|Evolution in the News - July 2015|
|by Do-While Jones|
The dinosaur body temperature compromise remains controversial.
In this month’s feature article, we said that, sixty years ago, dinosaurs were known to be cold-blooded reptiles. Back then it was inferred that dinosaurs had to have been cold-blooded because they were so big they could not possibly eat enough food to nourish their huge bodies. Furthermore, big, warm-blooded animals produce lots of internal heat when moving, with proportionally less surface area to allow that heat to escape, so they would have had a “heat shedding” problem. Dinosaurs could not be warm-blooded.
A few years later, some scientists said dinosaurs could not be cold-blooded because a large cold-blooded animal could not absorb enough heat from the environment to remain active, especially in cold weather. Therefore, they must have been warm-blooded, like birds.
We examined this long-standing controversy last August 1, partly because of an article that ran in Science just a few weeks previous to that newsletter. That Science article stated,
Over the past few decades, the original characterization of dinosaurs by early paleontologists as lumbering, slow-metabolizing ectotherms [cold-blooded animals] has been challenged. Recent studies propose that dinosaurs were capable of an active lifestyle and were metabolically similar to endothermic [warm-blooded] mammals and birds.
… recent advances in metabolic theory provide a theoretical framework for evaluating metabolic rate on the basis of growth.
We used a comparative approach to characterize the energetics of dinosaurs and other extinct taxa. We examined the empirical and theoretical relationship between growth and resting metabolic rate, using a broad database of major vertebrate clades, and used our results to examine the energetics of Mesozoic dinosaurs. From empirical studies, we constructed ontogenetic growth curves and determined a maximum rate of growth for each species. Environmental temperature was standardized by only considering growth rates in ectotherms from tropical and subtropical climates or from laboratory settings between 24° and 30°C, comparable to temperatures experienced by dinosaurs during the Mesozoic.
Our results find that mass-independent growth rates in dinosaurs were intermediate to, and significantly different from, those of endothermic [warm-blooded] and ectothermic [cold-blooded] taxa (table S2). Although some dinosaur growth rates overlap with high-power ectotherms or low-power endotherms, they cluster closest to energetically and thermally intermediate taxa, such as tuna (Fig. 2). Further, our analyses uphold the somewhat surprising finding that feathered dinosaurs, including protoavian Archaeopteryx, did not grow markedly differently from other dinosaurs (Fig. 4). It appears that modern avian energetics did not coincide with feathers or flight, which is consistent with fossil evidence that modern bone histology in birds did not appear until the late Cretaceous.
Past work has often struggled to fit dinosaurs into a simple energetic dichotomy; our work suggests that an intermediate view is more likely. Although dinosaur growth rates vary, they cluster most closely to those of thermally intermediate taxa (Figs. 1 and 2), which we term mesotherms. Mesothermic tuna, lamnid sharks, and the leatherback turtle rely on metabolic heat to raise their body temperature (Tb) above the ambient temperature (Ta) but do not metabolically defend a thermal set point as endotherms do. This reliance on metabolic heat distinguishes them from other large homeothermic reptiles, such as crocodiles, which bask to elevate Tb. 2
At the risk of oversimplifying their argument, here’s what they claim in plain English. Bones have growth rings sort of like tree rings. They claim to be able to determine the rate of growth of an animal by looking at these rings, and they believe they can accurately correlate body temperature with growth rate, assuming what the atmospheric temperature was at the time. Their conclusion was that dinosaurs were passive, cold-blooded creatures most of the time; but they could burn fats and sugars when necessary to become active, warm-blooded creatures for short periods of time.
(In Jurassic World, the fictional dinosaur Indominus rex was just the opposite. Most of the time, she was warm-blooded; but she could intentionally become cold-blooded in order to hide from infrared cameras by assuming the same temperature as the environment.)
Grady’s medium-temperature (neither warm-blooded nor cold-blooded) metabolic compromise prompted this reaction from D’Emic and Myhrvold:
Grady et al. (Reports, 13 June 2014, p. 1268) suggested that nonavian dinosaur [that is, dinosaurs that weren’t actually birds] metabolism was neither endothermic nor ectothermic but an intermediate physiology termed “mesothermic.” However, rates were improperly scaled and phylogenetic, physiological, and temporal categories of animals were conflated during analyses. Accounting for these issues suggests that nonavian dinosaurs were on average as endothermic as extant placental mammals. 3
The body of the rebuttal goes on to say,
Improper conversion from annual to daily time scales leads to underestimation of the growth rates of extinct animals such as dinosaurs.
Estimating the duration of growth within a year from bone tissue is a complex topic for future skeletochronological research to address. Just as scaling down from annual to daily growth rates is not straightforward, scaling up of daily growth rates can be misleading.
In other words, the regression computed by Grady et al. [figure 1 in (1)] conflated clades with other categories of animals. A fairer comparison would be to compare clades only, which would mean including birds within Dinosauria, a category that would overlap the distribution of mammalian data (Fig. 1D). 4
D’Emic said that Grady’s analysis is wrong. He used a mathematical argument only someone with a PhD in statistics could understand (which we mercifully did not inflict upon you). Grady responded with an equally obtuse mathematical argument, accusing Myhrvold of misapplying statistical techniques, with a small concession at the end.
Subsequent analyses by Myhrvold to reassess the relationship between basal metabolic rate and growth are conceptually flawed.
Characterization of dinosaur growth offers a rare metric for assessing paleoenergetics because growth is a continuous, quantitative trait that can be directly linked to metabolism and body temperature. Our analyses suggest that dinosaurs were energetically intermediate and that mesothermy was likely widespread. Despite our disagreements, we concur with both that dinosaur growth studies have room for improvement. 5
We did not report this earlier because although Science received the comment by D’Emic and Myhrvold for publication on 15 August 2014, and Grady responded on 5 September 2014, both letters weren’t published until 29 May 2015. We don’t know why, after holding these comments for so many months, Science finally decided to print them.
Although we don’t know why Science waited so long to publish the rebuttal and re-rebuttal, we can tell you why we mention them now. Quite simply, the June newsletter was already finished on 29 May, and we knew the July newsletter would be all about dinosaurs. That explains our timing; but not the reason for publishing this column.
Here’s the take-away from the whole discussion: Scientists still don’t agree as to whether dinosaurs were warm-blooded, cold-blooded, or mesothermic—and they probably never will because there aren’t any dinosaurs left (except birds ) whose temperatures can be measured. It is all unverifiable speculation—which isn’t science. As long as somebody is willing to finance the research, there are people who will take the money and come up with a new conclusion.
(And, as an aside, the fact that one has to assume a radically different climate before humans started causing climate change, strongly suggests that climate change might not be the result of human activity—but nobody really wants to open that can of worms! )
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Disclosure, August 2014, “Dinosaur Delusions”, http://www.scienceagainstevolution.info/v18i11f.htm
2 John M. Grady, Science, 13 June 2014, “Evidence for mesothermy in dinosaurs”, pp. 1268-1272, http://www.sciencemag.org/content/344/6189/1268.full
3 M. D. D’Emic, Science, 29 May 2015, “Comment on ‘Evidence for mesothermy in dinosaurs’”, p. 982, http://www.sciencemag.org/content/348/6238/982.2.full?sid=940b2687-402e-4cd8-bdff-9cf5a5dac524
5 Grady, et al., Science, 29 May 2015, “Response to Comments on ‘Evidence for mesothermy in dinosaurs’”, p. 982, http://www.sciencemag.org/content/348/6238/982.4.full?sid=940b2687-402e-4cd8-bdff-9cf5a5dac524