A cow has four stomachs, right? That's what we learn at school, isn't it? Wrong! Cows are ruminants and rather than having four stomachs, their one stomach has four separate sections – the rumen, the reticulum, the omasum and the abomasum – leading to the common misconception that they have four stomachs.
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The first two chambers, the rumen and reticulum, are often so similar that they’re called a ruminoreticulum. They’re the main site of microbial activity and where the products of fermentation are absorbed (some microbes produce methane gas, which cows famously release by belching). The third chamber, the omasum, has leaf-like ‘lamellae’ that increase surface area for absorption and serve as a sieve to retain fibre for regurgitation. The fourth chamber, the abomasum or true stomach, is an acidic environment where food is digested before the resulting nutrients are absorbed in the intestine.
How do cows digest their food?
First, they masticate thoroughly. Their name comes from ‘ruminare’, Latin for ‘to chew over again’: a mixture of food and saliva forms cud as it’s repeatedly mashed, swallowed and regurgitated, which turns plant fibres into tiny particles.
The second stage of digestion is fermentation, the biochemical reactions that convert complex carbohydrates into small, energy-rich molecules such as sugars and volatile fatty acids under oxygen-free (anaerobic) conditions. In plants, cell walls are reinforced with cellulose, a carbohydrate that’s broken down by the enzyme cellulase. The metabolism of a mammal can’t make cellulase, but symbiotic microorganisms in its digestive system can. And in ruminants, those microbes inhabit a specialised stomach.