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Two ways to rent a stomach

Why Do Grass-Eaters Have Such Different Insides?

A cow chews all day and a horse gallops off after a quick mouthful. They eat the same grass. The difference is where each of them keeps its microbes.

A cow Keeps its microbes infront of the stomachA horse Keeps them behind it
One grass, two completely different machines.

  1. Step 1 of 5

    No animal alive can digest grass.

    Grass, lockedshut withcellulose Only microbescarry the key

    Not one. Grass locks its goodness inside cellulose and no animal makes anything that cuts it, so every grass-eater on Earth is really a container for borrowed microbes that can.

  2. Step 2 of 5

    There are only two places to keep them.

    Front fermenter Tank first, stomachsecondBack fermenter Stomach first, tanksecond

    Either the tank comes before the stomach, so everything is fermented first and then digested, or it comes after, so everything is digested first and then fermented. Almost every grass-eater is one or the other.

  3. Step 3 of 5

    In front is thorough, and slow.

    What it gets from a mouthfulVery little iswastedHow quickly it can move onHours ofstanding andchewing

    Everything passes the microbes before anything is absorbed, so almost nothing is missed. The animal then digests the microbes themselves, which is where a surprising amount of its protein comes from. The price is hours a day standing still and chewing.

  4. Step 4 of 5

    Behind is wasteful, and fast.

    Fermenting behind thestomachEats fast, stays mobile, can gallopA lot passes straight through unused
    An elephant puts away 150 kg of plants a day and digests under half of it.

    A horse ferments what is left after digestion, so a lot goes straight through unused. In exchange it can eat quickly, keep moving and run — and an elephant simply eats more, passing so much undigested that whole seeds sprout from its droppings.

  5. Step 5 of 5

    A rabbit gets round it by eating the meal twice.

    Grass goes through once The tank sitspast everythingthat absorbs So the softpellets goround againTwice

    Its tank is at the far end, past everything that absorbs, so the first pass wastes most of the goodness. So it produces a special soft pellet, eats it straight away, and runs the same meal through from the top a second time.

    This is why a rabbit that grows too stiff or too fat to reach its own back end becomes seriously ill — it is being starved by a meal it cannot collect.

The short version

Every grass-eater rents microbes, and whether the tank sits before or after the stomach decides how much it gets from a mouthful and how fast it can move.

Try it yourself

Look at a horse and a cow in the same field. The cow will be lying down chewing; the horse will be walking and eating at the same time.

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