Salamanders stand apart from most vertebrates in ways that go well beyond regrowing lost limbs. Their bodies are built around a suite of anatomical compromises and innovations that let them breathe through bare skin, detect chemicals by tapping their noses on surfaces, and store spring-loaded tongues that fire faster than any muscle should allow. With roughly 800 known species spread across habitats from mountain streams to underground caves, the group has evolved an unusually wide range of body plans while keeping a basic form that has barely changed since the age of dinosaurs.
Breathing Without Lungs
Amphibian skin is not just a covering. It contains a dense network of blood vessels that can exchange oxygen and carbon dioxide directly with the environment, handling anywhere from none to all of a salamander’s oxygen needs and at least a fifth to all of its carbon dioxide output, depending on the species and its activity level.1Endothelial Biomedicine. Skin Breathing in Amphibians This arrangement comes with a cost: the skin must stay thin and moist, which ties most salamanders to damp or aquatic habitats and makes them vulnerable to drying out.
The most dramatic version of skin breathing shows up in the plethodontids, the largest family of salamanders, which have lost their lungs entirely. Nearly two-thirds of all living salamander species are lungless and rely on their skin and the lining of their mouth for every breath they take.2PubMed Central. Convergent evolutionary reduction of atrial septation in lungless salamanders What makes this even stranger is that plethodontid embryos actually start building lungs during development. Transient lung buds form early on, complete with the genetic signals that normally guide pulmonary growth, but the tissue undergoes programmed cell death before the animal hatches.3PubMed Central. Developmental basis of evolutionary lung loss in plethodontid salamanders The genes for making lungs have persisted for at least 25 million years, and possibly more than 60 million, even though no adult plethodontid has used them. Evolution switched off the later construction steps while leaving the blueprint largely intact.
Losing lungs has knock-on effects throughout the body. The heart, for instance, no longer needs to route blood to a pulmonary circuit, so the wall dividing the atrium into left and right halves has partly dissolved. Lungless salamanders from two separate evolutionary lineages have independently arrived at the same heart layout, with a reduced atrial septum and incomplete separation of oxygenated and deoxygenated blood.2PubMed Central. Convergent evolutionary reduction of atrial septation in lungless salamanders That convergence suggests the redesign is not accidental but a predictable consequence of doing away with lungs.
A Heart That Keeps Blood Sorted Without Walls
Even in salamanders that do have lungs, the heart has only a single ventricle rather than the divided ventricles found in mammals and birds. You might expect that oxygenated blood returning from the lungs or skin would mix freely with oxygen-depleted blood coming back from the body, but research on axolotls shows that the two streams remain surprisingly well separated as they pass through the ventricle. The structural trick relies on the geometry of the trabeculae, the spongy ridges inside the chamber, rather than on a physical wall. Blood of different oxygen levels flows through distinct pathways and exits through separate routes.4PubMed Central. Opportunities and short-comings of the axolotl salamander heart as a model system of human single ventricle and excessive trabeculation The system is not as efficient as a four-chambered heart, but it works well enough for an animal with low metabolic demands and multiple gas-exchange surfaces.
Limb Regeneration and the Blastema
Salamanders are the only tetrapods, the group that includes all four-limbed vertebrates, that can fully regrow a lost limb as adults. When a limb is amputated, a wound epithelium quickly covers the site, and cells beneath it begin to accumulate into a mound of progenitor cells called the blastema.5PubMed Central. The axolotl limb blastema: cellular and molecular mechanisms driving blastema formation and limb regeneration in tetrapods The blastema is not a homogenous lump. Multiple cell types migrate to the injury, including epithelial cells, immune cells, nerve fibers extending new growth cones, and connective tissue cells. Some of these revert to an embryonic-like state through a process called dedifferentiation, effectively winding back the clock on their identity so they can be reprogrammed.6PubMed. The cellular and signaling dynamics of salamander limb regeneration
Nerve supply is critical. If the nerves serving the stump are cut, the blastema fails to form properly. The interaction between regenerating nerve axons and the wound epithelium appears to be what sustains the burst of cell division needed to rebuild the limb.7PubMed. Mechanisms underlying vertebrate limb regeneration: lessons from the salamander Once the blastema reaches a certain size, it becomes self-organizing and can pattern the missing structures on its own, even if transplanted to a different location on the body. The regenerated limb is not a scar or a rough approximation; it contains bone, muscle, tendons, nerves, and blood vessels in the correct spatial arrangement.
Humans share most of the genes involved, which is partly why the axolotl has become such a fixture in regenerative medicine research. The question is not whether the genetic toolkit exists in mammals but why it stays silent after injury. Understanding what signals activate the blastema pathway, and why they stop short in other vertebrates, remains one of the more active areas of developmental biology.
Spring-Loaded Tongues and Flexible Teeth
Plethodontid salamanders have turned their tongues into projectile weapons. In some species, the tongue can be launched from the mouth at extraordinary speed to snag prey at a distance, and the whole strike can happen in a few milliseconds. The mechanism behind this is not raw muscle power. Evolutionary changes in the shape and arrangement of the tongue’s skeletal elements and neural control have converted the system from a conventional muscle-driven apparatus into a spring-powered one. Elastic energy is stored and then released all at once, which makes the strike both faster and less sensitive to temperature than a purely muscular movement would be.8PubMed Central. Evolution of a high-performance and functionally robust musculoskeletal system in salamanders For a cold-blooded animal hunting in cool, damp environments where muscles contract sluggishly, that thermal robustness is a significant advantage.
The teeth play their own role in holding onto prey once it is caught. In species like the red-backed salamander, each tooth has an uncalcified zone along its posterior wall that acts as a living hinge. The asymmetry of this hinge means the tooth tip can flex backward but not forward, so a struggling insect pushed deeper into the mouth meets less resistance than one trying to wriggle out.9PubMed. Structure of the radially asymmetrical uncalcified region of the teeth of the red-backed salamander, Plethodon cinereus (Amphibia, Plethodontidae) If a tooth does break off under stress, the hinge allows the crown to snap away cleanly from the bone pedestal beneath it, avoiding damage to the jaw. Replacement teeth grow in behind the old ones, so losing a few during a meal is not a problem.
Chemical Sensing Through Nose-Tapping
Salamanders navigate their world heavily through chemical cues, and plethodontids have evolved a delivery system found in no other salamander family. A pair of tiny grooves runs from the upper lip down to the nostrils. These nasolabial grooves work like capillary tubes: when the animal taps its nose against a surface, fluid wicks up the groove and is channeled directly to the vomeronasal organ, a specialized sensory structure in the nasal cavity that detects pheromones and other nonvolatile chemicals.10PubMed. Chemical access to the vomeronasal organs of a plethodontid salamander
In most vertebrates with a vomeronasal organ, the structure sits fairly deep in the nasal passage and receives chemical stimuli somewhat indirectly. In plethodontids, the organ has migrated forward so that its front edge sits right where the nasolabial groove ends. The sensory lining is thickest at the anterior tip, right where the groove delivers its payload, and thins out toward the back.11PubMed. Organization of the vomeronasal organ in a plethodontid salamander Experiments with radiolabeled tracers confirmed that material placed at the base of the groove accumulates heavily in the vomeronasal organ but not in the main olfactory tissue, and that blocking the groove prevents delivery.10PubMed. Chemical access to the vomeronasal organs of a plethodontid salamander The result is a precision chemical-sampling tool that lets these small, mostly nocturnal animals identify territorial boundaries, locate mates, and assess the quality of potential food without relying on airborne scent.
Hearing Without a Visible Ear
Salamanders lack the tympanic membrane, the eardrum, that frogs and most other land vertebrates use to pick up airborne sound. For a long time this raised the question of whether they can hear at all. They can, though their sensitivity is limited to low frequencies. Measurements across eight species found responsiveness to airborne sound from roughly 50 to 1,200 Hz and to ground-borne vibration across a similar range.12The Journal of Experimental Biology. Seismic sensitivity and bone conduction mechanisms enable extratympanic hearing in salamanders The mechanism is unusual: sound pressure at threshold levels is enough to set the entire head vibrating slightly, and the inner ear’s sensors detect those tiny translational movements of the skull. Neither the lungs nor the mouth cavity amplify the signal, as some researchers had proposed. Sensitivity to ground vibrations probably plays a complementary role in natural settings, where substrate-borne cues from approaching predators or conspecifics may be at least as informative as airborne sound.
The Lateral Line and Electroreception
Aquatic and larval salamanders retain a lateral line system, the same sensory apparatus that fish use to detect water movement. In axolotls and other aquatic species, mechanoreceptive neuromasts sit exposed on the skin surface rather than inside canals, and they are arranged in lines along the head and trunk that correspond to the canal positions in fish.13PubMed. Pit organs in axolotls: a second class of lateral line neuromasts Alongside the neuromasts are electroreceptive ampullary organs, clustered on the head, that can pick up weak electric fields produced by the muscle activity of nearby prey.14PubMed. Electroreceptors and mechanosensory lateral line organs arise from single placodes in axolotls Both types of sensor originate from the same embryonic cell clusters, the lateral line placodes, but differentiate into functionally distinct organs. Salamanders that metamorphose and move onto land typically lose these organs, which only function in water. Permanently aquatic species keep them throughout life.
Walking With a Tail That Compensates
Salamanders walk with a sprawling gait that involves lateral undulation of the trunk, a pattern inherited from early tetrapods. The tail is not just trailing behind, though. Experiments on tiger salamanders fitted with restrictive shells that limited trunk bending showed that the animals compensated by increasing the curvature of their tails, without changing their limb joint angles at all.15PubMed Central. Turtling the Salamander: Tail Movements Mitigate Need for Kinematic Limb Changes during Walking in Tiger Salamanders (Ambystoma tigrinum) with Restricted Lateral Movement The tail, in other words, is an active partner in locomotion, helping to shift the body’s center of mass and maintain stride efficiency even when the main body axis cannot flex normally. This kind of modular compensation between body segments is a useful feature for animals that navigate tight spaces in leaf litter and rock crevices.
Defensive Anatomy Beyond Poison Glands
Many salamanders have granular (poison) glands embedded in the skin alongside the mucous glands that keep them moist. These glands are filled with protein-rich granules and can release toxic or distasteful secretions when the animal is harassed.16PubMed Central. Morphology of the Cutaneous Poison and Mucous Glands in Amphibians with Particular Emphasis on Caecilians (Siphonops annulatus) Bright belly colors in species like fire salamanders serve as a warning to predators that the skin is not safe to bite.
The Iberian ribbed newt takes defense a step further with an anatomical trick that seems almost reckless. Its ribs are sharpened and can be pushed outward through the body wall, piercing through specialized skin warts on their way out. Those warts produce toxic secretions, so the protruding rib tips become venom-coated spines. The effect makes the newt both painful to bite and physically difficult to swallow.17Freshwater Biology. Contextualising the bizarre: The integrated functioning of rib puncture as an antipredator defence in the Iberian ribbed newt (Pleurodeles waltl) The wounds heal quickly afterward, aided by the same robust tissue-repair capacities that underlie salamander regeneration more broadly.
Paedomorphosis and Flexible Life Cycles
Most amphibians go through metamorphosis, transforming from an aquatic larva into a terrestrial adult. Many salamander species have found another option: they skip the transformation entirely and become sexually mature while retaining larval features like external gills, tail fins, and aquatic body proportions. This strategy, called paedomorphosis, is driven by changes in thyroid hormone signaling pathways.18PubMed. Salamander paedomorphosis: linking thyroid hormone to life history and life cycle evolution The axolotl is the most famous example, but paedomorphosis has evolved independently in several lineages.
Paedomorphic salamanders are often described as larvae that can reproduce, but this framing can be misleading. They are better understood as adults that are larval in form. They possess some genuinely larval characteristics but also show features of mature animals, including fully developed gonads and, in some cases, adult-like patterns of limb development that help distinguish where the larval period ends and reproductive adulthood begins.19PubMed. Paedomorphic salamanders are larval in form and patterns of limb emergence inform life cycle evolution In some species, the choice between metamorphosing and remaining paedomorphic is facultative, meaning environmental conditions like pond permanence or population density can tip the balance. This flexibility gives salamanders an unusual degree of life-cycle plasticity compared to most vertebrates.
Reproductive Anatomy and Sperm Storage
Salamander reproduction is unusual among vertebrates in that most species practice internal fertilization without copulation. Males deposit a spermatophore, a small gelatinous packet capped with sperm, on the ground or substrate, and the female picks it up with her cloaca. What she does with those sperm next is the interesting part: females in seven of the ten salamander families possess cloacal glands called spermathecae that can store viable sperm for extended periods.20PubMed. Comparative biology of sperm storage in female salamanders
In the red-backed salamander, the spermatheca is a compound tubular gland in the roof of the cloaca whose epithelial lining actively secretes material around the stored sperm, apparently maintaining them in a viable state.21Journal of Morphology. Sperm storage in the spermatheca of the red-back salamander, Plethodon cinereus (Amphibia: Plethodontidae) In smooth newts, the spermatheca consists of a mass of tubules opening into the posterior cloaca, and sperm are most abundant during the peak of breeding season. Multiple males can contribute sperm to the same female, setting the stage for sperm competition within the storage organ.22Acta Zoologica. The Cloaca and Spermatheca of the Female Smooth Newt, Triturus vulgaris L. (Amphibia: Urodela: Salamandridae) The duration of storage varies across species, from weeks to months and, in some plethodontids, potentially across breeding seasons.
Males in the plethodontid family have their own specialized anatomy for courtship. Many possess a mental gland, a pad or cluster of secretory units on the chin that develops only in reproductively active males. During courtship, the male rubs or presses this gland against the female, delivering pheromones that influence her receptivity. The gland’s shape varies considerably across lineages, ranging from large pads to fan-shaped or even bifurcated forms, and the ancestral delivery behavior was head-rubbing, though some species have since lost this behavior or evolved alternatives.23PubMed Central. Phylogeny of Mental Glands, Revisited In at least one species, the mental gland contains hundreds of individual simple glands packed into an oval pad that appears only during the breeding season and regresses afterward.24Amphibia-Reptilia. The mental gland of Bolitoglossa nicefori (Caudata: Plethodontidae)
Why Salamanders Matter to Vision Science
The large, accessible cells of the salamander retina have made these animals unexpectedly important in the history of vision research. Salamander photoreceptors and retinal neurons are substantially bigger than those found in mammals, which made them some of the first vertebrate visual cells to be studied with electrophysiology techniques in the mid-twentieth century. Much of what we know about how vertebrate retinas process light, from how individual photoreceptors respond to single photons to how circuits of retinal ganglion cells encode visual information, was first worked out in tiger salamanders and mudpuppies.25PubMed Central. What the salamander eye has been telling the vision scientist’s brain. The salamander eye is not representative of all vertebrate eyes in its structure, but its outsized cells provided a window into general retinal principles that held up across species. In this sense, the anatomy that makes salamanders unusual is exactly what made them useful as a scientific tool.