Dinosaurs may have run like emus by keeping one foot on the ground

If you want to imagine a dinosaur running, then perhaps look at emus. They probably shared a similar posture, which makes it most energy efficient to keep one foot on the ground when running at an intermediate pace.

Pasha van Bijlert at Utrecht University in the Netherlands and his colleagues wanted to understand why birds have a style of running that is so different to humans.

For instance, emus (Dromaius novaehollandiae), which can sprint at up to 50 kilometres per hour, always have one foot on the ground when running at intermediate speeds. This was thought to require much more energy than the “aerial” running style, in which both feet are off the ground at the same time, which is seen in other bipeds, such as people.

To learn more, the scientists built a computer model based on an emu and used physics simulations to measure the energy output of different running styles, while varying the bird’s anatomy.

“These are all changes you can’t really make in a real bird, but they represent differences between different bird species,” says van Biljert. “For example, quails are very crouched and their tendons probably don’t store a lot of energy. Emus are, comparatively, pretty upright, but their tendons store a lot of energy.”

We tend to think of running as always having an aerial phase, says van Biljert. “However, [for emus] it goes like this: walking, grounded running, aerial running. So they have an extra running style at intermediate speeds. Grounded running appears to be a wasteful running style, because it costs more energy than aerial running.”

But in some cases, grounded running can optimise energy use, he says. “If your muscles are strongest in very crouched postures, and an upright posture is impossible, then there is a range of running speeds over which grounded running is actually your best choice. Humans don’t do this, because we can stand up straight, but birds can’t.”

Fossil evidence suggests that some dinosaurs had similar postures to the large running birds alive today. Velociraptors and Deinonychuses had relatively thin tails and long, feathered forelimbs, resulting in a forward shift to their centre of mass and a more crouched posture, says van Biljert. This resembles the stance of emus, he says.

“When explaining dinosaur locomotion to people who haven’t spent a lot of time thinking about such things, I jokingly like to say that the closest thing to a non-avian theropod dinosaur would be an emu with a crocodile tail.”

Jacqueline Nguyen at the Australian Museum in Sydney says the researchers’ model allowed them to conduct a virtual experiment that provides new insights.

“The authors suggest that grounded running in birds may have first evolved within non-avian dinosaurs,” says Nguyen. “This running mode may have been another bird feature that was inherited from their dinosaur ancestors, like bipedality, modified leg bones and feathers.”