Dozens of animal groups split their lives between land and water, spanning nearly every major branch of the animal kingdom. Amphibians are the most famous example, but the roster also includes certain fish, many reptiles, a surprising number of mammals, various birds, and a range of invertebrates from crabs to water beetles. What makes these animals remarkable is not just that they can survive in both environments but that their bodies have had to solve fundamentally different physical problems in each one, from breathing and seeing to moving and staying hydrated.
Amphibians Are the Textbook Answer, and for Good Reason
The word “amphibian” literally means “double life,” and the roughly 8,000 known species of frogs, toads, salamanders, newts, and caecilians embody that name. Most begin as fully aquatic larvae with gills and a tail, then undergo metamorphosis into air-breathing adults capable of living on land. This transformation is one of the most dramatic physical overhauls in the animal world. The skin alone goes through extensive rearrangement during the shift from water to land, changing in structure, physiology, and even immune function to handle life in open air.1PubMed Central. The Complex Bridge between Aquatic and Terrestrial Life: Skin Changes during Development of Amphibians
The heart changes, too. In salamanders that undergo metamorphosis, the heart’s electrical activity shifts to support higher heart rates and stronger contractions needed for a more active terrestrial lifestyle. Calcium flow into heart cells increases, boosting the force of each beat, while potassium currents speed up the heart’s electrical reset so it can cycle faster.2PubMed. The metamorphosis of amphibian myocardium: moving to the heart of the matter These are not superficial changes. The animal is essentially rebuilding its cardiovascular system to work in a medium where gravity suddenly matters much more.
Even after metamorphosis, amphibians remain tied to moisture. Their skin is permeable, which helps them breathe (many amphibians absorb a substantial portion of their oxygen directly through the skin) but also means they lose water rapidly when exposed to dry air. Research shows that amphibians face extremely high water costs just from breathing. They can adjust blood flow to the skin’s capillary beds to regulate the trade-off between gas exchange and water loss, and some species alter the lipid composition of their skin to reduce permeability when conditions are dry.3Integrative and Comparative Biology. Amphibians Exhibit Extremely High Hydric Costs of Respiration Still, amphibians maintain normal water balance whether the surrounding medium is water or air, using integrated mechanisms that control skin permeability, bladder-wall water reabsorption, and urine production.4Biological Reviews. 200 Years of Amphibian Water Economy: From Robert Townson to the Present
Fish That Climb Out of the Water
It sounds like a contradiction, but a number of fish species routinely leave the water and survive on land for hours or even days. Mudskippers are the best-known examples, spending much of their time on mudflats, climbing mangrove roots, and even fighting territorial battles out in the open air. But the amphibious habit is scattered across many fish families, from lungfish that breathe air during droughts to various gobies, blennies, and eels that move between pools or cross short stretches of land.
The central challenge for any fish on land is desiccation. Fish cannot prevent water loss through physiological means the way mammals can. Instead, amphibious fish resist drying out through skin modifications and behavioral strategies, such as staying near water or burrowing into moist substrate. The more terrestrially adapted species can tolerate substantial water loss and may even use evaporation to cool themselves, a crude form of thermoregulation.5Fish and Fisheries. Adaptations of amphibious fish for surviving life out of water
Breathing on land requires its own set of tricks. Freshwater species may breathe through their skin or gills, while some have evolved specialized chambers branching off the gill cavity. Marine species use a wider range of solutions, including modified gills, specialized throat linings, and even the intestine. In all cases, the key feature is increased blood vessel density in whatever tissue handles gas exchange, allowing more blood to pick up oxygen during air exposure. The more amphibious a fish species is, the better adapted its fins are for support and movement on land, with structural changes to the pectoral and pelvic fins and well-developed muscles that allow effective locomotion on solid ground.5Fish and Fisheries. Adaptations of amphibious fish for surviving life out of water
The giant mudskipper offers a particularly vivid example. Its mouth lining is heavily vascularized with tiny distances between air and blood, functioning almost like a lung. When exposed to air, it reduces ammonia production (ammonia is toxic and hard to excrete without water), and it acidifies the water in its burrow with carbon dioxide to reduce ammonia toxicity. Its skin is loaded with cholesterol and saturated fatty acids that decrease the skin’s permeability to gases and water, helping it resist drying out.6PubMed. Air breathing and ammonia excretion in the giant mudskipper, Periophthalmodon schlosseri
Semi-Aquatic Mammals
Mammals that split time between land and water include otters, beavers, platypuses, hippos, and the pinnipeds (seals, sea lions, and walruses). Each group has taken a different approach to the dual-environment problem, and none has fully abandoned one medium for the other.
Hippos are perhaps the most dramatic example. They spend most of the day submerged in rivers and lakes, emerging at night to graze on land. Their semi-aquatic lifestyle, large body size, specialized gut, and partially webbed feet combine to make them powerful ecosystem engineers. On land, hippos create closely cropped grazing lawns with distinct plant communities and alter the spatial extent of wildfire, which shapes the demographics of woody plants along riverbanks. In water, they deposit enormous quantities of nutrient-rich dung, stimulating aquatic food chains and changing water chemistry in ways that affect many other organisms.7PubMed Central. Are hippos Africa’s most influential megaherbivore? A review of ecosystem engineering by the semi-aquatic common hippopotamus
All mammals that dive share a reflex that overrides normal body functions during submersion: the heart slows, blood vessels constrict to shunt blood away from the extremities, and the body conserves its stored oxygen for the brain and heart. This diving response is most pronounced in large aquatic mammals, but it exists in all vertebrates, including humans. Pelagic mammals have layered additional adaptations on top of this reflex, such as elevated oxygen-carrying capacity in the blood and muscles, but the basic response of slowed heart rate and vasoconstriction is the same neural reflex seen in terrestrial species.8PubMed Central. The mammalian diving response: an enigmatic reflex to preserve life?
Reptiles That Straddle Both Worlds
Crocodilians, many turtles, and some snakes and lizards divide their time between water and land. Crocodiles and alligators are built for long submersions, basking on riverbanks, and ambush hunting in shallow water. One of their most striking adaptations is cardiovascular. Unlike diving birds and mammals, crocodilians can actively shunt blood away from the lungs even though they have a fully divided heart with four chambers. They achieve this through an “extra” aorta leaving the right ventricle, a small opening connecting the two aortas (the foramen of Panizza), and connective tissue structures in the pulmonary outflow tract that can restrict blood flow to the lungs. During prolonged submersion, this system lets them redirect blood into the body’s circulation and ensures blood still reaches the brain and coronary arteries even when the left side of the heart is essentially shut down.9PubMed. Dynamic anatomical study of cardiac shunting in crocodiles using high-resolution angioscopy
Marine reptiles like sea turtles and sea snakes face an additional problem: salt. Seawater is much saltier than their body fluids, so they need a way to excrete excess sodium. Research on marine amniotes (mammals, birds, and reptiles that live in the ocean) found that species with wider geographical ranges tend to encounter a broader range of water salinities, and those same species have higher osmoregulatory ability, measured by the sodium concentration in fluids expelled from salt-excreting organs like salt glands.10Proceedings of the Royal Society B: Biological Sciences. Osmoregulatory ability predicts geographical range size in marine amniotes In other words, the ability to handle salt determines how far these animals can roam.
Waterbirds and Their Feather Engineering
Penguins, ducks, geese, cormorants, pelicans, herons, and many shorebirds all depend on water for feeding but nest and rest on land. The critical adaptation that lets most waterbirds move between the two environments is their plumage. For over a century, scientists assumed that the oil from the uropygial gland (the preening gland near the tail) coated feathers in a waterproof layer. More recent work has shown this is only part of the story. The feather’s hierarchical structure, with its barbs and barbules creating a fine porous surface, inherently repels water with or without oil.11Journal of Avian Biology. What do we really know about the water repellency of feathers? The architecture itself traps a thin layer of air against the feather surface, preventing water from soaking through. This is why a duck can pop out of a lake and shake itself nearly dry in seconds, while a cormorant, whose feathers are structured differently and allow partial wetting, has to stand with its wings spread to dry off.
Diving birds face the same oxygen-conservation challenge as diving mammals. Penguins in particular have evolved dense bones that reduce buoyancy, flipper-like wings for powerful underwater swimming, and countercurrent heat exchangers in their flippers and legs to minimize heat loss in cold water while still allowing blood flow.
Invertebrates on the Boundary
The land-water divide is crossed by many invertebrates, too. Land crabs are a striking case. Species like Christmas Island’s red crabs spend most of their adult lives on the forest floor but migrate to the sea to breed, and their larvae develop in the ocean before returning to land. Physiologically, terrestrial crabs are surprisingly similar to their aquatic relatives despite having very different access to water and ions. They lose water quickly through evaporation and excrete salts in their urine. There is no single physiological solution to living on land. Instead, each crab species assembles a “composite habitat” by exploiting the temporal and spatial variability of moist microhabitats, balancing water losses with gains from dew, rain, puddles, or damp soil.12Integrative and Comparative Biology. Water and Solute Balance in the Transition to Land
Dragonflies and many other insects spend their larval stages in freshwater and their adult lives in the air. Water beetles and water boatmen are fully aquatic as adults but can fly between ponds. Certain snails, flatworms, and even some spiders (the diving bell spider breathes underwater using an air bubble held by its body hairs) live at the boundary between land and water. The invertebrate world contains far more examples of amphibious living than the vertebrate world, though they receive less attention.
Seeing and Hearing in Two Worlds
One of the biggest challenges for any animal that moves between water and air is sensory. Light and sound behave very differently in the two media, and a sensory system optimized for one environment tends to perform poorly in the other.
Vision is the most obvious problem. A curved cornea acts as a powerful lens in air but becomes nearly useless underwater, because water and corneal tissue have similar refractive indices. The result is that land animals become far-sighted when they dive, and aquatic animals become near-sighted when they surface. In some fish, this shift in focusing power amounts to as much as 20 to 30 diopters, an enormous optical change.13Nature. Air and Water Vision of the Atlantic Flying Fish, Cypselurus heterurus Aquatic mammals have evolved several workarounds for this. Most are sharply focused while submerged and have mechanisms to correct the resulting near-sightedness when they surface.14PubMed. Adaptive features of aquatic mammals’ eye Seals, for instance, can dramatically change the shape of their lens to refocus, while cetaceans rely on a slit-shaped pupil that acts differently in bright air than in dim water.
Hearing presents a parallel puzzle. Sound travels about four times faster in water than in air, and the mechanisms for detecting it differ completely. The standard mammalian ear is designed for air: sound waves hit the eardrum, the middle ear bones amplify them, and the signal passes to the fluid-filled cochlea. Underwater, this system does not work well because the impedance mismatch between water and the air spaces in the ear changes. Ringed seals can hear nearly as well as fully aquatic mammals underwater and nearly as well as fully terrestrial mammals in air. Researchers believe the seal ear works in the conventional way above water, but underwater, swelling of tissue in the ear canal and middle ear may create a functionally fluid-filled pathway that better matches the surrounding water.15Journal of Experimental Biology. Amphibious hearing in ringed seals (Pusa hispida): underwater audiograms, aerial audiograms and critical ratio measurements
Different pinniped species have struck different balances. Testing of California sea lions, harbor seals, and northern elephant seals revealed that the sea lion hears best in air, the harbor seal hears almost equally well in both media, and the elephant seal’s auditory system is tuned for underwater function at the expense of aerial sensitivity.16The Journal of the Acoustical Society of America. Low-frequency amphibious hearing in pinnipeds: Methods measurements noise and ecology A broader comparative analysis confirmed that pinnipeds have not simply sacrificed aerial hearing to improve underwater reception. Despite hearing underwater nearly as well as fully aquatic whales and manatees, many seals and sea lions retain hearing in air that rivals that of their fully terrestrial carnivore relatives.17PubMed. Comparative assessment of amphibious hearing in pinnipeds
The Locomotion Trade-Off
An animal built for speed in water is often clumsy on land, and vice versa. This is not just casual observation; it is a measurable trade-off. In a study of dusky salamanders, researchers found a strong negative correlation between an individual’s sprinting speed on land and its fast-start speed in water. Salamanders that were quick terrestrial runners tended to be slower swimmers, and those that were fast swimmers were slower on land. Semi-aquatic species performed better on land, while the more fully aquatic species performed better in water.18PubMed Central. Trade-offs between aquatic and terrestrial locomotion and functional parallelism in Desmognathus salamanders This kind of trade-off likely explains why truly amphibious animals are generalists rather than specialists: they are competent in both environments but rarely the fastest or most efficient in either one.
The body plans that work best on land (long legs, rigid skeletons, lightweight frames) are not the ones that work best in water (streamlined shapes, flexible bodies, dense or neutral buoyancy). Animals that use both environments tend to arrive at compromise morphologies. Otters have webbed feet and dense fur for swimming but can still run capably. Crocodiles have laterally flattened tails for powerful swimming and legs positioned underneath the body that allow a “high walk” on land. Penguins traded flying in air for flying through water, using their stiff flippers to generate thrust underwater but waddling awkwardly on ice.
Ecological Roles at the Land-Water Border
Animals that cross the boundary between land and water often serve as living pipelines for nutrients, linking ecosystems that would otherwise be largely separate. Hippos are a dramatic example. They graze on land at night and defecate in rivers during the day, acting as conveyor belts that move carbon and nutrients from savanna grasslands into aquatic systems. Research has shown that hippo inputs to rivers, including both excretion and egestion, represent a significant source of nutrients for aquatic food webs in sub-Saharan Africa, comparable to other major nutrient sources.19Freshwater Biology. The hippopotamus conveyor belt: vectors of carbon and nutrients from terrestrial grasslands to aquatic systems in sub‐Saharan Africa
Semi-aquatic species and turtles more broadly serve as important links between aquatic and terrestrial ecosystems by transporting nutrients across boundaries. Global populations of large aquatic herbivores are declining, though some show striking local recoveries that have dramatic consequences for the systems they inhabit.20Ecography. Assessing the role of large herbivores in the structuring and functioning of freshwater and marine angiosperm ecosystems When these animals disappear from a landscape, the nutrient transport between land and water slows or stops, and both ecosystems change in ways that cascade through food webs.
Water Sources as Parasite Hotspots
The places where land animals and water meet are not just ecologically productive. They are also where parasites concentrate. A study in a semi-arid savanna used an experimental approach, draining some water sources while leaving others filled, to test how shared water holes affect parasite transmission among large herbivores. When water was present, the density of parasitic worm eggs in the surrounding soil was roughly 16 times higher in wet soils near filled water sources than at drained ones. Even in dry soil, parasite egg density near water sources averaged about four and a half times that of sites away from water.21Nature Communications. Water sources aggregate parasites with increasing effects in more arid conditions For any animal that regularly commutes between land and water, these concentrated parasite loads at the water’s edge are a persistent cost of the amphibious lifestyle. In arid environments, where animals have fewer water sources to choose from, the effect is amplified because more individuals crowd around the same limited pools, depositing and picking up parasites in a tightening cycle.