The molehill is a familiar sight in lawns and fields, presenting a conical mound of loose soil that appears almost overnight. This distinct feature is the primary evidence of the presence of a subterranean mammal known as the mole. While many people assume the molehill is the entrance to the animal’s home or a lookout post, the mound is actually just the waste material from deep construction projects. Understanding the formation of these hills requires examining the mole’s extensive underground architecture and its highly specialized digging technique.
The Function of the Molehill
A molehill is fundamentally an exhaust vent, serving as a disposal chute for soil excavated during the construction of permanent tunnel systems. When a mole digs deep into the subsoil to establish its living quarters, it must displace the removed earth upward. This excess soil is pushed vertically up a shaft until it bursts onto the surface, creating the characteristic conical pile.
The primary function of the mound is waste disposal, but it also serves a secondary purpose as a passive ventilation shaft for the deep tunnel network below. By pushing soil out, the mole maintains a channel that helps regulate the atmosphere in its permanent living spaces. This process contrasts sharply with the mole’s superficial feeding activity, which often leaves only a raised ridge, or “mole run,” and no visible hill.
The material of the molehill itself is typically subsoil, which is often a different color or texture than the topsoil, helping to distinguish it from the mounds of other burrowing animals. These hills are not merely small piles; established molehills can be significant, sometimes reaching nine inches in height and two feet in diameter. The amount of soil displaced indicates that the mole is engaged in a major excavation project, suggesting the establishment or repair of deep, permanent underground structures.
The Different Types of Tunnels
Moles create complex, multi-layered networks that can be broadly categorized into three distinct types based on depth and purpose. The most visible structures are the superficial feeding tunnels, which are shallow pathways just beneath the grass roots used for hunting invertebrates. These often result in the raised ridges seen across a lawn, which are temporary and only involve pushing the soil up and sideways rather than ejecting it.
The second type is the main transit tunnel, which is a semi-permanent highway used repeatedly for travel between different feeding areas and the central nest. These are typically deeper, extending down to a depth of 15 to 50 centimeters, and are maintained and patrolled regularly. Because they are used over a long period, their construction requires significant displacement of soil, often leading to the creation of smaller molehills along the main route.
The deepest and most permanent structures are the nesting and living chambers. These chambers are often located three feet or more below the surface, providing insulation from frost and predators. The massive amount of earth that must be moved to excavate these deep, stable rooms, along with dedicated food storage “pantries,” is what generates the largest and most prominent molehills. These mounds are the unavoidable byproduct of establishing a secure, deep-seated home base.
Specialized Digging Mechanics
The sheer volume of earth a mole can move is enabled by anatomical specializations. The mole’s forelimbs are its primary excavation tools, having evolved into broad, paddle-like appendages. This structure gives the mole immense mechanical advantage for pushing soil in a powerful, lateral swimming motion.
The skeletal framework features a robust sternum and an enlarged shoulder girdle that anchors the powerful chest muscles. The humerus, the upper arm bone, is short and broad with exaggerated muscle attachment sites, and uniquely possesses a rotation or “torsion” that allows the forelimb to rotate efficiently. This humeral rotation digging technique enables the mole to loosen soil by rotating its spade-like paws outward and backward.
Once the soil is loosened and gathered behind the animal, the mole turns around in the newly excavated tunnel to complete the disposal process. Using its head and the paddle-like forelimbs, the mole compacts the loose earth into a plug. It then pushes this plug of soil vertically up the pre-dug shaft, forcing it onto the surface to create the signature mound.