Moles are subterranean mammals often thought to be blind. While they do possess eyes, these organs are highly specialized and differ significantly from typical mammalian vision. These adaptations allow moles to thrive in dark, underground environments where traditional sight offers little advantage.
The Eyes of Moles
Moles possess two eyes, like most mammals, but these organs are tiny and often inconspicuous, sometimes hidden by fur or covered by skin. For example, the Iberian mole has permanently closed eyes due to fused eyelids. Despite their small size, mole eyes contain typical mammalian structures like a cornea, lens, and retina. However, their internal structures, such as lens fibers, show developmental defects, resulting in disorganized cells.
This structural arrangement prevents moles from forming detailed images or perceiving sharp visuals. Instead, their limited vision primarily detects changes in light and darkness. This light detection is crucial for sensing exposure to sunlight, which could indicate a breach to the surface, or for regulating their body clocks. Their vision allows for basic light perception.
Navigating Without Clear Sight
Given their reduced visual acuity, moles rely heavily on other highly developed senses to navigate their subterranean world and locate prey. Their sense of touch is particularly refined, primarily through their sensitive snouts. The tip of a mole’s nose is equipped with thousands of microscopic tactile structures known as Eimer’s organs. These dome-shaped mechanosensory units are densely packed, with star-nosed moles possessing an extraordinary 25,000 such organs on their unique, tentacled snout, containing 100,000 nerve endings.
Eimer’s organs function like highly sensitive fingertips, allowing moles to explore their surroundings and discriminate between different objects, textures, and even vibrations in the soil. The star-nosed mole’s tactile fovea, located on its central rays, enables it to rapidly identify and consume small prey within milliseconds. Beyond touch, moles also utilize an acute sense of smell to locate food sources like earthworms and insects, and they can detect subtle changes in soil types. Their hearing is also acute, allowing them to perceive vibrations and sounds underground, further aiding in prey detection and tunnel navigation.
Evolutionary Path to Unique Vision
The unique visual system and enhanced alternative senses of moles are outcomes of evolutionary pressures associated with their subterranean lifestyle. Living almost entirely underground, where light is absent, renders complex, image-forming vision largely unnecessary. Maintaining and protecting large, sensitive eyes in a digging environment would be energetically costly and make them susceptible to damage from dirt and debris.
Natural selection favored the reduction of eye structures while simultaneously promoting the development of other senses that are more beneficial in a dark, confined space. This evolutionary trajectory led to moles having structurally underdeveloped lenses and abnormal expression of some eye development genes, resulting in their limited vision. Instead of investing in sight, moles evolved to excel in touch, smell, and hearing, enabling them to efficiently forage, build intricate tunnel systems, and avoid predators in their dark habitats.