Sonic isn’t your typical hedgehog. He boasts incredible superspeed that lets him launch off springs, sprint over water and even clear loop-de-loops. In the Sonic the Hedgehog games, our favorite blue hedgehog runs through these loops to complete levels. If he enters one too slowly, though, he risks falling off the track.
 
Animals — including hedgehogs — don’t typically run through loops. Instead, climbing creatures rely on specialized limbs to scale vertical surfaces. That leaves few role models in the animal kingdom for Sonic’s upside-down antics. But roller-coaster carts, race cars and even advanced parkour runners can clear loops. 

​​​Sonic could take advantage of the same physics for his stunts, says ​​Paolo Segre. He studies biomechanics at the University of Wisconsin–Green Bay. The key to clearing a large loop is “speed, speed, speed,” Segre says. “That’s all that really matters — and some decent shoes.”  

As Sonic approaches the top of the loop, two downward forces act on him: gravity and the “normal force” exerted by the track. Together, these forces create a centripetal force that keeps Sonic on his circular path through the loop. Sonic’s inertia, which is his resistance to a change in motion, helps him maintain his high speed as he moves through the loop. That allows him to clear the top without falling off the track. 

Like a roller-coaster car, Sonic needs to reach a minimum speed to do this. But that speed would barely be a light jog for such a fast hedgehog, says Segre. Sonic can sustain speeds of some 1,200 kilometers (760 miles) per hour. That’s about the speed of sound.  

Geckos have toe pads covered in tiny bristles called setae. Made of keratin, these tiny hairs create weak molecular bonds that help the small lizards stick to vertical surfaces.Freder/iStock/Getty Images

A hairy situation 

Loops pop up in most Sonic the Hedgehog levels. But loops are rare in nature, says ​​Mostafa Hassanalian. At the New Mexico Institute of Mining and Technology in Socorro, Hassanalian designs robots inspired by animals. A lack of natural loops means most animals haven’t evolved to run through these obstacles, he says. Instead, animals such as insects and geckos use specialized limbs to climb vertical surfaces.  

Many insects run up surfaces using tiny claws and hairs on their legs that grip little pits and cracks. Geckos have thousands of tiny fibers under their feet that bond with a surface’s molecules. Called van der Waals forces, these weak bonds are easily broken when the gecko lifts its foot. 
 
Sonic’s hair may also help him clear loops — but not by gripping the track. Hedgehog quills are actually hardened, modified hairs. In the real world, these needle-like hairs protect hedgehogs from predators. Sonic’s quills, though, may make him more aerodynamic, notes Hassanalian.  
 
Speedy animals such as cheetahs and horses need to reduce drag, or resistance to their motion caused by moving through air. An object’s shape determines how much drag it experiences. Sonic’s quills might reduce drag on his body by preventing large vortices, or whirls of air, from whipping up around him. The quills’ jagged edges could cut these whirls into smaller vortices less likely to disturb airflow, says Hassanalian.  
 
He likens it to the vortex generators found on a race car. As a car speeds along, air passes along its surface. If this flow drifts away from the car, it can make the airflow turbulent, resulting in increased drag. Vortex generators are fins on a car that create small vortices, which redirect air back toward the car’s surface. That helps stabilize the car at high speeds.  

Hedgehogs haven’t evolved to zip through loops. These small insectivores can only move at a top speed of about six kilometers (four miles) per hour. Philartphace/E+/Getty Images

Running up that hill 

The physics behind race cars don’t just apply to Sonic. Birds also take cues from racing to scale vertical surfaces. Most birds tackle tricky inclines using wing-assisted incline running, says Segre. ​​When running up a slope, two-legged animals angle their bodies parallel to the surface. But this makes it hard to maintain the proper grip needed for climbing. 
 
So what can birds do? Flap their wings. “The wingbeats push them into the ground, kind of like the spoiler on a race car,” says Segre. Like vortex generators, a spoiler is a device that affects how air flows over a vehicle. This disrupted airflow helps create a downward force that presses race cars into the ground, improving grip, or traction, between the tires and the road.  

“Once [birds] have that traction, they can actually use their legs to run,” Segre says. This lets birds navigate different terrain, forage for food and escape predators. “We especially see it in baby birds who are not yet good at flying.” 

In experiments, this clever trick even let birds run up inverted walls — surfaces that slant toward the climber as they rise. No other two-legged animal manages this feat. “We see tons of climbing animals that are [four-legged],” says Segre. This is because four-legged animals can “keep their body right up against the wall like a rock climber.” By keeping their center of gravity close to the surface, they’re less likely to tip over backward. Wing-assisted incline running helps birds hug similarly close to steep surfaces. 
 
Still, even wing-assisted birds aren’t able to run upside down like Sonic. And their tactic for scaling inclines isn’t what Segre would expect for our blue hero. While birds mostly use their legs to ascend, their wings are essential to getting enough traction. “This isn’t really what Sonic does,” says Segre. “He just uses pure speed.”  

You can use basic rules of physics to calculate how fast Sonic would have to travel to clear loops in the real world.