Rhyme and Reason: Gulp by Mary Roach

Rhyme and Reason is where the excerpts I enjoy, from books I’m currently reading, come to live.

Mary Roach approaches the subject of food and what happens to it once it’s eaten with a mix of unbridled delight, ceaseless curiousity, infectious enthusiasm and a whole lot of hilarity. Which makes reading her book an incredibly enchanting experience. I’ve become a huge fan of her zealous writing and her ability to make science so much fun. I can’t wait to read everything else she’s written, which include books about dead bodies, sex, space and the afterlife.

Gulp deals with a myriad of topics you wouldn’t expect to find in a book about digestion including, as this particular excerpt shows, the existence of dragons in mythology. As the author says in the introduction, “I don’t want you to say, ‘This is gross.’ I want you to say, ‘I thought this would be gross, but it’s really interesting.’ Okay, and maybe a little gross.


Snakes don’t belch, but they can, under certain circumstances, create an inflammable eructation of literally mythical proportions. First, a little background: Many plant-eating animals lack rumens, so some fermenting takes place in the cecum, an anatomical pouch at the junction of the small intestine and the colon. These same plant-eaters – horses, rabbits, koalas, to name three – tend to have a larger-than-life cecum. Pythons and boas do too, which struck Stephen Secor as odd, because they’re carnivores. Why, he wondered, would a meat-eater need a vegetation disgestion unit? Secor theorised that perhaps these snakes had evolved ceca as a way to digest and take advantage of plant matter inside the stomachs of their prey.

To test his theory, Secor fed rats to some of the pythons in his lab at the University of Alabama and hooked them up to a gas chromatograph. He tracked the hydrogen level in their exhalations as they digested whole rats over the course of four days. He did see a spike, but it appeared long before the rat arrived at the python’s cecum. Instead, Secor suspected, the hydrogen spikes were the result of the decomposing, gas-bloated rat bursting inside the python. ‘One thing led to another.’ (Secor’s way of saying he popped a bloated rat corpse and measured the hydrogen that came off it.) Suspicion confirmed. The hydrogen level was ‘through the roof’. Secor had stumbled onto a biological explanation for the myth of the fire-breathing dragon. Stay with me. This is very cool.

Roll the calendar back a few millennia and picture yourself in a hairy outfit, dragging home a python you have hunted. Hunted is maybe the wrong word. The python was digesting a whole gazelle and was in no condition to fight or flee. You rounded a bend and there it was, Neanderthal turducken. Gazython. The fact that the gazelle is partially decomposed does not bother you. Early man was a scavenger as well as a hunter. He was used to stinking meat. And those decomp gases are key to our story. Which I now turn over to Secor.

‘So this python is full of gas. You set it down by the campfire because you’re going to eat it. Somebody kicks it or steps on it, and all this hydrogen shoots out of its mouth.’ Hydrogen, as the you and I of today know but the you and I of the Pleistocene did not know, starts to be flammable at a concentration of 4 percent. And hydrogen, as Stephen Secor showed, comes out of a decomposing animal at a concentration of about 10 percent. Secor made a flame-throwery whooosh sound. ‘There’s your fire-breathing serpent. Imagine the stories that would generate. Over a couple thousand years, you’ve got yourself a legend. He did some digging. The oldest stories of fire-breathing dragons come from Africa and south China: where the giant snakes are.

Gulp: Adventures on the Alimentary Canal by Mary Roach