One of the most intriguing stars in the universe is right here on Earth: the eleven pairs of pink fleshy appendages ringing the snout of the star-nosed mole. From its appearance and location, one would think this star might be a supersensitive olfactory organ, helping the nearly blind mole negotiate its subterranean environment, or an extra hand for grasping prey or manipulating objects. Some researchers have hypothesized that the star detects electric fields, thus acting as a kind of antenna. But in reality, the star is an extraordinary touch organ with more than 25,000 minute sensory receptors, called Eimer’s organs, with which this hamster-sized mole feels its way around.
Under a microscope, the Eimer’s organs appear in a honeycombed pattern of tiny epidermal “domes,” each sensitive to the slightest touch. Although the star is less than half an inch across, its surface is supplied with more than 100,000 large nerve fibers. By comparison, the touch receptors in the human hand are equipped with only about 17,000 of these fibers. Imagine having six times the sensitivity of your entire hand concentrated in a single fingertip.
Together with Jon Kaas, also of Vanderbilt University, I have been investigating how the star-nosed mole (Condylura cristata) uses this exquisitely sensitive organ to explore its dark, damp world. (This North American species is unique in its preference for wetlands, where it digs tunnels and forages mostly in mud and water.) First the mole The fleshy pink “fingers” on the snout of the star-nosed mole point to this animal’s unique evolutionary history. samples an area by touching the ground with all twenty-two appendages. Its brain processes this information in less than a twentieth of a second. If one of the appendages detects anything of potential interest (often an unfortunate earthworm or other invertebrate), the mole moves its nose slightly to bring the lowermost central pair into contact with the object. The Eimer’s organs on this pair are particularly well supplied with nerves and can provide the animal with a higher-resolution “image,” enabling the mole to know whether it has encountered something good to eat or should keep searching. For small prey, the entire process—from first touch by peripheral appendage to swift ingestion—takes just about a fifth of a second.
The star-nosed mole continuously scans its environment with its nose, much as we constantly shift our eyes to perceive the world around us. Usually humans and most other visual mammals initially detect the important parts of a scene through peripheral vision and then shift their eyes so that the central part of the retina, the fovea, can provide a more detailed image. (If you’re not convinced of this, try reading this sentence without moving your eyes.) The visual areas of the brain—particularly those in the cerebral cortex—that are devoted to processing information from this tiny but vital region are much larger than the areas that handle information from the lower-resolution, peripheral regions of the retina.
[pagebreak][media:node/1405 full caption]The touch centers of the star-nosed mole’s brain are organized similarly, with much more space in the cerebral cortex assigned to dealing with input from the central appendages of the star than from each of the less important peripheral appendages. Handling information this way conserves neural tissue, because it concentrates most of the brain’s computing power on only a small part of the sensory world at any given moment. Some scanning time might be saved if large areas of the brain received high-resolution data from the entire star (or from your entire visual field), but to do this, the brain would have to be gigantic. In addition to learning how this mole’s remarkable star works, we have been trying to determine how it evolved. For clues to the history of bony structures, one can turn to the fossil record, but there are no bones in the star and no fossilized mole noses. So we have turned to another, less direct place to look for evolutionary clues: embryonic development.
Most animal appendages (including antennae, wings, legs, fins, and arms) start out as simple extensions of the body wall—essentially as direct outgrowths of the embryonic tissue. Moreover, similar genes are expressed during the early development of appendages in animals as different as humans, fish, birds, and insects. This suggests that a basic “program” for appendage outgrowth evolved hundreds of millions of years ago and has been redeployed many times in the course of evolution.
But what about the star-nosed mole’s novel snout appendages? While we do not yet know the genes involved, we have been able to document the mechanics of the star’s development. As it turns out, the star’s appendages develop unlike those in any other animal, suggesting that it had unique precursors and an entirely independent evolutionary history.
Working in collaboration with Kaas and Glenn Northcutt, of the University of California, San Diego, I examined star-nosed mole embryos at various stages of development. We quickly found that all but the very earliest embryos have a protostar (as well as huge embryonic forelimbs destined to become the digging arms of the adult mole), but that instead of forming as outgrowths of the embryonic nose, the star’s twenty-two appendages first appear as slight, elongated swellings on the embryonic face. In later stages, when the swellings are more pronounced, it almost looks as if the star has been folded back against the side of the face. This impression is not quite accurate but does foreshadow events to come.
During most of the mole’s embryonic development, nothing separates the swellings from the side of its face. But just before birth, a new layer of epidermis grows underneath the swellings. At this point, the appendages become separate cylinders, though they are still attached to the face by this new skin.
Shortly after birth, the back end of each cylinder detaches from the face and swings forward, remaining attached only at its front end (a bit like peeling a banana). What was once the hindmost part of each cylinder thus becomes the forward-facing tip.
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The developmental sequence of the star differs from the way other animal appendages are known to form. In his book The Blind Watchmaker, biologist Richard Dawkins gives examples of unusual developmental sequences in other animals, such as the sole, a bony flatfish that spends most of its life lying on one side and has both its eyes on the upward-facing side of its head. As a young sole develops, it essentially “pulls” one eye across its face, grotesquely distorting its The mole uses the 25,000 minute touch receptors on its exquisitely sensitive star to find its way around its subterranean world. skull and facial musculature. This awkward process makes sense only when we appreciate that flatfish were not designed carefully and then created, but rather evolved from upright fish that had symmetrical bodies. As Dawkins points out, the ancestors of flatfish must have begun their evolutionary journey by lying on their side on the ocean floor—a position that would have resulted in one eye facing the bottom. In each subsequent generation, the most successful offspring would have been those in which the eye shifted slightly closer to the other side of the face. Today we can read the course of this evolution in the development of a flatfish.
What clues does the unusual development of C. cristata’s star provide about the evolution of this animal? Perhaps the ancestors of the star-nosed mole had strips of Eimer’s organs along the side of the face, and perhaps, in the course of evolutionary time, these strips slowly elevated and eventually “peeled” forward to form separate appendages. If so, the stages of this evolutionary sequence may have been conserved in the present sequence of embryonic development. While this explanation of the scenario seems reasonable, we can’t be certain of it without further evidence. Hoping that an analysis of living species could provide insights into the star-nosed mole’s past, we began to examine other moles from around the world.
Our studies showed that nearly all of the roughly thirty species of mole have some Eimer’s organs on the tip of the snout (usually 1,000 to 2,000, distributed evenly around the nostrils) but no indication of starlike appendages. Three species in the genus Scapanus from western North America, however, seemed to provide what we were searching for. The coast mole (S. orarius), for example, has—in addition to Eimer’s organs around its nostrils—small, raised modules of these sensory receptors surrounding the center of the nose. Moreover, the adult snout of the coast mole bears a strong resemblance to the early embryonic snout of the star-nosed mole, with “proto-appendages” pointing backward and adhering to the sides. This arrangement is just what we would predict for an ancestor of the star-nosed mole.
The coast mole, of course, is not ancestral to the star-nosed mole; in fact, the two are not even especially closely related. But the existence of the coast mole’s proto-appendages supports the proposition that the ancestors of the star-nosed mole had similar structures. Embryonic development in the coast mole stops at the proto-appendage stage but continues in the star-nosed mole, leading to the unfolding of separate appendages that form the marvel of sensitivity we see today.
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Eastern Mole, Scalopus aquaticus Unless you make a concerted effort to find them, you are unlikely to encounter star-nosed moles, which spend most of their time mucking about in the muddy soil of wetlands and stream banks. (In fact, it is the wetness of their environment that allows them to sport their extravagant star; moles rub their noses against the soil continually and dirt would quickly damage their delicate fleshy appendages.) But if you live in a house with a yard, you may well be familiar with the tunnels and mounds of some of North America's six other species of mole. The architects themselves seldom emerge above ground, but if you do come across one—unearthed perhaps by a neighborhood dog or deposited on your doorstep by the family cat—you can easily identify it. A mole is a fairly small mammal (about the size of a hamster) with tiny eyes that are often completely hidden by fur, but the dead giveaway is the animal's gigantic, heavily clawed forelimbs, or arms, which are held close to the side of the body.
Huge muscles, connected to massive rectangular humeri (equivalent to your upper arm bones), power the mole's arms. And its clavicle (collarbone), which in most mammals articulates, or connects, with the scapula (shoulder blade), articulates directly with the humerus. These anatomical oddities give the animals their neckless appearance but also power the distinctive breaststroke motion with which the moles dig. (Most other digging animals burrow with their feet positioned below their body.) Moles can generate tremendous force as they tunnel through the soil. In a mole-ridden lawn, you may actually hear the sound of roots breaking as the animals make their inexorable advance through the ground.
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Moles dig two kinds of tunnel: shallow surface runs, which appear as ridges in the ground, and excavations that may be several feet deep. Ridges form as a mole pushes the overlying soil upward, as if it were traveling under a carpet. If you step on a ridge you will feel yourself sinking into the ground. Soil farther below the surface can't be so easily compressed, so to dig deep, moles have to push soil right out of the tunnels. They do this at the nearest convenient openings, forming the famous mole hills from which mountains are made.