Planet Earth


By Jeffrey KlugerJul 1, 1994 12:00 AM


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Most pet species--as any pet owner can tell you--do not generally take the trouble to age. The majority of nonhuman creatures we bring into our homes live their lives in one of only two states: a) healthy and hearty and fresh from the pet store, or b) dead. There is no slow slide into old age in a birdcage, no sudden appearance of golf shoes and lime green slacks in the hamster community, no crotchety grumbling about the menu during feeding time in the aquarium.

Male guppy: Have I eaten this before, Ethel?

Female guppy: Yes, you have, dear.

Male guppy: Did I enjoy it?

Female guppy: No, you didn't, dear.

Male guppy: Maybe I'll just wait for the pound cake and Sanka.

But when it comes to the world's favorite pets--dogs--things are different. Man's best friend ages in much the same way as man himself does. For every human septuagenarian suddenly struggling with fading eyesight, brittle bones, and an inexplicable taste for Tony Bennett, there's an aging dog experiencing many of the same problems. Among the most bothersome symptoms of old age plaguing both species is loss of hearing. Though most of us expect our great-grandparents to suffer from hearing disabilities, we have higher hopes for our Great Danes. According to veterinarians, however, any older dog can expect to lose at least some of its auditory abilities before it shuffles off this canine coil.

Timmy: Lassie, Farmer Johnson's barn is on fire and his cows are trapped inside!

Lassie: You say Lyndon Johnson's eating a tire and his lapels are way too wide?


Lassie: Fine, thank you. And you?

All of this may soon change. Now in development at Auburn University in Alabama is the first hearing aid ever designed exclusively for dogs. In a pet-product industry that has already concerned itself with whether your dog is getting enough cheese in his Gaines Burgers (answer: no; but if you've ever been face-to-face with the family poodle and inhaled at the precise moment he exhales, a lump of sharp cheddar would not be the first thing you'd think of offering him), canine hearing aids seem long overdue. It was not until a few years ago, however, that the folks at Auburn took the first steps toward making the earpiece for dogs a functioning reality. Since then the Alabama inventors have gone a long way toward making Rin Tin's tin ear a thing of the past.

The dog hearing aid was the brainchild of Curtis Smith, an Auburn audiologist. Smith came by his interest in canine audiology in 1988 in a least-scientific, most-personal way: a friend's dog started going deaf. In humans, the early signs of hearing loss are usually easy to spot. When your grandfather starts listening to the McLaughlin Group at volumes that annoy blue whales off Antarctica, or answers the question "How are you?" with "tuna fish," chances are his ears are lying down on the job. With dogs, things are a little less clear.

"A dog can't tell you when he's losing his hearing," says Smith. "But there are ways you can test him. The most obvious one is simply to call his name. If your dog usually answers to your voice and one day you find yourself yelling 'C'mere, Blue! Fetch, Blue!' and he doesn't respond, chances are he's got a hearing loss."

Chances, of course, could also be that his name isn't Blue, but in the case of Smith's friend's dog, the problem lay elsewhere. "Deafness in dogs is just like deafness in humans," he says. "It can be caused by three things: an infection, some kind of trauma or injury, or nerve degeneration. The first two problems can develop at any time; the third is mostly a function of age."

Dogs, like humans, other mammals, reptiles, and birds, owe their ability to hear to an organ called the cochlea, a hollow structure in the inner ear shaped like a snail's shell. Inside the cochlea are hair cells, each sprouting microscopic bristles that vibrate in response to sound waves. The hair cells send their signals through the auditory nerve to the brain. As a creature gets older, its hair cells die and their signals to the brain fade.

For the hard-of-hearing hound, life can be a lot more difficult than for the hard-of-hearing human, and for a very simple reason: dogs rely on their ears far more than we do. If you're crouching in the grass in defense of your territory, you need to be able to hear any interlopers. If you're stalking through a field in search of prey, you need to be able to hear it move. If you're planning to eat the Federal Express man, you need to hear him coming up the walk.

For these reasons, nature endowed the canine cochlea with more snaillike turns than the human cochlea; more turns mean more hair cells, and more hair cells mean better hearing. The average dog can detect sounds between a low frequency of 30 hertz and a high of 50,000. Humans range from 20 hertz to 20,000. (Dogs, for the record, are not the auditory champs. Bats can hear an even wider range of frequencies, as can whales. The all- time winners of the aural Super Bowl, however, are dolphins, whose upper limit has been estimated at 225,000 hertz. The all-time losers, on the other hand, are turtles and snakes, which come from the factory equipped with almost no hearing at all.)

After Smith's friend's dog's ears began to fail him, Smith visited the dean of Auburn's college of veterinary medicine to ask if any of the resident professors had been conducting work on canine hearing that might be of help. The dean said that indeed someone had been and introduced Smith to Arvle Marshall, who had written several papers on the subject.

Although Marshall had not made many strides in treating hearing loss in dogs, he had come up with an interesting way of diagnosing it. Subject dogs were brought into his lab and fitted with lightweight headphones (and, if they requested, Rollerblades, Spandex pants, and inverted baseball caps). Next, electrodes were attached to their heads and connected to a device that detected brain waves. Short clicks of various intensities were then played through the headphones, and Marshall could see on a computer screen which sounds elicited a response--if not a request for the original cast album. Most dogs cooperated in the painless procedure; those that didn't were given a mild tranquilizer to make them a bit more docile. But regardless of whether the subject dogs were sober or sedated, the results their brains yielded were the same.

"There is a specific brain wave associated with auditory stimulation whether a dog is aware he's hearing a sound or not," Marshall explains. "When you see it appear on the screen, you know a tone has been heard; when you don't, you know it hasn't. By cranking the intensity of the tones up or down, I found I could determine the limit of a dog's auditory range and discover how much hearing he had lost."

When Smith brought his friend's dog to visit Marshall, it turned out that the aging canine had indeed lost a considerable amount of hearing. The two researchers decided to work together to invent a device that would give him back at least part of what he'd lost. The first amplifier Marshall and Smith came up with was little more than a small human hearing aid wrapped in a piece of foam plastic and inserted in a dog's ear canal. On the surface, the idea seemed sound, but the subject dogs in Smith and Marshall's lab didn't agree. No sooner was the hearing aid inserted than the dogs would shake and paw their head until the device flew out. They would proceed to sniff around on the floor for it and then, well, eat it.

"Clearly," Smith says, "this design was not a good one."

For the next two years Smith and Marshall experimented with a number of models that were smaller, lighter, and, they hoped, less intrusive, but nearly all of them wound up as microchip snack chips. Finally, the researchers concluded, if dogs objected to having an earpiece placed in their ear, why not put it somewhere else?

Their new design called for attaching the hearing aid hardware to the dog's collar. The amplified sound would then travel through hollow, almost weightless tubes to the dog's ears, where the tubes were held in place with a piece of foam plastic--it's the same sort of arrangement that was once used on airplanes to carry music from an outlet on a seat's armrest to a passenger's ears. As long as their subject dogs didn't begin listening to a medley of Burt Bacharach hits or selections from Mantovani, Marshall and Smith figured, hearing would be restored.

"The lighter the object in the ear canal was, the less we figured dogs would object to it," Smith says. "When we tried this model, the dogs weren't completely happy with it, but they at least seemed able to tolerate it."

To persuade a dog to do more than just tolerate the hearing aid, Marshall and Smith have developed a training regimen designed to acclimate it to the new gadget. As with all other training routines, this one involves teaching the dog to associate wearing the amplifier and tubing with something pleasant or reinforcing. For a human trying to get used to a hearing aid, the ability to hear Tom Brokaw discuss guaranteed health care without having to ask, "Did he say galvanized menswear?" would be reinforcement enough. For dogs, however, Smith and Marshall had to find a reward that was a bit more primal.

"You have to train a hard-of-hearing dog to wear the device a little at a time," Smith says. "Start off at just a few minutes and build up to half an hour a day, then an hour, and so on. The whole time the dog is wearing it, you should be petting him, talking to him, feeding him his favorite food. Basically, he has to make the association that when the hearing aid is inserted, something good is about to happen. Within three or four weeks he should be able to wear it all day."

Certainly there are flaws in Smith and Marshall's system. Though some dogs are bright enough and trainable enough to learn to wear the hearing aid easily, others may never get the hang of it. Does your pet a) react to the presence of an intruder in the house by bringing him a rawhide chew, b) bark at lawn flamingos, or c) bark at the lawn? If you answered yes to any of the above, you probably shouldn't expect great results from a dog hearing aid. Nevertheless, in the two years since Marshall and Smith developed their hearing aid, over 50 dogs have been fitted with the apparatus, nearly all successfully.

Despite the early success of the product, Marshall and Smith do not see it as an auditory panacea. For one thing, since most people have trouble training their dogs to obey such fundamental commands as "sit," "stay," and "please don't drink out of the toilet," it is only the most devoted dog owners who would have the patience to work with their pets for the month it would take the animals to grow accustomed to the device.

Less troubling but still significant is that the hearing aid amplifies sounds only in the 500 to 6,000 hertz range, a frequency span in which many, but by no means all, audible sound waves fall. "We can help a dog hear some things," Smith says, "but many things he'll still miss."

A final problem has do to with the dog's outer ear, or pinna. In humans the pinna remains stationary at all times (or at least in most humans; for the purposes of their research, Smith and Marshall excluded Soupy Sales, Jerry Lewis, and Shemp). In dogs, however, this additional flap of sound-catching flesh moves in a wide range of directions to improve the reception of incoming sounds, and the constant scanning can gradually work a hearing aid loose. This, Smith and Marshall think, is another reason that their device should be sold only to the most attentive dog owners, ones who will be conscientious about reinserting the earpiece when it falls out.

Despite such minor drawbacks, Smith and Marshall have begun to explore other ways to improve animal hearing. They've designed another hearing aid for one of humankind's other favorite animals, the horse. After all, horses can lose their hearing just as easily as dogs or humans, and the results can be just as maddening.

Horse: So anyway, I'm at this party talking to Dick Cavett and I say to him, "Dick--"

Owner: I said "canter," not "banter"!

Smith and Marshall can look forward to an expanding market for their future inventions. Just as improved nutrition and better health care are leading to an increasingly older human population, so too is better veterinary care leading to an increasingly older pet population. As it does, specialists like Drs. Smith and Marshall could wind up doing more for interspecies communication than Dr. Dolittle ever did. It's all well and good to talk to the animals, but it helps if they know what you're saying.

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