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.
Which mammals are ruminants?
There are around 200 living species from six families of even-toed ungulates (hoofed animals). Three have numerous well-known members: bovids (including antelope, goats, cattle and sheep); cervids (from red deer and muntjac to reindeer and moose); and giraffids (including okapi).
The three remaining families have only a few species: antilocaprids (pronghorn), moschids (musk deer) and tragulids (chevrotains or ‘mouse deer’). The defining feature of ruminants, the stomach, first evolved 32-39 million years ago (MYA).
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 a ruminant's 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.
Any other distinctive features ruminants share?
Ruminants are more closely related to whales (also even-toed ungulates!) than to other herbivores. Both groups have specialised teeth: instead of upper incisors, for instance, many ruminants have a thick, flat dental pad for grinding plants.
Another characteristic is bony cranial appendages – protrusions that scientists call ‘headgear’. Those differ among four ruminant families: bovids have horns; cervids have antlers; giraffes have ossicones; and pronghorn have, well, pronghorns. Headgear originated 21-23 MYA and so the relatively primitive tragulid family never had any, whereas the ancestors of moschids lost their antlers later in their history.
Do the different ruminant species have different diets?
Yes. Grass and roughage eaters like cattle will graze, rest and ruminate for long periods while high-fibre food (such as cellulose in grass) is slowly digested in a large rumen, before nutrients are absorbed in a long intestine.
By contrast, ‘concentrate selectors’ like deer choose readily-digestible material (fruits, shoots and leaves) and exploit bacteria that quickly degrade plant cells to liberate nutritious contents. In between are ‘intermediate mixed feeders’ like gazelles.
From tropical forest to Arctic tundra, ruminants have colonised most terrestrial habitats, thanks in no small part to becoming the ultimate plant-eating machines.
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