New clues about dino speed come from birds strutting through mud

Ask Google “how fast were dinosaurs,” and you’ll quickly get a bunch of answers. It’s said that Tyrannosaurus rex could have run a five-minute mile — and Velociraptors could have gone twice as fast. But a new study suggests these numbers might not be quite right.

What is a dinosaur?

The problem is that scientists estimate dinosaurs’ speed based on tracks they left in wet ground millions of years ago. And for decades, researchers have used a fairly simple formula to link dinos’ stride to their speed. Experiments with birds strutting through mud now show that relationship is more complex than expected.

Researchers shared the findings June 25 in the journal Biology Letters.

Muddy math

Peter Falkingham led the research. He’s a paleobiologist at Liverpool John Moores University in England. Over a video call from a fossil dig near Oxford, England, he points his phone down to show a fossilized footprint that could fit both of his feet multiple times. It was probably left by a large sauropod. These were long-necked dinosaurs that looked a little like giraffes.

“The faster you move, the longer the stride you take,” says Falkingham. In the 1970s, a zoologist named Robert McNeill Alexander created equations to calculate animals’ speeds using the distance between their footprints. Now, any scientist who found dinosaur tracks could, in theory, find out how fast it went.

“It’s easy to plug numbers into the equation and get a number out,” Falkingham says. “You feel like you’ve done a calculation, so it must be correct.” But that may not be true.

To come up with his speed equations, Alexander studied how mammals walk. Dinosaurs, though, are much more similar to modern birds, says Jonathan Codd. He is a physiologist at the University of Manchester in England. The mammals Alexander watched were also walking on hard, dry ground. But animals only leave footprints in wet soil, Codd points out.

For the math to come out right, researchers also need to plug in how far an animal’s hip was from the ground. That’s often unknown when all scientists have left of a dino is footprints. 

All told, Alexander’s formula can give a rough idea of how fast an animal was moving, Falkingham says. But those numbers are not as set in stone as scientific studies often make them seem. “I got grumpy at reading all these papers,” Falkingham says. So, he decided to put the math to the test.

Scientists often use fossilized dinosaur footprints to calculate how fast these animals moved. But those estimates might not be right. Greg Willis/Wikimedia Commons (CC BY-SA 2.0)

A fowl experiment

Falkingham dug up videos he recorded over 10 years ago. The clips showed helmeted guinea fowl (Numida meleagris) walking in mud. He originally took the videos because he wanted to study how the birds’ feet moved in and out of the muck.

This was messy business. Falkingham had to clean the birds’ feet after every experiment. He also had to make his own mud using poppy seeds and tiny glass bubbles. That’s because he was using X-rays to “see” the bird’s foot through the mud.

“It took a long time to mix, so you don’t want to be mixing it for every experiment,” Falkingham says. But the birds kept pooping in his custom-made mud! “You’d make a piece, and you come back the next day and it’s molding and smelly.” 

On the last day of filming, Falkingham decided to take videos of two fowl walking around in the mud. If they stopped for too long, he prodded them using a stick with a glove on the end. He also took pictures of the footprints the birds left on the mud.

He gathered these final data just out of curiosity — but now they’ve come in quite handy.

To calculate how fast dinos were, scientists got some help from the extinct animals’ modern relatives: birds.

The numbers don’t add up

Falkingham and his colleagues recently looked back at the videos of the two guinea fowl. The birds’ actual speeds didn’t match those calculated from their footprints using Alexander’s method, the team found. In fact, estimated speed based on footprints could be up to 2.5 times as fast as the actual animals moved. This was probably because walking on soft, sticky ground slowed the birds down.

The equation had worked pretty well in a past study where guinea fowl were put to run on a treadmill. That’s because those animals were taking steps of the same size to run at a set speed. But the birds on mud could hold a constant speed even when they took steps of different sizes.

“That’s just how animals move when they’re out in the wild. They speed up, they slow down. They take longer strides when they don’t need to,” Falkingham says.

These new data are not the final answer to how fast dinosaurs could run, he adds. They just show that the lab math doesn’t add up in the real world.

So, could T. rex run faster than humans?

The calculated speeds for a T. rex range from 20 to 40 kilometers (12 to 25 miles) per hour. “Somewhere in there’s the answer, but you may just never know,” Codd says. We might have to accept that we can’t know everything about an animal no one has ever actually seen.