Thursday, May 10, 2012

Dinosaur Farts: Climate Driver or Just Gas?

Hmmm, how do you begin a serious discussion about dinosaur farts? Maybe I should call it flatulence? How about methane emissions from sauropod posteriors? To be honest, it hasn't ever been something I've thought about before. A correspondence paper, published this month in Current Biology,on this topic caught my attention, and it caught the attention of several news outlets. Makes sense, I suppose. As one article put it, "It sounds like perfect journalist bait." Obviously perfect blogger bait as well. So I wanted to take a closer look at this paper and see what it's really talking about.

The paper, by researchers David Wilkinson, Euan Nisbet and Graeme Ruxton, concerns the methane produced by sauropod dinosaurs and if it helped to drive Mesozoic climate warmth. You'll remember our talk about sauropods from the Antarctic Sauraopod: No Longer a Cold Case post from January. These were often really really big creatures with a high level of diversity and large geographic range. It has even been suggested that they may have been a keystone species in many Jurassic and Cretaceous ecosystems.

But let's first start of with a modern comparison, probably one with which you are familiar: Livestock. Ruminant animals (cows, sheep, buffalo, and goats) have a unique digestive system that can convert otherwise unusable plant material into food. This digestive system, produces methane, a potent greenhouse gas that can affect climate directly through its interaction with long-wave infrared energy and indirectly through atmospheric oxidation reactions that produce carbon dioxide. A paper published in the Journal of Animal Science in 1995 estimates that ruminant livestock can produce 250-500 L (66-132 gallons [US, liquid]) of methane per day. The U.S. EPA estimates this to contribute about 80 million metric tons of methane annually. Ruminant livestock are one of the largest methane sources in the world. If you look specifically at cattle (as the J. Anim. Sci article does), they typically emit six percent of their ingested energy as methane. In the U.S. (which as about 100 million cattle) this accounts for about 5.5 million metric tons of methane per year. That's 20 percent of the country's methane emissions! I bring up this comparison because it is directly compared to sauropod emissions in this paper.

Now, dinosaurs, particularly sauropods, are a lot bigger than cows. But in terms of abundance, they were probably less numerous, with only a few tens of individuals per square kilometer. The digestive biology of these animals has, for a long time, stumped paleontologists. Sauropods have small teeth that are shaped for gripping and clipping plants, but not really for chewing or mashing those plants. How these plants were broken down is a bit of a mystery. There have been several hypotheses from small, swallowed stones called gastroliths to microorganism-assisted fermentation. The latter, as Wilkinson, Nisbet and Ruxton point out, could be the methane producing mechanism in these animals. However, it is unlikely that these dinosaurs had an endothermic, mammalian-style metabolism. And very unlikely that they were ruminant herbivores. So the authors did some calculations for these dinosaurs based on modern non-ruminant herbivores where methane (litres per day) = 0.18 (body mass in kg)0.97. This means that for a 20,000 kg sauropod (about a medium sized Apatosaurus louise or "Brontosaurus"), the methane emission would be 2,675 liters per day from one animal. This scales up to 6.9 tonnes/km2 of methane per year and a global methane production of 520 Tg (520 million tonnes) annually. Their estimation is similar to the amount of methane humans are currently pumping into the atmosphere each year rather than cows specifically.

OK. So what do we take away from this? Many news outlets have seized onto this and made pretty outrageous claims ("Dinosaurs may have farted themselves to extinction"....uhh, wow). The fact is that we don't know for sure.We don't know what the digestive systems of these animals was like. We don't even know if they produced methane at all. These estimates were derived from a modern animal model based on the methane output of rabbits and guinea pigs fed a hay-only diet. They might make me hand back my degrees if I called that a really great animal corollary. But, as one article put it, "If you know about croc fart research, please chime in." Then there is the issue of sauropod abundance. The estimations of population size were derived from the fossil record of the Morrison Formation, a 150 million year old sedimentary rock sequence in the western U.S. and Canada. Is this Formation an accurate slice of a prehistoric ecosystem? While we are on the topic of ecosystem, you should also consider the Mesozoic era which had slightly shorter days, more land area, and a warm, moist climate that supported greater primary productivity. How does this factor in?

Ultimately, the paper doesn't say that dinosaurs farted themselves to extinction. It doesn't mention dinosaur extinction at all. What it actually says is that "methane was probably important in Mesozoic greenhouse warming" and that their "calculations suggest that sauropod dinosaurs could potentially have played a significant role in influencing climate through their methane emissions." That's it. Dinosaurs may have farted. And big dinosaurs may have farted bigger.

Read the correspondence paper for yourself here:

ResearchBlogging.orgWilkinson, D., Nisbet, E., & Ruxton, G. (2012). Could methane produced by sauropod dinosaurs have helped drive Mesozoic climate warmth? Current Biology, 22 (9) DOI: 10.1016/j.cub.2012.03.042

More information about modern ruminant livestock emissions can be found at:
U.S. EPA site on ruminant livestock
Johnson, K.A. and D.E. Johnson (1995) Methane emissions from cattle. Journal of Animal Science: 73(8), 2483-2492.

And a couple of good articles about this paper:
Smithsonian's Dinosaur Tracking blog's post "Media Blows Hot Air About Dinosaur Flatulence"
Pharyngula's post "The reports of dinosaurs dying of farts are greatly exaggerated"

(image from the American Museum of Natural History)

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