Subterranean mammals are a fascinating group of animals that spend most, or all, of their lives beneath the Earth’s surface. Found across various mammalian orders and families worldwide, they have developed extraordinary adaptations to survive in their unique, challenging environment. In their self-constructed burrows, they encounter conditions vastly different from those above ground, including darkness, limited ventilation, and distinct temperature and humidity levels.
Adapting to Life Underground
Life underground prompts subterranean mammals to develop specific physical and physiological modifications. Their bodies exhibit a streamlined, spindle-shaped form, tapering at both ends, which helps them move efficiently through dense soil. Many species possess powerful forelimbs equipped with large, strong claws, designed to loosen and excavate earth. Some, like certain rodents, utilize their continuously growing, sharp incisor teeth for digging, complementing or even replacing limb-based excavation.
Their fur is short, dense, and velvety, lacking a distinct nap, which allows them to move forward and backward in confined tunnels without resistance. Eyes are reduced or even absent, sometimes covered by skin, as vision is of limited use in darkness. External ears are also small or missing to minimize friction during burrowing.
Atmospheric conditions in burrows pose unique physiological challenges. Burrows can have significantly lower oxygen concentrations, sometimes as low as 6% (compared to 21% above ground), and much higher carbon dioxide levels, ranging from 0.5% to 13.5% (compared to 0.03% above ground). To cope with these hypoxic (low oxygen) and hypercapnic (high carbon dioxide) conditions, subterranean mammals have evolved specialized respiratory and circulatory systems.
Some species, like the blind mole-rat, have altered metabolic rates and adjusted thermoregulation, exhibiting lower body temperatures to reduce oxygen consumption. High concentrations of oxygen-carrying proteins, such as hemoglobin in the blood and myoglobin in muscles, enhance oxygen transport and storage. Some also show an increased capacity for bicarbonate in their blood, which helps buffer the higher acidity caused by elevated carbon dioxide levels.
Perceiving the Subterranean World
Without light, subterranean mammals have developed specialized sensory systems to compensate for reduced or absent vision. Their sense of touch is enhanced, with specialized receptors distributed across their bodies. Moles, for instance, possess thousands of tiny, dome-shaped mechanosensory structures called Eimer’s organs on their snouts. These organs are densely innervated by nerve fibers, allowing moles to rapidly discriminate between objects, sense minute surface features, and detect textures as they explore their surroundings by repeatedly touching their noses to the ground. Some species, like the star-nosed mole, have evolved elaborate fleshy appendages on their snouts, which are covered in these tactile organs, effectively acting as a “tactile eye.”
Olfaction, or the sense of smell, is highly developed. Their sophisticated olfactory systems enable them to create detailed “smell maps” of their environment, locating food sources like insects, worms, or plant tubers, and communicating with conspecifics. Some species, such as hedgehogs, also utilize a vomeronasal organ (Jacobson’s organ) in the roof of their mouth, which detects pheromones and specific chemicals, aiding in social communication and foraging. Hearing is also adapted to the underground environment; low-frequency sounds and vibrations travel more effectively through soil than higher frequencies. Many subterranean rodents have auditory systems biased towards these low frequencies, allowing them to perceive ground vibrations for communication and detecting the movements of prey or predators.
Evolutionary Marvels and Longevity
The evolution of subterranean mammals showcases convergent evolution, where unrelated species independently develop similar traits in response to environmental pressures. Across different continents, diverse mammalian groups—including rodents, insectivores, and marsupials—have evolved analogous physical forms, digging mechanisms, and sensory adaptations to life underground. For instance, the shovel-like paws of the African golden mole and the specialized digging forelimbs of the distantly related North American star-nosed mole illustrate how different lineages arrived at similar solutions for efficient burrowing. This pattern of shared adaptations, despite distinct evolutionary histories, highlights the influence of the subterranean habitat’s selective forces.
Some subterranean mammals exhibit longevity and resistance to age-related diseases. The naked mole-rat ( Heterocephalus glaber ) lives for over 37 years, which is long for a rodent of its size. This species rarely develops cancer, a phenomenon attributed in part to its production of high-molecular-mass hyaluronan (HMW-HA), a substance found abundantly in its tissues.
This HMW-HA is over five times larger than that found in humans or mice and plays a role in regulating cell proliferation, making naked mole-rat cells highly sensitive to contact inhibition, an anti-cancer mechanism. The blind mole-rat also produces high molecular weight hyaluronan, which may contribute to its longevity by acting as an antioxidant and increasing stress resistance. Beyond these unique physiological traits, subterranean mammals play an ecological role by aerating and mixing soil, which influences nutrient cycling and water infiltration within their ecosystems.