Science Puzzle
How Can a Cow Live on Grass?
Grass is mostly cellulose, the tough material that makes up plant cell walls. Your body has no way of breaking cellulose down: eat a bowl of grass and almost none of its energy would reach you.
Yet a cow can live on almost nothing but grass and build a body weighing more than half a tonne. How does it get anything out of a food we can't digest?
The Answer
The cow can't digest cellulose either. It keeps a vast population of microbes in its stomach that do the job for it.
A cow's stomach has four chambers. The first and biggest, the rumen, holds over 100 litres and works like a warm, sloshing fermentation tank. It's packed with trillions of bacteria, along with other microbes, which make enzymes that can break cellulose apart. They feed on the grass and release fatty acids as waste, and the cow absorbs those through the rumen wall as its main source of energy.
To help the microbes along, the cow brings mouthfuls of half-digested grass back up and chews them again, which is what "chewing the cud" means. Cows can spend around eight hours a day doing it. Further down, in the later chambers, the cow digests many of the microbes themselves, which gives it much of its protein.
There's a side effect. Some of the rumen's microbes produce methane, a powerful greenhouse gas, which the cow burps out: around 100 kilograms a year from a single cow. Scientists are testing feed additives, including a type of seaweed, that cut it down.
The principle: Symbiosis. Two kinds of living thing can depend on each other: the cow gives its microbes food and a warm home, and the microbes unlock energy from grass that the cow could never reach alone.