The Eastern Mole (Scalopus aquaticus) is a subterranean mammal found across the eastern and central United States, renowned for its powerful digging ability. This fossorial creature spends nearly its entire life creating extensive tunnel systems just beneath the soil surface. Given its dark, underground habitat, people often wonder about the state of its vision.
The Direct Anatomical Answer
The Eastern Mole does possess eyes, though they are highly reduced and practically invisible. These organs are considered vestigial, meaning they have lost most of their original function through evolution. The eyes are extremely small, often described as being the size of a pinhead, and are typically covered by a thin layer of skin and fur.
This layer of skin serves a protective function, shielding the delicate eye tissue from abrasive soil particles as the mole digs. Although the eyes cannot form sharp images, they retain some function in detecting light. The mole’s visual apparatus can perceive changes in light intensity, allowing it to distinguish between light and dark, which is likely used to detect breaches in its tunnel system or for time-keeping.
Sensory Adaptations for Subterranean Life
The mole compensates for its non-functional vision by developing an exquisitely sensitive sense of touch, which acts as its primary guide in the darkness of its tunnels. The mole’s pointed snout is the center of this sensory world. This snout is covered in thousands of specialized receptors known as Eimer’s organs, which are the most densely innervated touch receptors found in any mammal.
These bulbous papillae allow the mole to rapidly investigate its surroundings and locate prey, such as earthworms and insect larvae. Each Eimer’s organ contains multiple types of nerve endings, including those that respond to soft touch and those tuned to vibrations. This complex arrangement lets the mole discern minute surface features and textures as it taps its nose against the tunnel walls.
The Eastern Mole also exhibits a remarkable sensitivity to ground vibrations, often described as a seismic sense. The ability to detect subtle movements in the soil helps the mole track the movement of prey and quickly react to predators. This highly specialized somatosensory system effectively replaces the information that vision provides to above-ground animals.
The Evolutionary Reason for Reduced Vision
The reduction of the Eastern Mole’s eyes is a classic example of an evolutionary trade-off driven by a life underground. In a perpetually dark environment, maintaining complex visual organs is energetically expensive, offering no survival advantage. The resources and energy required to sustain a fully functional eye are better utilized elsewhere.
Natural selection favored moles that allocated resources to developing beneficial traits, like powerful digging claws and sensitive Eimer’s organs. The genes responsible for vision, no longer under strong selective pressure, accumulated mutations over time. This process is called regressive evolution, leading to the degeneration of the lens and retina and the eye’s small, subcutaneous position.
However, the continued retention of the rudimentary eye suggests it still serves a purpose related to light detection. This basic light sensitivity is crucial for regulating the animal’s circadian and circannual rhythms, which are the biological clocks that govern daily and seasonal cycles. Even a tiny, skin-covered eye can transmit light cues to the brain, informing the mole of the time of day and year for activities like breeding and foraging.