Rodents, rabbits, elephants, warthogs, narwhals, and several other mammals all possess teeth that grow continuously throughout life. So do some unexpected creatures outside the mammal family tree. The trait is more common than most people realize, and the specific teeth involved, whether incisors, tusks, molars, or entire sets, differ dramatically from one animal group to the next. What unites them is a shared biological trick: retaining active stem cells in the tooth that keep producing new material long after the animal has reached adulthood.
Rodents and Their Famous Incisors
Rodents are the most familiar example and the most studied. Every species in the order Rodentia, from mice and rats to beavers, squirrels, porcupines, and capybaras, has a pair of upper and a pair of lower incisors that grow without stopping. These teeth are open-rooted, meaning the base of the tooth never seals off the way a human tooth root does after it finishes forming. Instead, a region at the base called the cervical loop houses stem cells that continuously supply new cells to replenish the tip worn away by gnawing.1Frontiers in Physiology. An Evo-Devo perspective on ever-growing teeth in mammals and dental stem cell maintenance These stem cells give rise to the specialized cells that produce both the enamel coating on the front surface and the softer dentin behind it.
The enamel on a rodent incisor only coats the front face, which is what gives the tooth its chisel-like shape. As the softer back surface wears faster, the harder enamel edge stays sharp. Beavers take this a step further: their incisor enamel is reinforced with iron, which gives it an orange tint and makes it hard enough to shear through wood.2PubMed. Distribution of Elements in Beaver (Castor fiber) Tooth Enamel as a Sign of Environmental Adaptation: the Special Role of Fe, Co, Mg, and Fluorides (F(-))
The stem cell machinery behind rodent incisor growth involves an elaborate network of signaling pathways, including FGF, BMP, Notch, and Hedgehog signaling, as well as microRNAs and cell adhesion molecules.3PubMed Central. On the cutting edge of organ renewal: Identification, regulation, and evolution of incisor stem cells Block one of those signals and the balance between stem cell renewal and tooth growth breaks down. Experiments disrupting Notch signaling in mouse incisors, for example, caused massive cell death in the stem cell niche and halted enamel formation.4Differentiation. Notch signalling is required for the survival of epithelial stem cells in the continuously growing mouse incisor Modifying the FGF and BMP network can also change how large the incisor grows and even how asymmetric its shape becomes.5PubMed. The importance of signal pathway modulation in all aspects of tooth development
Most rodent species have continuously growing incisors but rooted, non-growing molars. A notable minority, however, also has ever-growing molars. Guinea pigs, chinchillas, and some vole species belong to this group. Their molars never form true roots and keep erupting, an adaptation generally tied to extremely abrasive diets.
Rabbits, Hares, and Pikas
Rabbits are often lumped in with rodents, but they belong to a separate order, Lagomorpha, which also includes hares and pikas. Like rodents, lagomorphs have continuously growing incisors, but with a key difference: rabbits have a second, smaller pair of upper incisors right behind the large front pair, sometimes called peg teeth. Rodents never have these. More significantly, rabbit cheek teeth (premolars and molars) also grow continuously, making their entire dentition open-rooted.6PubMed. Measuring Rabbit (Oryctolagus cuniculus) Tooth Growth and Eruption by Fluorescence Markers and Bur Marks A network of nerves threads through the constantly elongating cheek teeth to supply sensation as they grow.7PubMed Central. Is rabbit dentine innervated? A fine-structural study of the pulpal innervation in the cheek teeth of the rabbit
This is why dental problems rank among the most common health issues in pet rabbits. Without the tough, gritty vegetation that wild rabbits spend hours grinding down each day, captive rabbits fed primarily pellets or soft hay can develop acquired dental disease, a progressive condition that starts with root elongation and cascades into misaligned teeth, overgrown crowns, oral ulcers, and in severe cases, bone infections and jaw fractures.8PubMed Central. Dental Disease in Rabbits (Oryctolagus cuniculus) and Its Risk Factors—A Private Practice Study in the Metropolitan Region of Chile The root elongation typically precedes all other visible problems, so by the time an owner notices their rabbit eating less or drooling, the disease has been advancing for some time.9Journal of Exotic Pet Medicine. The Progressive Syndrome of Acquired Dental Disease in Rabbits
Tusks That Keep Growing
Several mammals have tusks, which are essentially specialized teeth that protrude well beyond the mouth, and many of these tusks grow throughout the animal’s life.
Elephant tusks are elongated upper incisors. They are open-rooted, socketed deep into the skull, and contain a funnel-shaped pulp cavity at their growing end, situated just below the floor of the nasal cavity.10Journal of Mammalogy. Injury to and Rate of Growth of an Elephant Tusk New ivory is laid down at this interior tip, pushing the tusk outward over time. Both African and Asian elephants have continuously growing tusks, though in some Asian elephant populations the males may have only small tushes or none at all. Interestingly, elephant molars use a completely different strategy: instead of growing continuously, they are replaced by a kind of conveyor belt system in which new molars emerge from the back of the jaw and gradually push forward, shoving worn-out teeth out the front. An elephant goes through about six sets of molars in a lifetime.
Warthogs have two pairs of tusks, upper and lower, which are actually their canine teeth. The upper canines grow to about 20 to 50 centimeters and curve upward around the snout, while the shorter lower ones stay around 10 centimeters and are used for defense. The lower tusks stay sharp through a self-sharpening mechanism: they constantly rub against the upper pair, grinding one edge thinner.11Journal of the Mechanical Behavior of Biomedical Materials. Structure and mechanical properties of selected biological materials Hippos similarly have continuously growing canines and incisors, and wild boar tusks follow the same pattern.
The narwhal’s spiral tusk is one of the more unusual examples. It is actually a left upper canine tooth that grows straight through the lip and can reach nearly three meters in length. Unlike most mammal teeth, the narwhal tusk appears to function as a sensory organ. Ocean water enters through porous cementum on the surface, travels through a network of open tubules that run the length of the tusk, and reaches sensory nerve endings near the pulp. Those signals travel via the trigeminal nerve to the brain, effectively turning the tusk into a giant environmental sensor.12PubMed. Sensory ability in the narwhal tooth organ system
Sloths, Armadillos, and the Enamel-Free Dentition
The superorder Xenarthra, which includes sloths, armadillos, and anteaters, has a dental setup that looks nothing like what you would expect from a mammal. Most xenarthrans that have teeth at all (anteaters are completely toothless) possess a simplified set that lacks enamel entirely. Their teeth are primarily composed of dentin, are peg-shaped and uniform, and grow continuously throughout life.13Nature. The hidden teeth of sloths: evolutionary vestiges and the development of a simplified dentition Sloths have about 18 teeth, all of which are open-rooted and ever-growing. Nine-banded armadillos can have far more, sometimes over 30, and all keep growing as well.
The absence of enamel means these teeth wear down relatively fast, which is exactly why continuous growth matters. A sloth’s leaf-heavy diet or an armadillo’s insect-and-grit meals would grind non-growing, enamel-free teeth to nubs in short order. The continuous growth compensates for the softer material.
Naked Mole-Rats and Digging With Your Teeth
Naked mole-rats deserve their own mention because they have pushed the continuously growing incisor to its functional extreme. These colonial, underground-dwelling rodents dig extensive tunnel systems almost entirely with their teeth, not their claws. Their incisors protrude outside the lips, so they can chew through soil and tubers without getting dirt in their mouths.14Journal of Zoology. Eruption rates of the mandibular incisors of naked mole-rats (Heterocephalus glaber) The rate of incisor growth in naked mole-rats is remarkably fast, matching the extreme wear their tunneling lifestyle imposes. Their entire social structure depends on it: worker caste animals do the bulk of the digging, and the teeth are the primary excavation tool.
Why Evolution Favored Teeth That Never Stop
Continuously growing teeth did not appear all at once. In rodents, the fossil record shows a gradual increase in molar crown height over roughly 50 million years, with intermediate stages of progressively taller teeth before fully ever-growing (hypselodont) molars finally appeared around 18 million years ago during the Miocene. The expansion of hypselodont species then came in waves, with radiations at around 14, 8, and 5 million years ago.15PubMed Central. Continuously growing rodent molars result from a predictable quantitative evolutionary change over 50 million years
The driving force behind this trend appears to be diet and habitat. Animals that eat grasses and other vegetation containing abrasive particles, particularly silica phytoliths and grit picked up from the ground, experience extreme tooth wear. Studies of large herbivores have shown that species consuming more C4 grasses (the type dominant in warm-season grasslands) and excreting more silica in their feces tend to have higher-crowned teeth.16Proceedings of the Royal Society B: Biological Sciences. Another one bites the dust: faecal silica levels in large herbivores correlate with high-crowned teeth The logic is straightforward: if your food wears your teeth down faster than a fixed tooth can last a lifetime, evolution favors teeth that keep growing. Open grasslands spread during the Miocene, which lines up neatly with the timing of the hypselodont rodent radiations.
Abrasion is not the only pressure, though. Animals that use teeth as tools, whether for digging tunnels, gnawing through wood, or fighting rivals, face the same problem. The naked mole-rat’s digging lifestyle and the beaver’s wood-felling habit both demand teeth that can keep up with extreme wear. Tusked animals face a different calculus: a tusk used for display, combat, or sensory perception benefits from lifelong growth because a bigger tusk serves the animal better, and breakage can be partially compensated by continued growth from the base.
How the Brain Manages Constant Growth
Continuously growing teeth create a maintenance problem that goes beyond biology at the cellular level. If a rodent’s incisors grow but do not wear evenly, the teeth become misaligned and useless, or worse, grow into the skull. Recent research has revealed that rodents possess a dedicated brainstem circuit specifically for managing this. Neurons in the spinal trigeminal nucleus oralis relay tactile feedback from the teeth to both motor circuits that execute gnawing and motivational circuits that drive the animal to gnaw in the first place.17Neuron. A brainstem circuit for the motivation and execution of gnawing and dental alignment In other words, the brain does not just control the muscles of chewing; it actively motivates the animal to engage in the gnawing behavior needed to keep teeth properly shaped. Lose that circuit, and the teeth would overgrow even if the animal had access to plenty of hard material.
This helps explain a common observation among owners of pet rodents and rabbits: these animals will gnaw on cage bars, wooden blocks, or anything available even when they are not hungry. The behavior is not just boredom or food-seeking. It is partly driven by a neural imperative to wear down teeth that will not stop growing.
Beyond Mammals
Continuous tooth growth is not limited to mammals, though the mechanism differs in interesting ways once you leave that group.
Sharks are the most famous tooth-replacers in the animal kingdom, but their system works differently from the mammalian one. Instead of a single tooth that keeps elongating, sharks constantly regenerate entire teeth from a dental lamina, a band of tissue behind each row that functions as a tooth factory. Stem cells marked by Sox2 sit in slow-cycling niches within this lamina and fuel the production of new teeth throughout the shark’s life.18PubMed Central. Sox2+ progenitors in sharks link taste development with the evolution of regenerative teeth from denticles Intriguingly, these progenitor cells share a developmental origin with taste bud cells, and cells from the superficial taste-tooth junction actively migrate into the deeper lamina to contribute to new teeth.19PubMed Central. Shark tooth regeneration reveals common stem cell characters in both human rested lamina and ameloblastoma Humans retain a resting version of this lamina, but it has long since lost the ability to activate and produce replacement teeth under normal conditions.
Sea urchins offer yet another model. They possess a set of five teeth arranged in a structure called Aristotle’s lantern, and these teeth continuously replace material lost through abrasion as the urchin scrapes algae off rocks. Each tooth contains a gradient of developmental stages, from newly forming mineral plates at the immature end to a highly dense structure at the grinding tip.20PubMed Central. Sea urchins have teeth? A review of their microstructure, biomineralization, development and mechanical properties The teeth are also self-sharpening, an engineering solution that has attracted interest from materials scientists studying how to make tools that maintain their own cutting edge.
Many reptile and fish species replace teeth throughout life as well, though the specifics vary enormously. Crocodilians cycle through thousands of teeth over a lifetime in a pattern of shedding and regrowth. Some fish, like piranhas, replace teeth in coordinated quadrant-by-quadrant bursts. These are all forms of continuous dental renewal, but the cellular machinery and evolutionary history behind each one differ from the mammalian stem-cell-driven growth seen in rodents and rabbits.
What Rodent Teeth Are Teaching Us About Human Dentistry
The stem cells that power rodent incisor growth have become one of the most studied models in regenerative medicine, and the reason is practical: researchers want to know whether the same machinery could someday be coaxed into action in human teeth. Single-cell profiling of mouse incisors and human wisdom teeth has revealed that certain pulp cell populations in growing human teeth share gene expression patterns with the stem cell populations in the mouse incisor’s growth zone. In particular, a cell type marked by the gene Smoc2 in human dental pulp tends to express the same markers seen in the active stem cell region of a mouse incisor, suggesting that a similar maturation hierarchy exists in humans, though it is not activated for continuous growth.21Nature Communications. Dental cell type atlas reveals stem and differentiated cell types in mouse and human teeth
This does not mean regrowing a lost human molar is around the corner. The gap between identifying shared molecular markers and actually reactivating a dormant growth program in adult human teeth is vast. But the rodent incisor gives researchers a living mammalian system in which the full cycle of stem cell maintenance, tooth mineralization, and enamel formation plays out continuously and can be experimentally manipulated. Understanding what keeps those stem cells active in a mouse, and what shuts them down in a human, is one of the central questions driving the field. The shark dental lamina research feeds into this too, offering a second evolutionary template for how nature solved the problem of lifelong tooth production. Whether either model leads to clinical therapies in people is still speculative, but the biology is concrete and actively being mapped.