Certain species of fish and birds are able to locomote in both air and water, two fluid media with very different properties. A fluid is a particular phase of matter that deforms under shear stresses and includes any type of liquid or gas. Because fluids are easily deformable and move in response to applied forces, efficiently locomoting in a fluid medium presents unique challenges. Specific morphological characteristics are therefore required in animal species that primarily depend on fluidic locomotion. Because the properties of air and water are so different, swimming and flying have very disparate morphological requirements. As a result, despite the large diversity of animals that are capable of flight or swimming, only a limited number of these species have mastered the ability to both fly and swim. These species demonstrate distinct morphological and behavioral tradeoffs associated with transitioning from air to water and water to air.
Flying birds Of extant species, only insects, birds, and bats are capable of true, sustained, powered flight.
Aerodynamic principles
For birds to achieve flight, they must first successfully overcome the downward pull of gravitational forces by producing lift. This is achieved by positioning or moving their wings in a way that makes the pressure below the wings higher than above the wings. This pressure differential creates a net upward force on the wing, thus allowing the bird to lift off the ground, provided the upward force exceeds its weight.
To achieve powered flight, forward flight birds must counteract drag forces that act opposite the direction of flight. Drag forces acting on a flying animal are composed of parasitic drag on the body fuselage and induced drag on the wings, both acting against the relative direction of flight. Adjusting the angle of attack while wing beat frequency is held constant, birds are able to direct a component of the lift produced by their wings backwards, thus producing propulsive force known as thrust.
Evolution of flight
There are many competing theories explaining the evolution of avian flight. The most widely accepted theories include:
Cursorial model: wings evolved as a stabilization mechanism for progressively longer jumps in running bipeds. Arboreal model: the earliest ancestors of birds were gliders rather than true fliers. Much like modern-day flying squirrels, early avian ancestors were thought to climb up trees and then glide down from the tree tops. Pouncing Proavis model: Similar to the arboreal model, this model proposes that early predators attacked from above and evolved wings to assist in stabilizing their descent when pouncing on prey. Wing-assisted incline running (WAIR): suggests that wings evolved to provide additional downward force to increase traction during fast ascent of steep slopes in chicks. Novel association of locomotor modules: A theory introduced by Gatesy and Dial in 1996 attempts to explain how birds were capable of developing wings that eventually led to the ability of true flight. They introduce the idea of "locomotor modules" as anatomical groupings (i.e. two legs) working together as a single functional neuromuscular unit for locomotion. The authors suggest that early Theropods (evolutionary precursors to birds) began with a single locomotor module consisting of the two hind-limbs coupled with the tail. Over time, these animals developed a second locomotor module that could be independently controlled: the fore-limbs, which eventually evolved into functional wings and further decoupled tail function from the hind-limbs, creating an additional (tail) module. This decoupling allows modern birds to have the freedom to independently coordinate their three locomotor modules (wings, legs, and tail) in novel ways, thus accounting for the extreme diversity seen in the avian taxa.
Adaptation As is true for any structure shaped by natural selection, bird anatomy has evolved to fit a particular species' behavior and lifestyle. For example, birds that live in dense forests and require high maneuverability and precise landing capabilities tend to have wing shapes and body plans that reduce stability to allow the execution of fast turns and sudden accelerations. Seabirds, on the other hand, tend to fly for extended periods in open air because land masses are distantly separated and floating on the surface of the water can be metabolically costly due to the temperature differential between air and sea water. As a result, large sea birds rely mostly on soaring flight because it allows these animals to achieve relatively continuous lift without the added metabolic cost of flapping their wings. Because birds fly at an angle relative to the wind during dynamic soaring, they must be able to achieve flight speeds greater than this head wind. Consequently, birds that rely on dynamic soaring tend to have low wing loadings and high aspect ratios. In other words, gliding birds have wing shapes that allow them to behave more like fixed wing aircraft and rely mostly on passive gliding. Albatrosses have the largest wingspan of any extant bird, evidence of their primary reliance on aerodynamic and slope soaring techniques to achieve their extremely long migration patterns.
In contrast, thermal soaring birds, such as Rüppell's vultures, tend to have much smaller wing loadings and aspect ratios. Because the fastest rising air occurs in the center of a thermal, these birds optimize their flight behavior by achieving very tight turning radii. In other words, these birds tend to have smaller wings relative to body mass, which renders them less stable in gliding but gives them much more maneuverability so that they are capable of executing very tight turns.
Swimming fish While some aquatic animals move by "walking" along the ocean floor or burrowing, the predominant mode of fish locomotion is swimming, achieved by exerting force on the surrounding water which, by Newton's 3rd law, results in a reactive force that propels the animal forward.
Hydrodynamic principles Similarly to the aerodynamics of flight, powered swimming requires animals to overcome drag by producing thrust. Unlike flying, however, swimming animals do not necessarily need to actively exert high vertical forces because the effect of buoyancy can counter the downward pull of gravity, allowing these animals to float without much effort. While there is great diversity in fish locomotion, swimming behavior can be classified into two distinct "modes" based on the body structures involved in thrust production, Median-Paired Fin (MPF) and Body-Caudal Fin (BCF). Within each of these classifications, there are numerous specifications along a spectrum of behaviors from purely undulatory to entirely oscillatory based. In undulatory swimming modes thrust is produced by wave-like movements of the propulsive structure (usually a fin or the whole body). Oscillatory modes, on the other hand, are characterized by thrust production from swiveling of the propulsive structure on an attachment point without any wave-like motion.
Median-paired fin
Many fish swim using combined behavior of their two pectoral fins or both their anal and dorsal fins. Different types of Median Paired Fin (MPF) gait can be achieved by preferentially using one fin pair over the other, and include:
Undulatory Rajiform: seen in rays, skates, and mantas when thrust is produced by vertical undulations along large, well developed pectoral fins. Diodontiform: in which propulsion is achieved by propagating undulations along large pectoral fins Amiiform: undulations of a long dorsal fin while the body axis is held straight and stable Gymnotiform: undulations of a long anal fin, essentially upside down amiiform Balistiform: both anal and dorsal fins undulate
Oscillatory Tetradontiform: dorsal and anal fins are flapped as a unit, either in phase or exactly opposing one another. The ocean sunfish is an extreme example of this form of locomotion. Labriform: oscillatory movements of pectoral fins and can be classified as drag based or lift based in which propulsion is generated either as a reaction to drag produced by dragging the fins through the water in a rowing motion or via lift mechanisms.
Body-caudal fin
Most fish swim by generating undulatory waves that propagate down the body through the caudal fin. This form of undulatory locomotion is termed Body-Caudal Fin (BCF) swimming on the basis of the body structures used.
Undulatory Anguilliform: seen in eels and lampreys, this locomotion mode is marked by whole body undulations in large amplitude wavelengths. Both forward and backward swimming is possible in this type of BCF swimming. Subcarangiform: similar to anguilliform swimming, but with limited amplitude anteriorly that increases as the wave propagates posteriorly, this locomotion mode is often seen in trout. Carangiform: body undulations are restricted to the posterior third of body length with thrust produced by a stiff caudal fin Thunniform: the most efficient aquatic locomotion mode. Thrust is generated by lift during the lateral movements occurring in the caudal fin only. This locomotion mode has evolved under independent circumstances in teleost (ray-finned) fish, sharks, and marine mammals.
Oscillatory Ostraciiform: the body remains rigid and the stiff caudal fin is swept in a pendulum-like oscillation. Fish using this type of BCF locomotion, usually rely predominantly on MPF swimming modes, with ostraciiform behavior only an auxiliary behavior.
Adaptation Similar to adaptation in avian flight, swimming behaviors in fish can be thought of as a balance of stability and maneuverability. Because BCF swimming relies on more caudal body structures that can direct powerful thrust only rearwards, this form of locomotion is particularly effective for accelerating quickly and cruising continuously. BCF swimming is, therefore, inherently stable and is often seen in fish with large migration patterns that must maximize efficiency over long periods. Propulsive forces in MPF swimming, on the other hand, are generated by multiple fins located on either side of the body that can be coordinated to execute elaborate turns. As a result, MPF swimming is well adapted for high maneuverability and is often seen in smaller fish that require elaborate escape patterns.
It is important to point out that fish do not rely exclusively on one locomotor mode, but are rather lomotor "generalists," choosing among and combining behaviors from many available behavioral techniques. In fact, at slower speeds, predominantly BCF swimmers will often incorporate movement of their pectoral, anal, and dorsal fins as an additional stabilizing mechanism, but hold them close to their body at high speeds to improve streamlining and reducing drag. Zebrafish have even been observed to alter their locomotor behavior in response to changing hydrodynamic influences throughout growth and maturation.
In addition to adapting locomotor behavior, controlling buoyancy effects is critical for aquatic survival since aquatic ecosystems vary greatly by depth. Fish generally control their depth by regulating the amount of gas in specialized organs that are much like balloons. By changing the amount of gas in these swim bladders, fish actively control their density. If they increase the amount of air in their swim bladder, their overall density will become less than the surrounding water, and increased upward buoyancy pressures will cause the fish to rise until they reach a depth at which they are again at equilibrium with the surrounding water. In this way, fish behave essentially as a hot air balloon does in air.
Swimming birds