The Physics of Motion: How Animals Fly, Swim, and Run

Fighting the Medium

Moving through a medium—air, water, land—means constantly fighting physics. Drag pulls you backward. Gravity pulls you down. Viscosity resists your motion. And the rules change completely depending on what you’re moving through.

Air is light and thin. It doesn’t provide much resistance at slow speeds, but it also doesn’t provide much to push against. Flying requires generating enough lift to overcome gravity while minimizing drag. Water is dense—about 800 times denser than air. That means more resistance, but also more substance to push against for propulsion. And land? Land is solid. You’re not pushing through it; you’re pushing against it, converting muscle power into explosive acceleration against a rigid surface.

Each medium demands different solutions. And evolution has found remarkably clever ways to optimize motion for each environment—often revealing fundamental physics principles in the process. Let me show you what I mean.

Fighting the Air to Stay Still

Start with hummingbirds. They’re tiny—about the size of your thumb, weighing a couple of pennies—but they can do something almost no other bird can: hover perfectly still in mid-air. Not gliding, not fluttering down, but actually maintaining position against gravity without any ground contact.

How? Figure-8 wing motion. Their wings move in a figure-8 pattern through the air, and here’s the key: they generate lift on both the downstroke and the upstroke. Most birds only get lift from the downstroke. But hummingbirds rotate their wings at the shoulder joint—they have incredibly strong shoulder and chest muscles for their size—so the upstroke also pushes air downward, creating lift.

Some species flap up to 80 times per second. Their hearts beat over 1,000 times per minute at peak activity. They consume oxygen 10 times faster than elite human athletes. The energy cost is astronomical—equivalent to 150,000+ human calories daily. But it works because it unlocks a food source other birds can’t access: nectar from flowers. While hovering, they can feed from blossoms that would be impossible to land on.

Now consider penguins. They’re birds too, but they can’t fly at all. Why? Because they optimized for water instead of air, and you can’t do both.

Wings for flight need large surface area, lightweight hollow bones, high wingbeat frequencies, and minimal body mass. Wings for swimming need reduced surface area, dense solid bones, low wingbeat frequencies, and greater mass for buoyancy control. These requirements are mutually exclusive. Physics doesn’t let you compromise.

Penguins chose water. They have heavy bones, reduced wingspan, greater weight, and muscles tuned for low-frequency propulsion underwater. They gave up flight entirely—not because they couldn’t evolve it, but because water and air demand opposite solutions. Evolution forced a binary choice.

This is what I mean about each medium having its own rules. Hummingbirds burn massive energy to stay still in air. Penguins abandoned air entirely for water efficiency. Same vertebrate body plan, completely different physics problems.

Pushing Water Without Wasting Energy

Let’s talk about swimming, where density creates both challenges and opportunities.

Manta rays demonstrate one elegant solution. Their pectoral fins make up 85% of body length—massive wings creating spans over 8 meters. They combine oscillations (vertical flapping) with undulations (traveling waves from body to wingtips). This dual motion generates thrust through vortex creation.

As the wings flap, they disturb water, creating rotating vortices that push against the surrounding fluid. The vortex generation converts wing motion into forward thrust with exceptional efficiency. The high aspect ratio—length versus width—provides stability like a tightrope walker’s pole, balancing the body during gliding.

The efficiency is remarkable. Mantas can cruise at 9 mph and sprint at 22 mph without excessive energy expenditure. Roboticists worldwide study this mechanism because it outperforms current underwater vehicle designs. The key is flexibility: the wings contain numerous separately controllable support structures, enabling precise vortex formation and maneuverability.

But there’s another swimming strategy: great white sharks use thunniform locomotion. Instead of whole-body undulation like most fish, they generate power through the caudal fin—the tail—while keeping the body relatively rigid.

Why? Because whole-body undulation becomes less efficient at high speeds due to increased drag. By minimizing lateral body movement, thunniform swimming reduces drag dramatically. The shark’s body stays streamlined, and the powerful tail provides all the thrust.

And here’s where material science enters. Sharks have cartilage skeletons, not bone. Cartilage is strong and dense but significantly lighter than bone. This reduces overall body mass, enabling faster swimming with less energy expenditure. The cartilage is somewhat flexible too, aiding swimming mechanics while maintaining structural integrity.

Great whites can hit 35 mph and migrate thousands of kilometers—Hawaii to California, South Africa to Australia. The cartilage-bone trade-off sacrifices some structural stiffness for mobility and efficiency. For apex predators requiring both burst speed for ambush and endurance for migration, reduced skeletal weight proves more valuable than maximum rigidity.

Two swimming strategies, two different trade-offs: mantas optimize for maneuverability with flexible vortex-generating wings; sharks optimize for speed and endurance with rigid bodies and lightweight skeletons. Same medium, different physics solutions.

Explosive Power on Solid Ground

Now to land, where motion means pushing against something that doesn’t move.

Lions are built for explosive acceleration. Their muscles are 50-80% type 2X fast-twitch fibers—the fastest contracting muscle type in the animal kingdom. Humans have about 5% or less. When you dissect lion muscle, it appears white with minimal red coloration, confirming the predominance of fast-twitch over slow-twitch fibers.

Type 2X fibers generate enormous force in minimal time. Lions can accelerate from 0 to 50 mph and leap 36 feet during prey capture. This isn’t endurance—it’s pure explosive power. The final attack phase after stalking demands rapid bursts before prey escapes to safe distances.

Why such extreme specialization? Because lions and their prey are locked in an evolutionary arms race of speed and acceleration. Zebras possess remarkably similar athletic capabilities. Predators must exceed prey performance to achieve viable hunting success rates. Any reduction in acceleration immediately reduces hunting success below sustainable thresholds.

The trade-off is endurance. Fast-twitch fibers fatigue quickly. Lions can sprint short distances but can’t chase prey for miles like wolves. Physics again: you optimize for power or endurance, rarely both.

And speaking of unexpected physics: remember penguins? They have one more trick for moving through water fast. Before diving, they fluff their feathers to trap air. During ascent, they release this air through plumage, forming a microbubble layer around their body.

This is air lubrication. The bubble layer reduces skin friction drag by up to 40%—exactly the same principle Russian torpedoes use. Penguins accelerate from 2 meters per second to 6 meters per second during ascent, providing enough momentum to leap onto sea ice. They’re using bubble physics to overcome fluid dynamics constraints.

Every Medium Has Its Rules

Here’s what physics teaches us about motion: there’s no universal solution. Each medium has fundamental constraints that determine what works and what doesn’t.

Air is light: you need mechanisms to generate lift on both wing strokes (hummingbirds) or you abandon it for denser fluids (penguins). Water is dense: you can use vortex generation for efficiency (mantas) or minimize drag with rigid bodies (sharks). Land is solid: you optimize muscle for explosive power (lions) or accept that you can’t sustain high speeds forever.

The animals that move well aren’t the ones that fight their medium’s physics. They’re the ones whose bodies speak the language of their environment. Hummingbird wings generate dual-direction lift because air is thin. Shark cartilage reduces weight because water density makes every gram matter. Lion fast-twitch muscles provide explosive power because prey won’t wait around.

And the trade-offs are real. You can’t hover like a hummingbird without burning insane energy. You can’t swim like a manta and sprint like a shark. You can’t accelerate like a lion and run marathons like a wolf. Physics doesn’t allow it.

What I love about this is how clearly it shows that “better” doesn’t exist in isolation. Motion efficiency depends entirely on context: what medium, what speed, what distance, what body size. Evolution isn’t optimizing toward some universal ideal. It’s solving specific problems under specific constraints.

That’s the real lesson. Next time you watch a bird hover or a fish swim or a predator sprint, you’re not just seeing biology. You’re seeing physics in action—millions of years of evolution finding solutions to fundamental physical constraints, one medium at a time.