by Richard Conniff
When humans run, we fly a little bit, with both feet airborne for part of the stride. When birds run, they tend to keep one foot firmly attached to the ground. At least in this one critical means of locomotion, they are more earthbound than humans.
Researchers have been puzzling for decades over the apparent paradox. Humans and birds are of course both quadrupeds, though of unconventional types: Bird forelimbs have evolved into wings, ours into arms. As a result, when walking or running, we both have to deal with the handicap of the bipedal lifestyle. This is—no surprise—more stressful and less stable than life on four legs.
When we run, humans adapt by going aerial because that gait is energetically optimal for us. Our upright posture puts the grounded portion of the running stride directly under the center of mass. Our muscles are tuned to exert maximum force against the ground at that point.
So why do birds prefer what researchers call “grounded running” when it appears to be energetically more costly? The authors of a new study in the journal Science Advances set out to answer that question.
To do so, they had to “decouple” the various factors involved in avian gait selection, not just energy expenditure, but gross morphology, posture, center of mass and how it moves, musculoskeletal stresses, and so on. All these factors interact in complex ways, and trying to separate out the effects in a living bird would be impossible, unethical, and possibly get your eyes pecked out.
Instead, Pasha A.M. van Bijlert, a biomechanist and paleontologist at Utrecht University, and his co-authors developed a computerized model of bird running. They based the model on emus, large flightless birds from Australia, primarily because previous studies of emu anatomy and locomotion gave their research a head start.
They hypothesized that grounded running is preferable for birds primarily because birds don’t stand upright as humans do. Their anatomy puts them in a crouching position. It might be difficult to see or imagine this. Emus—or especially their ratite relatives the ostriches, which can stand eight feet tall—don’t necessarily seem to be crouching. That’s because we tend to mistake the most visible joint in a bird’s leg as a sort of backward-bending knee.
In fact, that’s an ankle, and it may be easier to see it that way in this photo:
A birds real knees are actually hidden up in the feathers, with the short, powerful femurs pressed close to the bird’s torso. (Anyone who has ever hungrily eyed the thigh of a Thanksgiving turkey fresh out of the oven knows this in the gut, if not quite in the head.)
Other researchers have theorized that the hindlegs evolved in the knees-up position during the transition to flight, as the forelimbs grew larger to become wings, and the chest muscles massively expanded to become a powerful engine for the wings. That shifted the center of mass forward. The compressed position of the hindlegs may have provided increased postural support to keep the legs in the desired position in flight, whether on a short hop from one branch to the next, or on a migration over thousands of miles.
But the real advantage of that hindleg position and the resulting crouched posture was that it made it possible to get the feet forward, so birds could stand, stay balanced, and support the center of mass in the wings and chest.
So how do you run in a permanently crouched posture? Birds have an enhanced ability to store elastic energy in the tendons of their legs—more than twice as much in ostriches as in humans. One theory suggested that this might be a factor in making grounded running preferable. The Science Advances study showed, however, that even taking elastic storage of energy out of the equation, grounded running is still preferable.
Instead, the crouched position is the key to grounded running. It doesn’t just enable birds to keep their feet beneath their forward-weighted load. Their muscles have also evolved to generate the most powerful forces in a crouched posture. That makes it more energy efficient and less tiring for them to keep one foot in contact with the ground at all times. Grounded running also makes birds more stable and less susceptible to injuries.
Birds are of course living dinosaurs. They evolved within the lineage of maniraptoran theropods that also includes dinosaur celebrities like Velociraptor, Deinonychus, and Anchiornis. Even early non-avian theropods show changes in anatomical features that we now associate mainly with birds. The authors of the new study suggest that their work may help to understand, for instance, why a dinosaur we tend to think of in an upright stance, like Tyrannosaurus rex, might instead have preferred grounded running.
One final thought: In any case, humans probably shouldn’t get too smug about our brief interludes of aerial superiority. The fastest a human has ever run was the 44.72 kilometers per hour sprinter Usain Bolt once achieved in the 100-meter dash. (That’s 27.78 mph.)
An ostrich can routinely run faster than that, averaging 50-60 kilometers an hour. (That’s 30-37 mph.) And, silly walk notwithstanding, it can keep it up for a marathon.
NATURE’S MOMENTARY POWER TO DAZZLE
I was out walking last Sunday on a local trail. It was cool and cloudy, a start-of-fall kind of day in New England, and just a little gloomy. Then, at 4:36 p.m., the sun unexpectedly broke through. The forest lit up for a moment, and even though I have walked there dozens of times before, I saw, as if for the first time, how beautiful it is to be alive and outdoors.
Try it, please, today, and this weekend, too. The feeling will catch you by surprise and grow on you.
Here's a comment from Steve Brusatte, a paleontologist at the University of Edinburgh, and author of numerous books including The Rise and Fall of the Dinosaurs: A New History of a Lost World (2018):
"Nice article Richard ... it is a neat study. And jibes well with a lot of
what I've read recently while working on my next book, which is all
about bird origins and evolution. That crouched posture developed in a
transitional sequence as maniraptorans got bigger wings and flight
muscles, so it does seem to be a tradeoff with center of mass issues.
There were some scientists (John Hutchinson, Matt Carrano, and others)
who looked at the issue of limb posture in T. rex awhile back, and
concluded that it was a fully upright walker, not a croucher. So its
femur is a more traditional theropod dinosaur femur, its muscles more
traditional theropod dinosaur muscles, which make sense as it still
has the very long tail to which the main limb controller muscles
attached (that tail and its muscles are lost in birds). But...if this
new study is proposing that T. rex and other big theropods may have
employed something of a crouched posture or ground-based running, even
if it is just in some incipient stage, that would be really
intriguing. I'll have to take a closer look. Hope all is well. Cheers,
Steve
I've changed this article to reflect a correction from A.M. van Biljert, lead author of the Science Advances study. Small birds ALWAYS run with one foot firmly attached to the ground, and so do larger birds like ostriches when traveling at intermediate speeds. But at top speeds, ostriches and their ratite cousins switch to aerial running. "We show," van Biljert writes, "that this is partly attributable to how crouched the small birds are--if you’re very crouched, you spread out the range of speeds over which grounded running is optimal."