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October 3, 2005 6:29 AM   Subscribe

Does microbial activity in the gut of organisms contribute to their thermoregulation?

I've been wondering about the role of exothermic bacteria, present in the digestive system, and their role in thermoregulation.

Exothermic bacteria make compost piles hot -- sometimes over 160 degrees Fahrenheit. These same bacteria are already present in digestive systems of many animals. It might be fruitful to view the digestive system as a kind of compost pile (peristaltic action effectively “turns” the pile?). The gut is a good insulator, too, so what if these bacteria create heat that contributes to the host organisms metabolic heat? In fact, it may be that the host body has to cool the intestines, since these exothermic bacteria can generate so much heat.

We know that cows and other ruminants have bacteria that essentially cause fermentation in the rumen, and fermentation is an exothermic process.

There's an ongoing debate on whether dinosaurs were warm or cold-blooded, though that distinction is itself over-simplified, since there's evidence for both. What if their digestive systems functioned as hot composters, providing them with an internal source of heat energy for accelerated metabolism, while retaining mostly cold-blooded physiological features? This might go some way in explaining the polar dinosaurs.

This potentially has broad implications. I've been googling around this topic, and haven't found anything. Looking for input, and any links to more information.

I should be working on getting a job, but I can’t help but look into things like this which become interesting to me, for some reason.
posted by yesster to Science & Nature (7 answers total)
The fermentation of gut bacteria in ruminants definitely contributes heat, as you mentioned, so that does contribute to the organism's thermoregulation.

You raise an interesting question about dinosaurs, and "warm-blooded" versus "cold-blooded" organisms. Hopefully this isn't too far off topic...

Recently the standard classification of organisms as homeotherms and poikilotherms (warm and cold blooded, or "at ambient temperature" and "stable body temperature", respectively) have fallen out of favor for the terms endotherm, ectotherm, and heterotherm.

Many large pelagic fish (especially large sharks and tuna) have blood temperatures that average above ambient, but their temperature may not be consistent throughout their body. Bluefin tuna tend to keep their blood at 30 degrees C (+/- 2 degrees), while great white sharks are usually at 5-10 degrees C above ambient. Does this make them "warm-blooded", or just able to efficiently exhange heat? It's certainly a bonus for a large predator to be able to move quickly and keep their brain warm.

I would be willing to believe that large predatory dinos are similar, or even a smaller one like a Dimetrodon (perhaps the sail is for thermoregulation?) You're talking about the big vegetarians though...I think they'd have to have a pretty impressive compost pile going to generate enough heat, but if their metabolism was slow enough it might work.
posted by nekton at 9:06 AM on October 3, 2005

You might try PubMed and the combined journal resources of your local college/university library as a start, but I imagine doing calorimetric measurements of this kind would be difficult on a living organism, let alone extinct ones.

As you've noted, you might look at ruminants as a present-day model, but ruminants ferment grass to break down the plant cell wall and recover more nutrients; they are already warm-blooded and would seem to gain litttle survival benefit from exotherms.

You might think about taking stomach bacteria cultures and doing a lot of PCR ID experiments to get a statistical picture of the varied population in the gut. Then with some bioinformatics you'd search for analogous known exotherms and maybe build an enthalpic model for further testing, with the knowledge about those orthologous organisms.

Are there any other physiological characteristics in polar dinosaurs that could account for their survival, exothermic digestive bacteria aside?

Interesting question. Good luck.
posted by Rothko at 9:07 AM on October 3, 2005

This isn't much of an answer, but: does it matter? The problem large animals have isn't so much that they have a big pile of composting material in their gut, it's a problem of scale. They have a small surface area relative to their volume, and thus a problem with dissipating waste heat. Yes, decomposition of plant material in the gut would contribute to the problem, but I expect it would be a small portion of their total excess metabolic heat.
posted by sneebler at 9:24 AM on October 3, 2005

Good question. I can't add much to the answers above, except to note that (at least most) poikilotherms (sorry, not used to new terms yet) presumably don't derive a *lot* of heat from gut exothermy since they still rely on sunshine etc (visual: lizard sunbathing on rock).

Also, someone who says "I should be working on getting a job, but I can’t help but look into things like this which become interesting to me" should consider a career in research. Seriously. Email me if you think I might be of use in that (I'm a mol biol postdoc).
posted by sennoma at 9:41 AM on October 3, 2005

Thanks for the input so far.

Anecdotal: I built a compost pile last winter. It was about one cubic yard in size. After two days, it melted snow on top of it and around it. Stayed above 140 F for three weeks, without doing anything to it. That's a crapload of heat.

Also: is there any documentation regarding alcohol production in animal digestion? Fermentation occurs, we know, so are cows, horses, etc. always low-level intoxicated?

Hah! The "drunk sauropod" scenario!
posted by yesster at 1:17 PM on October 3, 2005

Great question.

I can't add to the discussion except to point out that Dimetrodon is not a dinosaur.
posted by luneray at 2:52 PM on October 3, 2005

Oh wow. Thanks, luneray!
posted by nekton at 4:59 PM on October 3, 2005

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