Star of the Swamp
The star-nosed mole has an incredible sense of touch that we humans can only envy
A neuroscientist is not the sort of person you'd expect to know how to catch a mole. Yet at least once a year, Ken Catania, a brain researcher at Vanderbilt University, ventures into the swamps of northwestern Pennsylvania to set his traps. Mole tunnels are painfully obvious on a suburban lawn, but in wetlands, finding them is a fine art. "You have to get a feel for where this animal would make its burrow," says Catania.
When Catania finds one of the creature's tunnels, he attempts to temporarily capture the mole. "Catching these guys takes hundreds of traps and checking them every few hours around the clock for a week," he says.
For Catania, not just any mole would inspire such effort. His quarry is the little-known, aptly named star-nosed mole. The animal for the most part is like a normal mole, except that surrounding its nostrils are 22 fleshy rays. They give the creature the surreal appearance of having miniature hands pasted to its nose--an effect that's intensified by the constant writhing of the tentacles. The star is not some vestigial oddity, research has shown, but a true neurological wonder. It provides the star-nosed mole with what may be the keenest sense of touch of any animal.
Scientists have been speculating for decades about the function of the mole's star, and they've relied mainly on hints from the way the animals live. Ranging from Canada down through the eastern United States as far south as Georgia, the star-nosed mole is remarkable for being the only mole that lives in wetlands. Though it is commonly found in these areas, the creature is rarely seen. Colonies of the moles dig tunnels through the mud, some leading to ponds, others to dead ends, and others to a nest of dry vegetation on land. The females give birth in the nest in late spring to babies, each of which is already bestowed with its own star. After three weeks, the young moles are then ready to begin finding food on their own.
The star is clearly crucial for this hunt. As the poor-sighted moles make their way down a tunnel, they sweep their tentacles back and forth with incredible speed. High-speed videos that Catania has made show that the tentacles can touch ten different objects in a second. As soon as they come in contact with prey--a worm or an insect--they snatch it up in a flash. Star-nosed moles can also swim (something that no other mole can do) and in the water they feed on small crustaceans and fish.
Some researchers have suggested that the moles use the star like a hand to touch surrounding objects. Others wondered if the star might have a more exotic function. The platypus (as well as some species of fish and amphibians) has special receptors that let it detect electric fields of other animals in water. The platypus uses its electroreceptors, located near the nose in its bill, to hunt in water. Perhaps, scientists speculated, the mole also has this sixth sense.
However, no one could say for sure what function the star served until someone looked at it up close. That's what Catania decided to do for his dissertation at the University of California at San Diego in the laboratory of noted neuroscientist Glenn Northcutt. "I just showed some pictures of the nose around the lab," Catania remembers, "and everybody got very excited."
Catania knew that electroreceptors have a very distinctive look. "There is a fairly large pore that leads to sensory cells and a conducting jelly in the pore, and you can see all this clearly under a microscope," he explains.
Catania could find nothing like them on the star when he looked at it under a high-powered microscope. Instead, he found, the star is covered with goose-bumplike structures called Eimer's organs. These organs are found on the noses of all moles, as well as some related insectivores like shrews. Each contains a cluster of nerves that make them particularly touch-sensitive.
But while Catania now doubts that star-nosed moles can detect electric fields, that doesn't mean that the creature's strange appendage is any less remarkable. Other species of moles have about 2,000 Eimer's organs on their noses, but the star-nosed mole has more than 25,000.
In order to handle the sensory blizzard that must be created by its star, the mole has an equally remarkable nervous system and brain. Collaborating with neuroscientist Jon Kaas at Vanderbilt, Catania found that more than 100,000 nerve fibers run from the star to the brain--almost six times more than come from the touch receptors in a highly sensitive human hand.
Such discoveries caused Catania to conclude that the star-nosed mole can distinguish things by sense of touch that people could only dream of recognizing. "A lot of things in a mole's environment probably have a distinctive texture at the microscopic level, and with one touch this animal knows exactly what it has touched," he says. "It's speculation based on the structure."
Given the advantages of having a star, why would only one species of mole have the appendage? The evolutionary explanation, Catania believes, has to do with how much abuse moles give their noses. Since they spend much of their time digging tunnels with their massive front paws, they rub their noses against the dirt almost continually. It's hard on their skin, and particularly on a delicate Eimer's organ. Under most conditions, then, the delicate, sensitive, blood-rich star would be damaged. In fact, Catania has found that the eastern mole, which often lives in dry soil, has actually evolved in the opposite direction of the star-nosed mole. Its Eimer's organs have degenerated to where they have fewer nerve endings.
The star-nosed mole, on the other hand, lives only in mud. "The moisture in its environment is critical," explains Catania. As a result, the star-nosed mole doesn't have to cope with nose burn, and so the evolutionary constraints that other moles have to deal with do not apply. "You ask, 'Why don't other moles have this star?'" says Catania. "Well, the fact is no other mole could have it."