Did dinosaurs have gut bacteria?

ZOE is running the largest ongoing scientific study of human nutrition in the world. As part of this research, we spend a great deal of time thinking about the gut microbiome and how it influences health.

Although ZOE’s focus lies squarely on improving human health, we can’t help thinking about how bacteria influence life more generally. We’re a curious bunch.

In this article, we attempt to answer a relatively left-field question: Did dinosaurs have a gut microbiome? 

Humans have trillions of bacteria and other microbes living in their guts. Collectively, these microbes are called the gut microbiome, and they play an important role in our health.

For instance, ZOE scientists have identified 15 “good” bacteria that are associated with good health and 15 “bad” bacteria associated with poorer health. 

If you would like to learn about the types of bacteria in your gut, start by taking our free quiz.

Now, to the matter at hand.

Walking with giants (and smaller beasts)

Dinosaurs first evolved around 230 million years ago. Around 65 million years ago, a rock the size of a small town cut their reign short. 

Although we don’t know how many kinds of dinosaurs evolved during their almost 200 million-year reign, scientists estimate there were thousands of species. 

Dinosaurs came in all shapes and sizes. They ranged from chicken-sized carnivores to herbivores the size of an office block.

It seems likely that at least some of these highly varied creatures benefited from gut bacteria. However, because bacteria are tiny — just a few thousandths of a millimeter across — and lack bony parts, they cannot leave a fossil trace. 

So, where do we begin? First, let’s answer this question…

Do all animals have a gut microbiome?

The majority of animals today do harbor bacteria in their gut. However, not all species have been shown to have a mutually beneficial relationship with microbes. 

For instance, while some ant species have a flourishing gut microbiome, others do not appear to have a particularly important gut microbiome.

According to one study of rainforest-dwelling species, ants that spend their lives off-ground in the trees are more likely to have a thriving gut microbiome than those living on the forest floor.

This, the authors explain, might be because ants living in the trees only have access to plant material, whereas ants on the ground have a wider variety of foods to choose from. 

Plant compounds are difficult for animals to digest, but our friendly microbes relish the task. ZOE recommends eating 30 different plants each week to help keep your gut microbiome healthy and happy.

It’s not just ants that lack gut bacteria. A paper published in 1978 suggested that certain crustaceans have no bacteria in their gut. And scientists have found similar results when studying 124 species of caterpillar. 

Other studies investigating butterflies showed that they do harbor gut bacteria. However, when their gut microbiome is disrupted, there is no detectable effect on the butterfly’s survival or development.

In contrast, disruptions to the human gut microbiome certainly can impact health. As one example, ZOE’s research has shown that the types of bacteria in our gut are associated with levels of glucose and fat in our blood.

Because bacteria are so abundant, it’s difficult for scientists to tell whether an animal has no gut microbiome or just very low levels of gut bacteria. However, other creatures that appear to have either no gut bacteria or a very limited gut microbiome include solitary bees, some beetles, dragonflies, stick insects, mud shrimp, and oysters.

A sliding scale

These examples, while fascinating, are few and far between. It’s also worth noting that all of the examples above are invertebrates — animals that lack a spine and are only very distantly related to dinosaurs.

With this in mind, it seems likely that dinosaurs might have resident bacteria. But perhaps some dinosaurs were less reliant on their gut microbiome than others.

Scientists think that, across the animal kingdom, there is a sliding scale of reliance on gut bacteria. 

This means that some animals, like us, are heavily dependent on their microbes for good health. In the middle of the scale, some species may tolerate bacteria living in their gut but derive little or no benefit. 

For instance, both the plant-eating brent goose and the insect-eating little brown bat have low levels of bacteria in their poop. This suggests that, perhaps, they are less reliant on their gut microbiome.

At the opposite end of the scale from humans, some animals may need to actively prevent microbes from settling in their gut to remain healthy. 

So, where do dinosaurs sit on this sliding scale?

Surviving dinosaurs

This statement may be a surprise to some of you: Birds are dinosaurs. 

As incredible as it may seem, scientists believe that birds evolved from small, feathered dinosaurs called maniraptoran theropods. As such, experts class birds as dinosaurs. 

Birds first flew onto the scene around 180 million years ago, long before that town-sized rock snuffed out the vast majority of dinosaurs. 

Perhaps our feathered friends would be a good place to begin our search for a dinosaur gut microbiome. Birds do, indeed, have gut bacteria.

This is a good start — the only living dinosaurs do have gut bacteria. But there is a fly in the ointment, and that fly has a tricky name: phylosymbiosis. 

In short, phylosymbiosis explains the phenomenon that closely related species tend to share similar gut microbiomes regardless of where they live and what they eat.

For instance, some non-human primates that are closely related have similar gut bacteria despite living in different environments and consuming different foods. This, scientists believe, is partly because evolutionary changes in the body make the gut particularly inviting for certain microbes and less inviting for others. 

Although there has been little research into our feathered friends’ guts, birds appear to display relatively weak phylosymbiosis. 

The power of flight

In one study, for instance, researchers investigated the gut microbiome of barn swallows. They compared the gut bacteria of swallows that migrated with individuals that stayed in one place all year. 

Both migrating swallows and stay-at-home swallows had similarly diverse gut bacteria. However, there was a large difference in the types of microbes present.

Some scientists believe this reduced phylosymbiosis in birds might be partly because they evolved the power of flight. Indeed, there is a similar lack of phylosymbiosis in bats — the only mammals to have evolved true flight.

Perhaps bird gut microbiomes, then, are a relatively new phenomenon. Could they have evolved since the demise of the dinosaurs? Or maybe they arrived on the scene after they evolved the power of flight and don’t represent the gut contents of the feathered dinosaurs that walked before them. 

Let’s look for clues in an animal that shared the Earth with dinosaurs.

Looking to crocs

Of all the animals alive today, crocodiles certainly look ancient, and they are. These reptiles and their close ancestors have been wallowing in murky water for around 247 million years. 

Importantly for this article, studies have shown that crocodiles do have a gut microbiome. They also show that their resident microbes are significantly different from mammals, fish, and other reptiles.

Crocodiles are, without overselling them, hardcore. Beyond surviving the meteorite that wiped out most of the dinosaurs, they can live in dirty water, eat rotten meat, handle exposure to heavy metals and radiation, and live for up to 100 years. 

Some authors believe that their gut microbiome might help them achieve these incredible feats. 

Because they have changed little since the time of the dinosaurs, perhaps they had a well-established gut microbiome before that fateful rock hit the planet.

Crocodiles are related to extinct dinosaurs — although probably not as closely related as birds — so can we assume that at least some dinosaurs had gut bacteria? Perhaps.

Meat-eater versus plant-eater

The type of food that an animal eats influences its reliance on gut bacteria and the type of bacteria that are present. Some species of bacteria are excellent at fermenting plant products, while others are better at breaking down more meaty substances.

Let’s see whether diet can shed some light on dinosaur gut bacteria.

As we already mentioned, dinosaurs came in all shapes and sizes. The sauropods were some of the most impressive. They hold the accolade of being the most enormous beasts to ever walk the earth. These long-necked behemoths include the family favorite, diplodocus. 

Sauropods could weigh up to 70 metric tons (more than 154,000 pounds) and devour leaves more than 65 feet (20 meters) above the ground — more than three times taller than a giraffe. 

Fueling a body of those proportions using plants alone is no mean feat. This has prompted paleontologists to assume that gut bacteria must have been working hard in the background.

Today, the largest herbivore on earth — the African elephant — would certainly struggle without its onboard microbes, as would the giraffe for that matter. 

Like most herbivores, elephants and giraffes do not have the enzymes necessary to break down fibrous plant material, such as cellulose and hemicellulose. Gut bacteria fill in the gaps.

This makes it highly likely that the largest of the extinct plant-eaters would have needed their help.

Some scientists have even wondered whether the methane produced by sauropod gut bacteria — emitted as farts — might have caused climate warming in the Mesozoic period. But, of course, there’s no direct evidence for this.

Turning to carnivores, it seems that, at least in today’s animals, they have a less varied and more changeable gut microbiome than herbivores. However, they do still need those microbes to keep them healthy.

For instance, polar bears — one of the largest living carnivores — have a thriving gut microbiome. Similarly, smaller meat-eaters, such as the European badger, are also home to gut bacteria. 

In fact, it seems that scientists are yet to identify any vertebrate carnivores without resident microbes.

Our ancient gut passengers

In short, we will never know for sure which dinosaurs had gut bacteria, which did not, and which had a gut microbiome that added little survival value. 

But we can make some relatively safe assumptions. Because virtually all other vertebrates today do depend on gut bacteria — or at least have some resident bacteria. And because the dinosaurs’ closest living relatives have a gut microbiome, it seems likely that most dinosaurs did, too.

Given that dinosaurs filled all the ecological niches available to them — swamps, waterways, oceans, savannahs, deserts, forests, and more — it would be incredibly surprising if gut bacteria weren’t present somewhere in their extended family.

It’s a wonderful thought that microbes have been assisting animals, perhaps since the first animals evolved, and they continue to keep us healthy to this day.

That such a vast array of animals have evolved an intimate relationship with gut bacteria underscores their vital importance in health. Today, ZOE scientists are steadily uncovering the many roles of gut bacteria in human health.

If you would like to learn more about the bacteria in your gut, take ZOE’s free quiz today.