The common perception that moles are completely blind is an oversimplification. Moles have not entirely lost their sight, but their eyes are significantly reduced in size and function, an evolutionary trade-off for a life spent almost entirely underground. This adaptation to a dark, subterranean environment has resulted in a visual apparatus that is considered vestigial, having lost its primary function of detailed sight. The mole’s sensory system is optimized for tunneling and foraging, making clear vision unnecessary for survival.
The Anatomy of Mole Eyes
The physical structure of a mole’s eye reflects its diminished role, appearing extremely small, often measuring only about one millimeter in diameter. For many species, such as the Iberian mole, the eyes are permanently covered by a layer of skin and fur. This covering acts as a protective barrier against soil and debris while burrowing, preventing the eyes from forming a clear image or recognizing patterns.
Internally, the ocular components are significantly compromised compared to surface-dwelling mammals. The lens may be disorganized, preventing proper light focus onto the retina, and the optic nerve is highly reduced, reflecting the minimal data it carries.
Despite these reductions, a mole’s retina contains the same types of photoreceptor cells—rods and cones—found in diurnal animals. However, the outer segments of these photoreceptors show anatomical regression, forming whorl-like structures instead of the neat stacks seen in animals with sharp vision.
Function of Limited Vision
While the eyes cannot form images, they retain the capacity for simple photoreception. The mole’s vision is limited to distinguishing the presence or absence of light, even when the eyes are covered by skin. This ability to sense light intensity is crucial for detecting breaches in the tunnel network. A sudden shaft of light signals an opening, which indicates vulnerability to predators like weasels or birds of prey, triggering an immediate photoavoidance response.
This limited light detection also plays a fundamental role in maintaining the mole’s circadian rhythm. The retina contains specialized cells with the photopigment melanopsin, which senses light levels but is not used for image formation. These cells connect directly to brain centers that regulate daily activity cycles and photoperiodicity, allowing the mole to recognize seasonal changes in day length. This recognition is necessary for triggering behavioral changes, such as foraging patterns and the timing of the mating season.
Sensory Compensation for Subterranean Life
The mole compensates for its lack of detailed eyesight through an extraordinarily sensitive sense of touch, centered on the snout. The tip of the nose is densely covered in thousands of microscopic sensory structures called Eimer’s organs, which are the primary tool for navigating, hunting, and understanding their environment. In a European mole, the snout may contain over 5,000 of these heavily innervated organs.
Eimer’s organs are domed, tactile papillae containing a column of cells associated with sensory receptors. These receptors allow the mole to detect minute skin indentations and vibrations. This complex sensory apparatus allows the mole to process tactile information in a manner functionally analogous to how other mammals process visual data.
The star-nosed mole, an extreme example, possesses around 25,000 of these organs on its unique, tentacled snout, making it one of the most densely innervated regions of skin in the animal kingdom. The mole uses these organs to rapidly survey its surroundings, identifying and assessing potential prey, such as earthworms and insect larvae, by touch within milliseconds.
In addition to this highly developed touch sense, olfaction (smell) plays a significant role in their subterranean world. Moles are capable of sniffing for prey in the soil and even underwater, effectively creating a three-dimensional map of their tunnel system and the location of food sources. The combination of hyper-sensitive touch and acute chemical detection allows the mole to thrive where eyesight is almost completely useless.