feet long – all shared the same physiology. Not counting the birds that carry on the Mesozoic legacy today, dinosaurs were an incredibly varied group of animals that ran around the Mesozoic world for over 180 million years. How each species fell along the continuum of ectothermy, mesothermy, and endothermy isn’t yet clear. But such arguments are a positive for paleontology. They remind us that dinosaurs were not monsters, but real animals who keep challenging us to find the right questions to draw the truth from their ancient remains.
D’Emic, M. 2015. Comment on “Evidence for mesothermy in dinosaurs.” Science. 348, 6238: 982. doi: 10.1126/science.1260061
Grady, J., Enquist, B., Dettweiler-Robinson, E., Wright, N., Smith, F. 2015. Evidence for mesothermy in dinosaurs. Science. 344, 6189: 1268-1272. doi: 10.1126/science.1253143
Grady, J., Enquist, B., Dettweiler-Robinson, E., Wright, N., Smith, F. 2015. Response to comments on “Evidence for mesothermy in dinosaurs.” Science. 348, 6238: 982. doi: 10.1126/science/1260299
Myhrvold, N. 2015. Comment on “Evidence for mesothermy in dinosaurs.” Science. 348, 6238: 982. doi: 10.1126/science.1260410