The Star-Nosed Mole (Condylura cristata)

The Star-Nosed Mole (Condylura cristata)

Star-nosed mole is a small mole found in wet low areas of eastern Canada and the northeastern United States, with records extending along the Atlantic coast as far as extreme southeastern Georgia. The star-nosed mole is easily identified by the twenty-two pink fleshy appendages ringing its snout, which is used as a touch organ with more than 25,000 minute sensory receptors, known as Eimer’s organs, with which this hamster-sized mole feels its way around. With the help of its Eimer’s organs, it may be perfectly poised to detect seismic wave vibrations.

The star-nosed mole is covered in thick, blackish-brown, water-repellent fur, and has large, scaled feet and a long, thick tail, which appears to function as a fat storage reserve for the spring breeding season. Adults are 15 to 20 cm (5.9 to 7.9 in) in length, weigh about 55 grams (2 oz), and have 44 teeth. The mole’s most distinctive feature is a circle of 22 mobile, pink, fleshy tentacles (called rays) at the end of its snout, from which it derives its name. These are used to identify food by touch, such as worms, insects and crustaceans.

It is a good swimmer and can forage along the bottoms of streams and ponds. Like other moles, this animal digs shallow surface tunnels for foraging; often, these tunnels exit underwater. It is active day and night and remains active in winter, when it has been observed tunneling through the snow and swimming in ice-covered streams. Little is known about the social behavior of the species, but it is suspected to be colonial.

The star-nosed mole mates in late winter or early spring, and the female has one litter of typically four or five young in late spring or early summer. However, females are known to have a second litter if their first is unsuccessful. At birth, each offspring is about 5 cm (2 in) long, hairless, and weighs about 1.5 g. Their eyes, ears, and star are all sealed, only opening and becoming useful about 14 days after birth. They become independent after about 30 days, and are fully mature after 10 months. Predators include the red-tailed hawk, great horned owl, various skunks and mustelids, and large fish, as well as domestic cats.

Vanderbilt University neuroscientist Kenneth Catania, who has studied star-nosed moles for 20 years, recently turned his research to the study of star-moles as a route to understanding general principles about how human brains process and represent sensory information. He called star-moles “a gold mine for discoveries about brains and behavior in general—and an unending source of surprises”.

Comparing the mole’s snout to vision, his research showed that whenever the mole touched potential food it made a sudden movement to position the smallest rays, the twin rays number 11, over the object for repeated rapid touches. He reports: “The similarities with vision were striking. The star movements resembled saccadic eye movements—quick movements of the eyes from one focus point to another—in their speed and time-course. The two 11th rays are over-represented in primary somatosensory cortex relative to their size, just as the small visual fovea in primates—a small region in the center of the eye that yields the sharpest vision—is over-represented in primary visual cortex.” He notes that some bats also have an auditory fovea for processing important echolocation frequencies, suggesting that “evolution has repeatedly come to the same solution for constructing a high-acuity sensory system: subdivide the sensory surface into a large, lower-resolution periphery for scanning a wide range of stimuli, and a small, high-resolution area that can be focused on objects of importance”

Based on the circular organization of the nerve endings and its innervation pattern in Eimer’s organs, Marasco proposed by mapping experiments that nearly all receptors in the star-nosed mole have a preference for a particular direction of applied stimuli. Thus, while one receptor elicits a strong response if compressed in one direction, it may stay “silent” when compressed in another one.

Evolution[edit]

The development of the star-like appendages suggests precursors with proto-appendages on an ancestor’s snout which became elevated over successive generations. Although this theory lacks fossil evidence or supporting comparative data, nearly all extant moles have sheets of the Eimer’s organ making up the epidermis of their snout around the nares. Also, recent studies of Catania and colleagues identified one North American species (Scapanus townsendi) with a set of proto-appendages extending caudally on the snout which exhibit a striking resemblance to the embryonic stages of the star-nosed mole, although Scapanus townsendii has only eight subdivisions on its face rather than the 22 appendages found on the star-nosed mole. Such change is of common occurrence in evolution and is explained by the advantage of efficiently adding modules to the body plan without need to reinvent the regulatory elements which produce each module. Thus, although the star is unique in its shape and size, it seems feasible that the structure is based on a more ancestral bauplan as it comprises similarities found in a wide range of other moles and also in the molecular structure of other mammals.

The picture which emerges suggests that the star-nosed mole is an extreme in mammalian evolution, having perhaps the most sensitive mechanosensory system to be found amongst the mammals.[13] The evolutionary process which led to elaboration of this star-like nose is based on two theories. One proposes the development of the structure of the star as a consequence of the selective pressure of the star-nosed mole’s wetland habitat. Wetlands have a dense population of small insects, so exploiting this resource requires a higher resolution sensory surface than that of other moles. Thus, a shift to the wetland environment may have provided a selective advantage for a more elaborate sensory structure. Furthermore, in wild caught moles of many species, the Eimer’s organs show obvious signs of wear and abrasion. It appears that constant and repeated contact with the soil damages the sensory organs, which have a thin keratinized epidermis. Star-nosed moles are the only species which live in the moist, muddy soil of wetlands where the less abrasive environment has allowed the delicate star-shaped structure to evolve.