Eat your greens! The secrets to how herbivores grow so large

The largest land animal that ever lived was a vegetarian. When a local farmer stumbled upon a thighbone in Patagonia, he initially assumed it was a gigantic segment of petrified wood. But as paleontologists from around the world began to excavate, they found the fossilised remains of perhaps the largest animal species ever to walk the earth.

Lying before them was a new species of Titanosaur, measuring 40 metres long, 20 metres high and estimated to weigh up to 90 tons.  What makes the existence of these lumbering behemoths all the more incredible is that they were generalist herbivores. They obtained all the energy necessary to reach such gigantic proportions from a relatively nutrient poor diet of plants.

In fact, the top 10 largest land animals to ever live were all vegetarians. The fossil record and Earth’s living biodiversity is littered with massive herbivores, so how do plant-eating animals reach such a size, and why?

If you’re a herbivore there’s three things you should worry about. Eating enough, reproducing, and doing those first two things before you get eaten yourself. Let’s find out how they do it.

Eating Enough


Herbivores are abundant and widespread, partly because there’s plenty of food. An old ecological rule says that for every trophic level you move up the food chain, 90% of energy is lost. So feeding from the most basic level—autotrophs that convert energy from the sun (plants to you and me)—is a good strategy as there’s plenty of energy. That’s why, for example, there are a lot less lions than there are wildebeest, because there’s a lot less lion food than there is wildebeest food. But that’s only useful if you can get energy out of the food you are eating.

One problem with eating plants is that cellulose—which makes up a large part of the plant cells, and is probably one of the most abundant chemicals on Earth—is hard to digest. This means that many animals can’t extract energy from it. Tackling this might be why some species grew so large, an adaptation to house all of the appropriate digestive organs. In the same way that effective carnivores need certain traits such as big teeth, quick reactions and perhaps claws or talons to feed, so too do herbivores have their own unique adaptations. The most important of these are stomachs (yes, plural!)

Digestive Adaptations


Whilst most omnivores and carnivores have just one, many specialist herbivores are ruminants with as many as four stomachs. Ruminants ferment their food prior to digestion, which often involves regurgitation and rechewing to help break down stubborn plant matter. Some of the largest ruminants are giraffe, yaks and even some kangaroos, but to explain the enormity of our Titanosaur and other ancient sauropods we have to think bigger.

‘When animals get bigger, digestive time goes up’ says Dr David Hone, a palaeontologist from Queen Mary University of London. ‘As you know, a large part of plants is cellulose which is hard to break down, but if you just increase the amount of time it spends in the body, it does eventually digest and then you get a massive return.’ This could go some way towards explaining the advantages of huge size.

Other scientists disagree and attribute a key limitation, amazingly, to chewing. If you evolve to chew your food, you have to devote a certain amount of your foraging time to chewing. ‘We think this is the reason why chewers—mammals, but also the chewing dinosaur groups—never surpassed the 15-20 ton threshold’ says Marcus Clauss a Professor of Comparative Digestive Physiology, Nutrition and Biology at the University of Zurich. Sauropods, like our enormous Titanosaur from Argentina didn’t chew, they just swallowed and let their digestive system do all the work.

Bigger is better (and more attractive)


Being bigger also offers obvious advantages when competing with others of the same species—termed intra-specific competition. Think for example, of competition over a food source or a harem of females.

This has to balance with the associated costs of being larger (needing more resources), but overall is a plausible explanation for slow increase in size over generations.

Interestingly, there’s evidence that human impacts on some wild species are actually forcing them to become smaller. Big horn sheep (Ovis Canadensis) in North America have been hunted as trophies for decades with hunters preferentially targeting the largest specimens. This is biasing the surviving gene pool towards smaller individuals, so each generation is on average getting slightly smaller.

Escaping the food chain


Ultimately, it’s predators that really worry herbivores. If you could exclude predators, then you could take all the time in the world for eating and reproducing. And that leads us to one of the coolest suggested adaptations. One way to reduce the number of things that can eat you is to grow big, really big.

‘Escaping predators is a big deal’ says Dr Hone excitedly. ‘Not being eaten will generally increase your reproductive output!’ 

Predators normally target prey that is smaller to avoid risk of injury or death. Very occasionally, a particularly bold (or desperate) pride of lions takes a small elephant, or a pod of orcas might take a young whale, but generally, size can be a herbivore’s best defence.

So, could that be the reason Titanosaurs grew so large? There were certainly some monstrous carnivores around at the time, but the largest T-Rex we know about weighed in at only seven tons, tiny in comparison. Surely they had nothing to worry about from predators.

Eventually though, their enormous bulk may have been a downfall. It’s thought that the fossilised individual dug up in Argentina probably became stuck in mud, perhaps during a storm. There it has lain for 95 million years, waiting to for someone to start unraveling its story.

Got a penchant for plant eaters? A leaning towards leaf eaters? Maybe a heart for herbivores? In which case, why not have a flick through our exclusive video streaming service, filled with plenty of planty documentaries from right across the world of wildlife. Available now.