Animal Locomotion

Swimming

Moving through water using fins, flippers, or body undulation
1,964 Animals
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Overview

Understanding This Category

Swimming is a mode of locomotion in which an organism self-propels through water by generating thrust against the surrounding fluid using body undulations and/or appendage strokes, while counteracting drag and managing buoyancy. It occurs at or below the water surface in fully aquatic and semi-aquatic organisms.

Swimming is movement in water by cyclical motions of body or limbs that push water back to create forward motion. Water resistance (drag) and lift affect swimming, so animals must produce thrust and stay streamlined. Buoyancy and body density affect posture and depth; animals use fins, flippers, swim bladders, fat, or lung volume to surface or dive. Fish use body and tail undulation (anguilliform to thunniform) to form vortices and rearward jet. Marine mammals, reptiles, and birds use appendage-driven oscillation or rowing (flukes in cetaceans, foreflippers, wing-propelled penguins). Semi-aquatic animals switch between surface paddling and submerged swimming. Speed, maneuverability, endurance and energy cost depend on shape and stroke style. Convergent traits include streamlined bodies, smooth surfaces, and narrow propulsors.

Etymology: From an Old English verb meaning "to move in water"; ultimately from an older Indo-European root meaning "to move" or "to swim."

Key Characteristics

Propulsion through water via body undulation and/or appendage strokes (fins, flippers, limbs, tail/flukes)
Thrust generated by accelerating water backward and forming wake vortices/jet-like flow
Hydrodynamic drag is the primary resistance; streamlining and stroke kinematics strongly affect efficiency
Buoyancy control and stability (depth, trim) are integral to sustained swimming
Occurs at the surface or fully submerged across aquatic and semi-aquatic animals
Performance involves trade-offs among speed, maneuverability, and energetic cost

Common Misconceptions

Mechanics

How It Works

Swimming is movement through a dense, viscous fluid where forward speed comes from producing net thrust that exceeds drag. The body (or appendages) accelerates water rearward; by Newton's third law the water pushes the swimmer forward. Because water's density is high, even modest limb or body motions can generate substantial forces, but the swimmer must minimize form drag (streamlining), skin-friction drag, and wave-making drag near the surface. Buoyancy and body mass distribution determine trim (pitch/roll) and how much lift or active control is needed to stay at a given depth.

Mechanically, swimming alternates between power production and recovery while maintaining stability. Many swimmers use oscillation (body undulation or fin/flipper beats) to create traveling waves that generate thrust via a combination of drag-based pushing and lift-based force on angled surfaces. Others use rowing-like strokes where a limb presents a broad surface during the power phase, then feathers to reduce resistance during recovery. Efficient swimmers coordinate axial muscles, tail/fin stiffness, and joint timing to keep thrust-producing surfaces at optimal angles of attack while preventing energy loss to excessive lateral slip or body yaw.

Depth control is intertwined with locomotion. Neutral or near-neutral buoyancy reduces the need for continuous lift, while negatively buoyant swimmers must generate upward lift with fins or body angle. Positively buoyant swimmers must angle thrust downward or use appendages to counteract rising. Many animals also modulate buoyancy (lungs/air sacs, oils, gas bladders) or use dynamic lift from fins to hold depth during sustained swimming.

Propulsion

Thrust is generated by accelerating water backward using body waves (axial muscle-driven undulation of trunk and tail), oscillating fins/flukes, or limb/flipper strokes. Forces arise from (1) drag-based pushing on broad surfaces and/or (2) lift-based forces from angled fins/flippers that shed vortices and create a rearward jet. Elastic tissues and fin stiffness can store and return energy between beats, improving efficiency.

Steering & Direction

Direction is controlled by asymmetries in force and surface area: differential stroke amplitude or timing between left/right propulsors, yawing the body to redirect thrust, and using control surfaces (pectoral/dorsal/anal fins, flippers, tail) as rudders. Pitch and roll adjustments (body bend, fin cant, flipper pronation/supination) manage depth changes and banked turns; near the surface, steering also accounts for wave drag and breaking the surface, while submerged steering relies more on fin-generated lift and body attitude.

Movement Cycle

A repeating stroke or undulatory cycle in which thrust is generated during a power portion and hydrodynamic losses are reduced during recovery, while posture and depth are stabilized. Timing often emphasizes smooth force production to limit unsteady drag and maintain body alignment.

1 Setup/Streamline (align body; set depth/trim)
2 Power Stroke/Downbeat (primary thrust generation; high force on propulsors)
3 Transition/Turnaround (reverse fin/limb direction; reorient angles of attack)
4 Recovery/Upbeat (reposition propulsors with reduced drag; maintain stability)
5 Glide/Coast (optional; exploit momentum and reduced drag before next cycle)

Variations

Anguilliform undulation (eel-like)

Large-amplitude waves travel along most of the body length; thrust is produced continuously along the body with high maneuverability but generally lower cruising efficiency at high speeds.

Carangiform/Subcarangiform undulation (fish-like)

Undulation is concentrated in the posterior body and tail; the head and trunk remain relatively stable, improving streamlining and cruising efficiency.

Thunniform caudal oscillation (tuna/dolphin-like)

Most motion is in a stiffened tail stock and high-aspect-ratio tail/flukes; generates lift-based thrust with high efficiency and high sustained speed.

Rajiform/Gymnotiform fin undulation (ray/knifefish-like)

A traveling wave passes along long fins (pectoral fins in rays and skates; anal fin in knifefishes), producing smooth thrust and exceptional station-keeping and maneuverability.

Oscillatory pectoral fin/flipper swimming (labriform/penguin-like)

Wing-like fins or flippers oscillate to generate lift-based thrust; efficient for agile swimming and, in some cases, high-speed underwater 'flight'.

Rowing/paddling (drag-based)

Limbs act as paddles: broad power stroke pushes water backward; recovery feathers the limb to reduce drag. Common in semi-aquatic mammals, amphibians, and many birds at the surface.

Surface swimming vs submerged swimming

Surface mode must contend with wave-making drag and may use higher stroke rates or different limb kinematics; submerged mode favors full streamlining and often relies more on lift-based propulsion and control surfaces for depth.

Jet propulsion

A cavity fills with water then contracts to expel a jet (e.g., squid, some jellyfish). Produces strong bursts and rapid maneuvering but can be less efficient for long-distance cruising.

Anatomy

Physical Structures

Fins or flippers (paired forelimbs and/or hindlimbs)

Generate thrust, steering, braking, and stabilization during strokes; enable maneuvering and station-holding in currents

  • Enlarged paddle/foil shape for lift-based propulsion
  • Webbing between digits or fused/flattened distal elements (flippers)
  • Flexible fin rays (in fishes) for fine control
  • Leading-edge thickening and tapered trailing edge to reduce turbulence

Tail fin (caudal fin) or tail fluke

Primary thrust generation via lateral (fish) or dorsoventral (cetaceans) oscillation; contributes to acceleration and sustained cruising

  • High-aspect-ratio shapes for efficient cruising or low-aspect for burst speed
  • Stiffened central support and flexible margins for vortex shedding control
  • Muscle-powered peduncle (tail base) optimized for force transmission

Axial body/trunk (undulatory body and core)

Transmits muscular waves along the body to produce thrust; maintains posture and reduces drag-inducing yaw/roll

  • Streamlined fusiform profile in many swimmers
  • Segmented myomeres (fish) or powerful epaxial/hypaxial muscle blocks
  • Reduced external protrusions; smooth body contouring

Propulsive paddles: webbed feet or hindlimbs

Rowing-style thrust and maneuvering, especially in semi-aquatic vertebrates; assists in diving and surfacing control

  • Interdigital webbing increases effective paddle area
  • Digit elongation and flattening; broad plantar surface
  • Feathered margins (in some birds) to reduce backstroke drag

Control surfaces: dorsal fin, pectoral fins, anal fin, flippers (as hydrofoils)

Stability (anti-roll/anti-yaw), pitch control, turning, and lift generation in lift-based swimming

  • Adjustable fin angles via intrinsic muscles
  • Stiffer leading edges; flexible trailing edges
  • Keel-like profiles that reduce side-slip during turns

Skin/integument (including scales, mucus, blubber, feathers)

Reduce frictional drag, protect tissues, and contribute to buoyancy/insulation

  • Mucus layer to lower skin friction and inhibit fouling (many fish)
  • Micro-structured surfaces that can modulate boundary layer flow (varies by taxa)
  • Blubber in marine mammals for buoyancy and insulation
  • Dense waterproof plumage and air trapping in aquatic birds

Buoyancy and depth-control organs (swim bladder or lung-derived buoyancy; fat stores)

Regulate buoyancy to minimize energy costs; enable neutral buoyancy and controlled ascents/descents

  • Gas secretion/resorption systems in swim bladders (teleosts)
  • Compressible air spaces and variable lung volumes in divers
  • Lipid-rich tissues (oils/fats) for buoyancy in some taxa
Musculature

Dominant axial musculature (epaxial and hypaxial muscles) for body undulation; segmented myomeres in fishes with strong lateral force production; caudal peduncle and tail-base muscles for power transfer to tail fin/fluke. In flipper/fin swimmers, robust shoulder/pectoral girdle and proximal limb muscles (e.g., pectoralis/supraspinatus-equivalents, latissimus/teres groups) for downstroke and recovery, plus rotator and stabilizer muscles for hydrofoil angle control. In paddling swimmers, strong hip extensors/flexors, thigh adductors/abductors, and distal limb musculature to fan webbing and optimize the power stroke, with core stabilizers to reduce unwanted roll/yaw.

Skeletal Adaptations

Swimming animals have streamlined skeletons with fewer bony bumps that cause drag. The backbone may be very flexible in anguilliform swimmers or partly stiff with a strong tail base in carangiform and thunniform forms. Shoulder and hip bones are wider for fin or flipper muscle attachment. In marine mammals and penguins, forelimb bones are shortened, flattened, and stiffened into flippers with less movement in end joints for efficient back-and-forth motion. Joints change to favor strong strokes and stable angles in water, with reinforced shoulders and constrained elbows/wrists. Caudal vertebrae support tail fins or flukes; strong tendons and ligaments transmit force and limit bending. Bone density varies: osteosclerosis (heavier bones) helps diving, while lighter skeletons help surface swimmers and flying animals.

Other Adaptations

Fusiform/streamlined body shape to reduce form drag
Reduced external appendages; retractable or minimized limbs when not used for propulsion
Countershading and camouflage patterns that also reduce visual detection in open water
Thermal insulation adaptations for cold water (blubber, dense fur, waterproof feathers)
Efficient oxygen storage and management for diving (large blood volume, high myoglobin, dive reflex in some taxa)
Valved nostrils/blowholes and protected airways for surface breathing while swimming
Sensory specializations for aquatic navigation (lateral line in fishes; vibrissae in some mammals)
Hydrodynamic posture control via adjustable fins/flippers and body stiffness modulation
Anti-fouling/low-friction surface coatings (mucus, specialized skin textures)
Energy-saving gaits such as burst-and-coast or gliding/porpoising depending on ecology
Performance

Speed & Capabilities

Speed

Routine cruising: ~0.3-2 m/s (1-7 km/h) for many fishes, turtles, seals; fast pelagic cruisers often ~1-4 m/s. Burst/escape sprinting: ~5-15+ m/s (18-54+ km/h) in highly specialized swimmers (e.g., tunas, some dolphins/sharks) for seconds.

vs Humans: Humans typically sustain ~0.8-1.5 m/s (recreational to strong lap pace), with elite sprint peaks ~2.0-2.4 m/s. Many aquatic animals exceed human speeds in both cruising (often similar-to-faster) and especially in burst sprinting (often several× faster).

Endurance

Highly variable by physiology and ecology. Fish and other water-breathers can cruise continuously for hours to days, with some migratory swimmers sustaining travel for weeks (at moderate speeds with feeding/rest patterns). Air-breathing divers (seals, otters, penguins, turtles, dolphins) are typically limited by oxygen stores underwater: vigorous swimming is sustainable for minutes per dive, while moderate-speed surface swimming can be sustained for hours with periodic breaths.

Energy Cost

Hydrodynamically efficient at moderate-to-large body sizes because buoyancy reduces the need to support body weight and streamlined shapes can lower drag; many large swimmers have very low energetic cost per distance. Efficiency drops sharply for very small swimmers where viscosity and drag dominate and where continuous high tailbeat/stroke rates are needed.

Generally lower cost of transport than terrestrial running and often comparable to or lower than flying for large animals; however, at small sizes swimming can be relatively expensive per distance compared with running due to high viscous losses. Within swimming, sustained cruising is far cheaper than burst sprinting (which is energetically costly and quickly fatiguing).

Limitations & Trade-offs

  • Requires a sufficiently deep/fluid aquatic medium; performance collapses in shallow water, dense vegetation, ice cover, or on land.
  • High drag makes sustained high speeds energetically costly; top speeds are typically only sustainable for seconds to minutes.
  • Acceleration and rapid direction changes are constrained by added mass of water and momentum; tight turns can be costly without specialized fins/body flexibility.
  • Air-breathing swimmers are limited by breath-hold duration when submerged; high-intensity swimming greatly shortens dive time.
  • Thermoregulation can be challenging in cold water; heat loss can limit duration without insulation or high metabolism.
  • Difficult to cross obstacles, steep gradients, or discontinuous habitats (e.g., dams, waterfalls, intermittent pools) without jumping/climbing adaptations.
Champions

Record Holders

Black marlin

Fastest fish (reported top speed)

~82 km/h (reported; exact maximum is debated)

Sailfish

Fastest fish (commonly cited)

~110 km/h (widely cited; evidence varies)

Common dolphin

Fastest swimming mammal (marine)

~55 km/h (reported)

Biomimicry

Nature-Inspired Technology

Swim fins (short-blade, long-blade, split fins)

Fish caudal-fin thrust and marine mammals' flipper strokes to increase effective paddle area and reduce fatigue while controlling drag.

Underwater foils and flipper-based propulsion for divers (e.g., monofins, dolphin-kick training tools)

Dolphin and whale tail-fluke oscillation and body undulation producing efficient lift-based propulsion.

Robo-fish / bio-inspired AUVs with undulating bodies

Eel-style anguilliform swimming and knifefish-style gymnotiform (ribbon-fin) swimming that trade top speed for maneuverability and low-noise motion in cluttered environments.

Torpedo-shaped submersibles and drag-reduced hull design

Streamlined fish and cetacean body forms that minimize pressure drag and flow separation during sustained swimming.

Riblet and micro-textured surfaces for drag reduction (swimsuits, coatings, marine applications)

Shark skin dermal denticles that can reduce skin-friction drag and influence boundary-layer behavior.

Hydrofoils on boats and boards (foil surfing, fast ferries)

Lift-based swimming and fin-generated lift in fast fish (e.g., tuna) and rays-using foils to raise the hull and reduce wetted area/drag.

Biomimetic fin propulsors for underwater robots (oscillating fins, flexible fin rays)

Flexible fin rays in fish that passively adapt to flow, improving thrust efficiency and maneuvering.

Paddle and oar blade shapes (spooned blades, wing paddles)

Aquatic birds' webbed feet and the concept of maximizing effective push phase while minimizing slip and wasted turbulence.

Buoyancy-control devices (diving BCDs, ballast systems in robotics)

Fish swim bladders and marine mammals' buoyancy management through lung volume and body composition to maintain depth efficiently.

Underwater acoustic stealth and sensing strategies in robots

Quiet propulsion and flow sensing seen in many swimming animals; lateral-line-like sensing inspires distributed pressure/flow sensor arrays for navigation.

Examples

Animal Examples

Iconic Examples

Bottlenose dolphin A classic fast, agile swimmer that uses powerful tail-fluke strokes and streamlined body shape for efficient propulsion.
Great white shark An iconic fish swimmer that relies on a stiff-bodied, tail-driven (thunniform) style for speed and long-distance cruising.
Atlantic bluefin tuna A hallmark of high-performance swimming with a rigid body, narrow tail base, and high-aspect caudal fin for sustained speed.
Green sea turtle Well-known flipper-powered swimming; the foreflippers generate thrust with a flying-like stroke while the body remains streamlined.
Emperor penguin A famous bird adapted to underwater 'flight,' using wing-like flippers for fast, maneuverable swimming.
Harbor seal A familiar marine mammal that swims efficiently using hind flippers and body undulation for thrust and steering.

Surprising Examples

Moose Despite being a large land mammal, it is a strong swimmer that can cross lakes and rivers and forage on aquatic vegetation.
Green iguana Commonly swims using lateral body and tail undulation, often dropping into water to escape predators.
African elephant Can swim using buoyancy and limb strokes; the trunk can function as a snorkel when partially submerged.
Leafcutter ant (worker) Can float and paddle across water during floods; ants can also form rafts, enabling short-distance aquatic movement.

Record Holders

Black marlin Fastest fish (reported top speed) ~82 km/h (reported; exact maximum is debated)
Sailfish Fastest fish (commonly cited) ~110 km/h (widely cited; evidence varies)
Common dolphin Fastest swimming mammal (marine) ~55 km/h (reported)

Found across: Bony fishes (teleosts) and cartilaginous fishes (sharks, rays), Marine mammals (cetaceans: dolphins/whales; pinnipeds: seals/sea lions; sirenians: manatees/dugongs), Marine reptiles (sea turtles; sea snakes; marine iguanas), Aquatic and semi-aquatic birds (penguins, auks; many waterfowl), Amphibians (frogs, salamanders, caecilians in aquatic phases), Aquatic invertebrates (crustaceans, cephalopods, jellyfish, many insect larvae), Semi-aquatic terrestrial mammals (otters, beavers, hippos, some ungulates like moose)

Ecology

Ecological Role

Common Habitats

River/Stream Flowing water favors streamlined propulsion and sustained station-holding or burst swimming to navigate currents, rapids, and riffles.
Lake Open water and variable depth allow efficient cruising, vertical positioning for feeding, and long-distance movement without terrestrial obstacles.
Pond Small, often vegetated waters reward maneuverable swimming for foraging among plants and avoiding ambush predators.
Wetland Shallow, patchy water creates a mosaic where swimming enables access to dispersed food and refuges while moving between pools and channels.
Swamp Flooded forests and slow channels favor swimmers that can maneuver through submerged roots and debris while exploiting rich prey resources.
Marsh Dense emergent vegetation favors slow, precise swimming and diving to exploit invertebrates, fish, and plant matter while staying concealed.
Bog Acidic, low-oxygen waters can limit competitors; swimming allows use of sparse open-water lanes and surface breathing strategies where needed.
Mangrove Tangled root networks create high shelter and prey density; swimming enables tight maneuvering and access to nursery habitats.
Estuary Brackish, tidally dynamic waters favor swimmers that can track salinity fronts and exploit seasonal prey pulses.
Coastal Nearshore zones combine currents, waves, and abundant prey; swimming supports commuting between feeding grounds and refuges.
Rocky Shore Wave-exposed, complex structure favors agile swimming for surge navigation, crevice foraging, and rapid retreat from predators.
Beach Surf zones require powerful swimming to handle breaking waves and exploit suspended prey or stranded resources near shore.
Coral Reef Three-dimensional structure and strong competition favor highly maneuverable swimmers for precise foraging, territory defense, and predator evasion.
Kelp Forest Dense kelp creates drag and visual cover; swimming enables weaving through fronds, ambush hunting, and efficient use of vertical habitat.
Open Ocean Pelagic environments favor energy-efficient cruising, long-distance migration, and wide-ranging prey search in low-structure habitat.
Deep Sea Dark, high-pressure waters favor swimmers that can patrol large volumes, conserve energy, and exploit sparse, patchily distributed food.
Seabed/Benthic Benthic zones favor swimmers that can hover, burst off the bottom, and forage along substrates while avoiding sediment plumes.
Fun Facts

Did You Know?

Many swimmers exploit vortices: undulating bodies and flapping fins shed swirling "rings" of water that, when timed well, act like a moving foothold to push against for extra thrust.

Some fish can fine-tune buoyancy on the fly-e.g., by adjusting gas in a swim bladder-so they don't have to waste energy constantly fighting to stay at a chosen depth.

Skin and surface texture matter: the tiny riblets on shark skin can reduce drag in certain flow conditions, and some fast fish actively manage a thin mucus layer that changes near-surface water flow.

Not all swimming is about pushing water backward-jellyfish often use "passive energy recapture," gliding on the wake they create, which can make their pulsing surprisingly efficient for their speed.

A number of semi-aquatic animals (like some lizards and insects) can "swim" without fully submerging, using surface tension as part of the support system-essentially running on a deformable water surface.

Efficiency: for many body sizes, moving through water can be more energy-efficient per distance than running on land because buoyancy offsets body weight-great for long-distance cruising despite water's higher drag.

Scale effect: at small sizes, water feels "thicker" (viscosity dominates), so tiny swimmers often rely on paddling or waving appendages rather than gliding-like trying to move through syrup compared with the inertia-dominated flow larger animals experience.

Speed: a top human sprint swimmer is on the order of a few meters per second, while some fast marine fish can exceed 10 m/s in short bursts-roughly the difference between a fast bicycle pace and highway-speed bursts in the same medium.

Swimming Animals

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