thumb|A specimen from the collection of the [[Muséum de Toulouse]]
The star-nosed mole (Condylura cristata) is a small semiaquatic mole found in moist, low elevation areas in the northeastern parts of North America. It is the only extant member of the tribe Condylurini and genus Condylura. It has more than 25,000 minute sensory receptors in touch organs, 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 nose is about in diameter with its Eimer's organs distributed on 22 appendages. Eimer's organs were first described in the European mole in 1872 by German zoologist Theodor Eimer. Other mole species also possess Eimer's organs, though they are not as specialized or numerous as in the star-nosed mole. Because the star-nosed mole is functionally blind, the snout was long suspected to be used to detect electrical activity in prey animals, though little, if any, empirical support has been found for this hypothesis. The nasal star and dentition of this species appear to be primarily adapted to exploit extremely small prey. A report in the journal Nature gives this animal the title of fastest-eating mammal, taking as little as 120 milliseconds (average: 227 ms) to identify and consume individual food items. Its brain decides in approximately eight milliseconds if prey is edible or not. This speed is at the limit of the speed of neurons.
These moles are also able to smell underwater, accomplished by exhaling air bubbles onto objects or scent trails and then inhaling the bubbles to carry scents back through the nose.
Ecology and behavior
The star-nosed mole lives in wet lowland areas and eats small invertebrates, such as aquatic insects (such as the larvae of caddisflies, midges, dragonflies, damselflies, crane flies, horseflies, predaceous diving beetles, and stoneflies), terrestrial insects, worms (such as earthworms, leeches, and other annelids), mollusks, and aquatic crustaceans, as well as small amphibians and small fish. Condylura cristata has also been found in dry meadows farther away from water. They have also been found in the Great Smoky Mountains as high as . However, the star-nose mole does prefer wet, poorly drained areas and marshes. 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. C. cristata is particularly adept at thermoregulation, maintaining a high body temperature in a wide range of external conditions relative to other Talpid moles. This explains its ability to thrive in cold aquatic environments. Little is known about the social behavior of the species, but it is suspected to be colonial.
This 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 long, hairless, and weighs about . Their eyes, ears, and star are all sealed, opening and becoming useful only 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, barn owl, screech owl, foxes, weasels, minks, various skunks and mustelids, and large fish such as the northern pike, as well as domestic cats.
Snout comparison to visual organ
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 the 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".
The star-shaped nose is a unique organ found only on the star-nosed mole. Living as it does, in complete darkness, the star-nosed mole relies heavily on the mechanical information of its remarkable, specialized nose to find and identify their invertebrate prey without using sight (since moles have small eyes and a tiny optic nerve). This organ is often recognized by its high sensitivity and reaction speed. In only eight milliseconds it can decide whether something is edible — in fact, this is one of the fastest responses to a stimulus in the animal kingdom and is the reason why the star-nosed mole was lately recognized in the Guinness Book of World Records as the world's fastest forager.
Anatomy and physiology
The star-nose is a highly specialized sensory-motor organ shaped by 22 fleshy finger-like appendages, or tendrils, that ring their nostrils and are in constant motion as the mole explores its environment. The star itself is across and thus has a diameter slightly smaller than a typical human fingertip. Nevertheless, it is much larger than the nose of other mole species, covering per touch compared to covered by the noses of other mole species. This structure is divided into a high resolution central fovea region (the central 11th pair of rays) and less sensitive peripheral areas. In this way, the star works as a "tactile eye" where the peripheral rays (1–10 on each side) study the surroundings with erratic saccade-like movements and direct the 11th ray to objects of interest, just like the primate's foveating eye.
Regardless of the anatomical position of the star as a distal (protruding or extending) portion of the nose, this is neither an olfactory structure nor an extra hand. The appendages do not contain muscles or bones and are not used to manipulate objects or capture prey. They are controlled by tendons by a complex series of muscles that are attached to the skull in order to perform a role that seems to be purely mechanical. For this purpose, the star also contains a remarkably specialized epidermis covered entirely by 25,000 small raised domes or papillae of approximately in diameter. however none contains as many as in Condylura. In comparison, this whole process takes 600 milliseconds in humans.
The importance of the star-like nose in the mole's lifestyle is evidenced in the somatosensory representation of the nose. Electrophysiological experiments using electrodes placed on the cortex during stimulation of the body demonstrated that roughly 52% of the cortex is devoted to the nose. This means that more than half of the brain is dedicated to processing sensory information acquired by this organ, even when the nose itself is only roughly 10% of the mole's actual size. Thus, it may be concluded that the nose substitutes for the eyes, with the information from it being processed so as to produce a tactile map of the environment under the mole's nose. As other mammals, the somatosensory cortex of the star-nosed mole is somatotopically organized such that sensory information from adjacent parts of the nose is processed in adjacent regions of the somatosensory cortex. Therefore, the rays are also represented in the brain. The inferior most sensitive pair of rays (11th) had a larger representation on the somatosensory cortex, even when these are the shortest pair of appendages in the nose of the star-nosed mole.
Other important fact of the representation of the star in the cerebral cortex is that each hemisphere had clearly visible set of 11 stripes representing the contralateral star. In some favorable cases, a smaller third set of stripes was also apparent; opposite to other body structures that have a unique representation, with each half of the body represented in the opposite cerebral hemisphere. Thus, opposite to other species, the somatosensory representation of the tactile fovea is not correlated with anatomical parameters but rather is highly correlated with patterns of behavior.
Frequency sensitivity
Among the receptors described, Marasco identified that there were receptors relatively unresponsive to compressive stimuli but were acutely responsive to any kind of stimulus that brushed or slid across the surface of the nose (Stimuli applied with large displacements and high velocity). In contrast, there were other receptors that responded robustly to small magnitude compression of any kind but were not responsive to sweeping stimuli. The receptors that were sensitive to sweeping were maximally activated across a broad range of frequencies from 5–150 Hz at large displacements ranging from 85 to 485 μm. Conversely, the receptors that respond to compressive stimuli showed a narrow peak of maximal activity at 250–300 Hz with displacements from 10 to 28 μm.
Directional sensitivity
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. During exploration, the mole's star-like appendage produces brief touches which compress Eimer's organ against objects or substrate. Although the star-like structure is not a chemoreceptor itself, it helps the star-nosed mole blow between 8 and 12 small air bubbles per second, each 0.06 to 0.1 mm in size, onto objects or scent trails. These bubbles are then drawn back into the nostrils, so that odorant molecules in the air bubbles are wafted over the olfactory receptors.
In 1993, Edwin Gould and colleagues proposed that the star-like proboscis had electroreceptors and that the mole was therefore able to sense the electrical field of its prey prior to mechanical inspection by its appendages. Through behavioral experiments, they demonstrated that moles preferred an artificial worm with the simulated electrical field of a live earthworm to an identical arrangement without the electrical field. They suggested, therefore, that the nerve endings in the star's tentacles are indeed electroreceptors and that the moles move them around constantly to sample the strength of the electromagnetic field at different locations as they search for prey.
The picture which emerges suggests that the star-nosed mole is an extreme in mammalian evolution, having perhaps the most sensitive mechano-sensory system to be found among mammals.