Every organism that has ever lived has faced the same fundamental problem: how to get from here to there. Nature’s solutions span an almost absurd range, from the snap of a Venus flytrap closing in a fraction of a second to an albatross riding wind shear across entire oceans without flapping its wings. What makes this diversity so striking is not just the variety of body plans involved but the physical ingenuity behind each one. Animals, plants, and even single-celled organisms exploit elastic energy, fluid dynamics, and gravity in ways that engineers are still working to replicate. The underlying physics changes dramatically depending on an organism’s size, its medium, and the speed it needs, which means the “rules” of movement are constantly being rewritten across the tree of life.
How Tendons Turn Runners Into Pogo Sticks
When you watch a kangaroo bounce or a gazelle sprint, what you are really seeing is a system built around springs. In humans and other running animals, tendons store elastic energy during each footstrike and release it during push-off, recycling mechanical work that muscles would otherwise have to generate from scratch. The Achilles tendon is the most studied example. Research shows that as running speed increases from a jog to a sprint, the contribution of tendon elastic energy to the total work of the ankle muscles climbs from roughly half to about three-quarters of the positive work produced.1Journal of Experimental Biology. Tendon elastic strain energy in the human ankle plantar-flexors and its role with increased running speed At higher speeds, the calf muscles behave more like rigid struts, holding steady while the tendon stretches and recoils, doing the bulk of the length change. The muscle is still active, but it is mostly generating force to load the spring rather than shortening to produce movement directly.
This arrangement has limits. The muscle energy cost of maintaining tension so the tendon can do its job is substantial and consistently exceeds the energy that the tendon returns.2PubMed Central. Achilles tendon strain energy in distance running: consider the muscle energy cost In other words, elastic storage saves energy compared with a system that has no springs, but it is not free. Anatomy matters too. People with shorter Achilles tendon lever arms store more elastic energy per kilogram at running and sprinting speeds, a relationship that vanishes at walking pace.3Scientific Reports. Shorter heels are linked with greater elastic energy storage in the Achilles tendon This may help explain why elite distance runners tend to have certain foot and ankle proportions, though the relationship between anatomy and performance is never as tidy as a single measurement suggests.
Size Changes Everything on Land
One of the most consistent patterns in animal movement is that larger animals pay less per kilogram to travel the same distance. The metabolic cost of transport for running birds and mammals drops in proportion to body mass raised to roughly the negative one-third power. A mouse burns far more energy per gram per meter than an elephant does.4Journal of Experimental Biology. Models and the scaling of energy costs for locomotion The mechanical cost of running, however, scales differently: it stays roughly constant per kilogram regardless of body size. That gap means the muscles of larger animals convert metabolic energy into mechanical work more efficiently, though the reasons are not fully settled. Possible explanations include differences in how muscle fibers are recruited, how much energy is lost as heat, and the longer ground-contact times that larger animals enjoy.
Speed, though, does not scale upward forever. Very large animals face a ceiling. Stride frequency generally decreases as body mass increases, and above a certain size, the time available to accelerate the body during each stride becomes the bottleneck.5PubMed. Theoretical considerations on maximum running speeds for large and small animals This is one reason the fastest land animals are mid-sized rather than the largest. Cheetahs, pronghorn, and certain greyhound-sized predators sit in a sweet spot where muscle power, limb length, and stride frequency align for peak velocity. An elephant-sized sprinter is a physical impossibility given the constraints of bone, muscle, and gravity.
Vortex Rings and the Physics of Swimming
Water is roughly 800 times denser than air, which means aquatic animals face enormous drag but can also push against a medium that pushes back hard. Fish exploit this by producing vortex rings with each flap of the tail or fin. These spinning doughnut-shaped parcels of water carry momentum backward, propelling the fish forward. Research has shown that many fish produce what fluid dynamicists consider optimal vortex rings, the geometry that yields maximum thrust for a given energy input.6PubMed Central. ‘Optimal’ vortex rings and aquatic propulsion mechanisms When fish accelerate, they do not simply make bigger vortex rings. Instead, they alter the shape and orientation of each ring to boost thrust without increasing ring size, a strategy that appears to be conserved across undulatory swimming vertebrates.7PubMed Central. Accelerating fishes increase propulsive efficiency by modulating vortex ring geometry
Some fish go a step further, recycling their own wake. When a fish coordinates its pectoral and caudal fins in a particular way, vortices shed by the pectoral fins travel along the body and reach the tail, where they momentarily boost pressure by as much as 64%. On average, this wake recapture raises propulsive efficiency by a few percentage points.8Royal Society Open Science. Fish can use coordinated fin motions to recapture their own vortex wake energy A few percent may sound modest, but for an animal that swims all day every day, those savings compound.
Drag reduction has its own elegant solutions. Shark skin is covered in tiny tooth-like structures called denticles, which feature grooved ridges known as riblets. These riblets interact with the turbulent layer of water flowing over the shark’s body to reduce skin friction.9PubMed. Experimental Studies of Bioinspired Shark Denticles for Drag Reduction When researchers 3D-printed artificial shark skin and tested it on a swimming model, the textured skin increased swimming speed by about 7% and lowered energy cost of transport by about 6% compared with a smooth surface at certain motion frequencies.10Journal of Experimental Biology. Biomimetic shark skin: design, fabrication and hydrodynamic function More recent work suggests the riblets may have originally evolved not to reduce drag outright but to offset the friction penalty that having scales in the first place creates, while also providing benefits like defense against parasites and anti-fouling.11Bioinspiration & Biomimetics. Hydrodynamic efficiency in sharks: the combined role of riblets and denticles
Soaring Without Flapping
Wandering albatrosses are among the most efficient travelers on Earth. They cross thousands of kilometers of open ocean while barely flapping, using a technique called dynamic soaring. The bird repeatedly climbs into the wind, turns to fly downwind at altitude, descends rapidly in the leeward direction, and curves back into the wind near the surface, completing a cycle roughly every ten seconds. Tracking data from wandering albatrosses in the southern Indian Ocean confirmed that the primary energy gain happens during the upper turn, where the bird crosses from the slower air near the water’s surface into the faster wind above.12Journal of Experimental Biology. Experimental verification of dynamic soaring in albatrosses
Dynamic soaring involves a trade-off. Maximizing the energy harvested from wind shear slows the bird’s progress toward any particular destination, while prioritizing directional travel reduces energy gain. Albatrosses manage this by dividing each flight cycle into distinct phases, spending more time harvesting energy when wind shear is weak and shifting toward directional flight when shear is strong.13PubMed Central. Optimal dynamic soaring trades off energy harvest and directional flight At very low wind speeds, below about 3 meters per second, the theoretical model predicts dynamic soaring should fail entirely, yet tracked albatrosses were observed flying in winds that light. They may supplement soaring with energy drawn from updrafts over ocean waves.14PubMed Central. Observations and models of across-wind flight speed of the wandering albatross At the other extreme, in strong winds above 8 meters per second, the same model predicts cross-wind airspeeds of up to 50 meters per second, but real albatrosses plateau at about 20 meters per second. They appear to deliberately limit their speed by adjusting their turning angles and soaring height to keep aerodynamic forces within safe limits for their wings.
Flapping flight involves its own fluid-dynamic trick: the leading-edge vortex. When an insect or hummingbird sweeps its wing forward, a rotating pocket of low-pressure air forms along the wing’s leading edge, generating lift well beyond what a fixed wing of the same shape could produce. Wing shape influences the strength of this vortex. Wings that are narrower near the base, a feature called petiolation, tend to produce larger, stronger leading-edge vortices and higher lift coefficients in the first half of the wing stroke.15PubMed Central. Petiolate wings: effects on the leading-edge vortex in flapping flight
Spring-loaded Strikes and Jumps
Some of the fastest movements in biology come not from muscles firing in real time but from stored elastic energy released in a burst. Mantis shrimp strikes, flea jumps, and the snap of a trap-jaw ant’s mandible all follow the same basic principle: muscles slowly load energy into an elastic structure (a biological spring), a latch holds the system in place, and when the latch releases, the spring drives the appendage far faster than any muscle could contract on its own.16Journal of Experimental Biology. Beyond power amplification: latch-mediated spring actuation is an emerging framework for the study of diverse elastic systems This framework, called latch-mediated spring actuation, is being found in a widening range of organisms. Recently, researchers described a jumping behavior in beetle larvae that uses a variant of this mechanism, in which the latch itself requires interaction with the ground surface to work properly, something not seen before.17PLOS ONE. A novel power-amplified jumping behavior in larval beetles (Coleoptera: Laemophloeidae) These findings suggest that the diversity of spring-latch systems in nature is still being underestimated.
Moving at Microscopic Scales
For bacteria, sperm cells, and other microorganisms, the physics of movement is unrecognizably different from what large animals experience. At their tiny scale, inertia is negligible and viscosity dominates. A bacterium that stops swimming comes to a halt almost instantaneously, as though moving through thick honey.18Reports on Progress in Physics. The hydrodynamics of swimming microorganisms This creates a problem famously described by the physicist Edward Purcell: a simple back-and-forth motion, like a scallop opening and closing its shell, produces zero net displacement in this regime because the forward stroke and the reverse stroke are perfectly symmetrical. Microorganisms solve this by using flagella and cilia that execute asymmetric, non-reciprocal motions, breaking the time-reversal symmetry that would otherwise trap them in place.19PubMed Central. Swimming by reciprocal motion at low Reynolds number A sperm cell’s whip-like tail, for instance, propagates waves along its length, and the drag forces on a slender body moving sideways versus lengthwise are different enough to generate net thrust from this wave.20PubMed. Modelling the fluid mechanics of cilia and flagella in reproduction and development It is an entirely different physics of propulsion from anything that works for a fish or a bird, a reminder that the rules of movement are dictated by scale.
How Plants Move Without Muscles
Plants lack muscles and nervous systems, yet some execute movements that rival the speed of animal strikes. The Venus flytrap closes its lobes in roughly 100 milliseconds. The mechanism relies on hydrostatic pressure built up inside the trap’s tissues before triggering. When a prey item brushes the trap’s sensory hairs, differential changes in tissue volume, with some regions swelling and others shrinking, cause the curved lobes to snap through to a concave shape. Full turgescence, meaning the tissues are pressurized with water, is a prerequisite for this snap to work properly.21PubMed Central. Snapping mechanics of the Venus flytrap (Dionaea muscipula) A dehydrated trap fails to close correctly. The trick is that the plant pre-loads energy into the structure through internal pressure, much like pulling back a bistable hair clip before letting it snap, rather than generating force in real time.
Passive movement in plants can be just as ingenious. A dandelion seed’s parachute, or pappus, does not simply slow its fall the way a miniature parachute would. Air flowing through the sparse filaments of the pappus creates a separated vortex ring above the structure, a stable low-pressure zone that dramatically increases drag relative to the seed’s weight. The angle at which the filaments fan out from the seed’s stem is critical. If the pappus were a flat disk, the vortex ring would not fully detach. The natural downward-angled geometry increases the radial pressure gradient near the filament bases enough to lift the vortex ring free, stabilizing the flight.22Physics of Fluids. The pappus angle as a key factor in the entire separation of a vortex ring from a dandelion seed’s pappus This enables dandelion seeds to travel extraordinary distances on gentle breezes.
When Thousands Move as One
Starling murmurations, schools of fish, and herds of wildebeest all exhibit collective movement that looks choreographed but arises from simple local rules. In starling flocks, researchers reconstructed the 3D positions and velocities of individual birds and found that speed fluctuations are correlated across the entire flock in a scale-free way. The correlation length is not fixed; it scales with the flock’s overall size, meaning a change in one bird’s behavior propagates to every other bird regardless of how large the group is.23PubMed Central. Scale-free correlations in starling flocks This gives each individual an effective perception range far larger than its direct line of sight, allowing the flock to respond almost instantaneously to a predator’s approach. The mathematical models that best explain this behavior show that local, pairwise interactions between neighbors are sufficient to produce the long-range order, with no need for a leader or global signal. Each starling tracks a fixed number of nearest neighbors regardless of how densely packed the flock is, meaning interactions are governed by who is nearby in rank order rather than who is within a fixed distance.24PubMed Central. Statistical mechanics for natural flocks of birds
V-formation flight in geese and other large birds serves a different collective purpose: energy savings. A bird flying behind and to the side of another can exploit the upwash from the leader’s wingtip vortices. Computational modeling of Canada geese in V-formation estimated that a trailing bird can save about 16% of the energy it would otherwise spend on generating lift.25PubMed. A modeling approach to energy savings of flying Canada geese using computational fluid dynamics The lead bird gets no such benefit, which is why formation flyers typically rotate positions.
The Stability–Maneuverability Trade-Off
Every body plan represents a compromise between being stable, meaning resistant to disturbance and efficient in a straight line, and being maneuverable, meaning able to change direction quickly. Among toothed whales, species with flexible bodies and mobile flippers can make very tight turns but at low turning rates. Fast-swimming species with stiffer bodies and less mobile flippers sacrifice tight turning radii for higher turning rates.26Integrative and Comparative Biology. Balancing Requirements for Stability and Maneuverability in Cetaceans The same trade-off appears in ancient organisms: 3D-printed models of extinct shelled cephalopods show that their shell geometry predictably influenced how quickly they could pitch and turn, with different shell shapes occupying different positions along the stability–maneuverability spectrum.27PubMed Central. Stability–Maneuverability Tradeoffs Provided Diverse Functional Opportunities to Shelled Cephalopods
Some animals, though, seem to cheat this trade-off. Research combining kinematic data from a fish, a mathematical model, and a biomimetic robot showed that producing and differentially controlling mutually opposing forces during swimming can generate passive stabilization and enhanced maneuverability at the same time.28PubMed Central. Mutually opposing forces during locomotion can eliminate the tradeoff between maneuverability and stability Opposing forces during movement are widespread across animal groups, and this work suggests they may serve a dual purpose that collapses what was assumed to be an unavoidable compromise.
Staying Upright on Rough Ground
Anyone who has stumbled on an uneven sidewalk knows that real-world terrain is full of surprises. Animals handle this with a mix of passive mechanics and rapid neural correction, and the balance between those two strategies is more tilted toward passive mechanics than you might expect. When guinea fowl were made to run over a sudden, unexpected drop in the ground, they maintained dynamic stability on 18 out of 19 attempts, stumbling only once and never falling.29Journal of Experimental Biology. Running over rough terrain: guinea fowl maintain dynamic stability despite a large unexpected change in substrate height Analysis of their limb mechanics revealed that the angle at which the leg contacted the ground after the perturbation explained about 80% of the variation in how the limb was loaded. Limb stiffness, which varied enormously between trials, had surprisingly little influence on the outcome. What did matter was posture-dependent work absorption: when the leg hit the ground at an unexpectedly steep angle, the joints absorbed energy, automatically slowing the bird without requiring a conscious decision to brake.30PubMed Central. Running over rough terrain reveals limb control for intrinsic stability The leg essentially acts as a self-correcting shock absorber, a design principle that roboticists have been working hard to replicate.
Temperature, Muscle, and the Limits of Performance
Movement depends on muscle, and muscle is sensitive to temperature. Within a specific thermal range, warming muscles up improves everything: force production, contraction speed, and power output all increase. But if temperature climbs too high, force and power drop off.31Conservation Physiology. The likely effects of thermal climate change on vertebrate skeletal muscle mechanics with possible consequences for animal movement and behaviour For cold-blooded animals in particular, environmental temperature directly sets the operating temperature of their muscles and therefore their capacity for movement. Studies of small grazers exposed to higher temperatures over long periods show that average speed increases with temperature, driven largely by metabolic rate, while more decision-based movement traits like how much time the animal spends moving are less affected.32PubMed. Long-term exposure to higher temperature increases the thermal sensitivity of grazer metabolism and movement The trajectories of warm-acclimated animals also tended to be straighter. As climate change shifts local temperatures, the movement capacity of entire communities of cold-blooded animals may shift in ways that ripple through food webs, affecting who catches whom and who escapes.
Dinosaur Gaits and Ancestral Constraints
Movement strategies are shaped not just by current physics but by evolutionary history. Many quadrupedal dinosaurs descended from bipedal ancestors, a transition that is rare among land vertebrates and has no modern analogue. This ancestral bipedality left a mark on how these animals moved. Limb-bone scaling in quadrupedal ornithischian dinosaurs reveals a range of stances and gaits rather than a single pattern. Trackway evidence shows that hadrosaurs, for example, placed their feet on the body’s midline in a narrow-gauged stance quite different from the wide-gauged sprawl of many modern large quadrupeds.33PLoS ONE. Limb-Bone Scaling Indicates Diverse Stance and Gait in Quadrupedal Ornithischian Dinosaurs
Forelimb posture tells a similar story. Analysis of forearm bone shapes across dinosaur groups found that no dinosaur clade could cross the radius over the ulna the way many mammals do. Their forearm bones remained parallel, meaning movements like the palm-down, elbow-out sprawl of a lizard were off the table.34PLOS ONE. Forearm Posture and Mobility in Quadrupedal Dinosaurs The bipedal origins of these lineages may have actually expanded the diversity of possible forelimb postures compared with what a simple lizard-versus-mammal dichotomy would predict, because the forelimb was freed from a weight-bearing role for millions of years before being pressed back into service. Reconstructing how extinct animals moved is always uncertain, but this work shows that assuming modern animals are the template can lead researchers astray. The constraints of ancestry, the physics of bone and joint, and the demands of body size all interact to produce movement solutions that are unique to their moment in evolutionary time.
From Biology to Soft Robots
Engineers have been raiding nature’s playbook for movement ideas with increasing sophistication. Translating biological principles into functional robots, especially soft robots that deform like living tissue rather than pivoting on rigid joints, requires coordinating actuation, fabrication, and power in ways that do not map neatly from biology. One recent project translated the stride of an elephant into a pneumatically actuated soft robotic leg. Because inflatable bending actuators behave nothing like mammalian joints and muscles, the team could not simply copy joint angles. Instead, they approximated the bending profiles of the actuators to match the overall kinematics of the elephant’s stride, producing a soft walker that moved at about 0.8 body lengths per second and could carry more than five times its own weight.35Integrative and Comparative Biology. First Steps of Transferring Animal Strides to a Biomimetic Soft Walker The gap between what biology achieves and what robots can currently replicate remains wide, but it is narrowing fastest in cases where engineers focus on replicating the functional outcome of a biological movement, such as a stable gait or efficient thrust, rather than copying the anatomy that produces it.