Wednesday, 3 August 2011

Return of the Warm Bloods


A couple of recent studies have raised the always interesting question of whether dinosaurs were endothermic or not. Robert Eagle et al of the California Institute of Technology in Pasadena used a technique known as clumped isotope thermometry to discover if the fossilised teeth of sauropods, such as Giraffatitan, Diplodocus and Camarasaurus, could be indicative of body temperature.

What is clumped isotope thermometry (CIT)? In simple terms, CIT measures the reaction and bonding of carbon and oxygen. The technique was being used to determine the temperature of the earth during the prehistoric past when checking for carbon dioxide that was contained within minerals. The isotopes carbon₁₃ and oxygen₁₈ will clump together in low temperatures.

This technique was adapted to measure the mineral bioapatite, which makes up the enamel in teeth and, because bioapatite acts as a thermometer, it was possible to analyze at what temperature the teeth were formed. The results reveal body temperatures in large sauropods ranging from 36° to 38° - exactly the range found in mammals.

Of course, this does not mean that this is conclusive proof of endothermy in dinosaurs but it is further evidence ( if indeed any was needed) that large dinosaurs had a high body temperature and confirms the generally accepted theory that large animals quite naturally have elevated body temperatures anyway. No, the problem is how did animals the size of sauropods actually cool down bearing in mind the variables in sauropod physiology?

In an earlier post I commented that sauropods are engineering marvels and that when the secrets of the sauropods are solved, then we will have answered nearly every other question posed by the dinosaurs all at the same time. Elephants, rhino, hippo and whales suffer from overheating and even sunburn if they are not careful. Elephants cool down because they can pass blood through their greatly enlarged ears and back into the body as it cools. They also wallow in mud and dust which is cooling and gives an element of protection for the skin. And, of course, they are not averse to standing under the odd tree when they can find them.

But sauropods were so big that it is extremely unlikely that they could do any of the above. If they got down to wallow in mud it would seem unlikely that they could ever get up again. They couldn’t cover themselves with dust or mud although there have been the odd theories suggesting that sauropods may have bore a similar trunk to elephants but this seems unlikely and was recently disproved, as do having large ears since only mammals have external ears.

So it seems likely that sauropods had some other way of keeping cool – a technique that is unknown to us at this time. Eagle et al speculate that they may have discharged heat through their long necks and tails or via special air sacs situated below the skin. Worth noting that none of the above take into account the environmental temperature at the time or, something that I’ve been considering for some time, the make up of the atmosphere which was considerably different from today.

More experiments using CIT are to be used on both small and medium sized dinosaur teeth and this will prove very interesting indeed. If the results display enamel forming temperatures that are also at elevated temperatures similar to the sauropods then this would be pretty compelling evidence for endothermy in dinosaurs as a whole.

What do bones tell us?

Another recently announced paper has also added weight to the argument that dinosaurs were endothermic. Roger Seymour et al from the University of Adelaide have discovered that the size of nutrient foramina in human femora is relative to the amount of aerobic activity that can be achieved. The authors have now applied this same technique to determine the aerobic activity of dinosaurs and, unlike the previous work, a range of dinosaurs of different shapes and sizes have been examined.

Blood is supplied by the main artery in the bone through the nutrient foramina to keep the bones in ideal condition and also to effect any repairs that are required. The more active the animal, the more conditioning and repair for the bone is needed, thus the greater the blood supply needs to be. So if the foramina are large, it would appear to show that a much greater degree of metabolic activity was achieved.

Before anything could be determined about dinosaurs, the authors measured multiple femora of extant mammals and reptiles of all sizes, from the smallest to the largest and the results were unequivocal – the larger the foramina, the higher the metabolic rate. Not only that, but the foramina in mammals are about ten times the size of those found in reptiles. So how did the bones of dinosaurs measure up?

Well it turns out that the foramina in dinosaur femora are relatively bigger than even mammals. This has surprised the research team because it indicates that dinosaurs were even more active than the mammals and provides yet more indication that dinosaurs were extremely active creatures and, in all probability, endothermic animals.

The theory regarding hot blooded dinosaurs was part of the original dinosaur renaissance way back in the seventies. It’s been interesting to follow the whole dinosaurian physiological and metabolic argument since Bob Bakker’s original (and groundbreaking) suggestion (Bakker 1972). We have gone through so many different studies and discussions that it has almost become accepted that dinosaurs were something in between ectothermic and endothermic – such as Scott Sampson’s Goldilocks hypothesis (Sampson 2009).

I used to think of this as a cop-out because, despite all the evidence to the contrary ie limb posture, bone histology, fast growth and world wide distribution of dinosaurs, it has generally been hard to accept dinosaurs as fully endothermic. I too have erred on the side of caution and settled for mesothermy – the safe option.

But now, with another two lines of evidence pointing strongly yet again to dinosaurs being highly active, fully developed endothermic animals, how long will it be before we come to accept endothermic dinosaurs? Indeed, how long will it be before we go to Bob Bakker and say “You know – you were right all along!” I hope I’m around to see it.

References

Bakker, R.T., 1972. Anatomical and ecological evidence of endothermy in dinosaurs. Nature 238:81-85.

Eagle, Robert A., Thomas T├╝tken, Taylor S. Martin, Aradhna K. Tripati, Henry C. Fricke, Melissa Connely, Richard L. Cifelli, and John M. Eiler.  Dinosaur Body Temperatures Determined from Isotopic (C₁₃-O₁₈) Ordering in Fossil Biominerals. Science 23 June 2011: 1206196 Published online 23 June 2011 [DOI:10.1126/science.1206196]

Sampson, S.D., 2009. The Goldilocks Hypothesis. In: Sampson, S.D., Dinosaur Odyssey. Fossil Threads in the Web of Life. University of California Press, pp. 175-191.

Seymour, R.S., S. L. Smith, C. R. White, D. M. Henderson, D. Schwarz-Wings. Blood flow to long bones indicates activity metabolism in mammals, reptiles and dinosaurs. Proceedings of the Royal Society B: Biological Sciences, 2011; DOI: 10.1098/rspb.2011.0968

3 comments:

Alessio said...

Wow, that's really interesting!
Regarding the way sauropods could have cooled down, mmh, maybe they had some sort of large dewlaps which acted like elephants' ears (or some other soft-tissue structure which didn't fossilized), what do ya think?

Mark Wildman said...

Hi Alessio! Well as I said in the post, the only animals today that have external ears are mammals and these are a unique mammalian adaption. However, that does not mean that it is beyond the realms of possibility that there may have been some form of "fleshy" appendage that may have helped in the cooling process, no matter how unlikely it appears.

Until more fossil evidence can be unearthed, then this train of thought must remain as nothing more than pure speculation.

Alessio said...

Agree. Too bad soft-tissues rarely fossilize, especially regarding big dinosaurs... Sigh, i'm still waiting for the day when paleontologists will unearth a Tyrannosaurus Rex "mummy" (or any other big theropod, for that matter).

However i think that probably many, many dinosaur species had ornamentations and other external, ehm, "accessories" that we wouldn't even guess by just looking at their skeletons; i mean, look at modern animals, especially birds (which are, afterall, dinosaurs themselves)! ;)

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