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.