WITH EVOLUTIONARY BIOLOGIST JV CHAMARY
INSTANT EXPERT
While a plant-based diet has health benefits for humans, unfortunately vegetation is largely indigestible to most mammals and passes through the body as ‘fibre’ – not the best way to get energy from food! Ruminants, however, can efficiently extract calories using physical and chemical processes to break down plant matter.
How do ruminants digest fibre?
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.
What’s special about the stomach?
It has multiple chambers. 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.
But other herbivores eat plants!
Yes, but they don’t digest vegetation as well as ruminants. The majority of mammals, from rabbits to horses, have stomachs with a single chamber (they’re monogastric) and fermentation occurs in the hindgut. Ruminants squeeze more energy out of food by fermenting plant carbohydrates in the foregut, before they reach the stomach. Some herbivores, such as camels, chew cud and have multi-chambered stomachs too – they’re called pseudoruminants.
Which mammals are ruminants?
There are around 200
