Biologists have been a little more successful in understanding how cats purr. In February 1991, physiologists Dawn Frazer and David A. Rice of Tulane University in New Orleans, La., and G. Peters of the Alexander Koenig Museum in Germany reported that purring consists of low frequency sound produced by vibrations of the cat's larynx (voice box) during inhaling and exhaling. These researchers recorded the sounds through instruments that they placed on various parts of the cat's body. They found that the sounds were not linked to the sounds of the cat's breathing pattern, which is why purring is continuous. Purring may in fact occur simultaneously with other vocalizations.

Scientists do not know the purpose of purring, however, especially as it might have served the cat's wild ancestor. Purring appears to be a sign of contentment, yet even cats that are ill and in discomfort may purr. Cat owners recognize that purring occurs when cats are around people, but the presence of people is not essential for purring. Cats may purr during mating —and kittens, while nursing. But cats of any age do not purr while they are sleeping.

Perhaps more intriguing than purring is the cat's ability to survive falls. The research of veterinarians Wayne Whitney and Cheryl Mehlhaff at the Animal Medical Center in New York City shed light on this ability in 1987. The cat's habit of falling out of open windows provided the researchers with an opportunity to study 115 cats that had fallen from high-rise apartments in New York City. The average fall was 5.5 stories. Of the 115 cats studied, 90 per cent survived, including one cat that fell 32 stories onto a sidewalk and suffered only a mild chest injury and a chipped tooth. Interestingly, cats that fell from 9 or more stories suffered fewer injuries than those falling from lower heights. Among cats that fell from 9 to 32 stories, only 5 per cent suffered fatal injuries, but 10 per cent of those that fell from 7 or fewer stories died.

How do cats manage to take falling so easily? For one thing, in comparison to human beings, a cat is much smaller and lighter. Also, a cat has more body surface area in proportion to its weight than a human being has. This increase in surface area results in greater air resistance, which slows the fall. The important thing, however, is that a falling cat apparently positions itself to form a sort of parachute. Less than one second after it starts to fall, a cat quickly rights itself in midair with all four legs pointing downward. The cat's inner ears act like an internal gyroscope, telling the cat which direction it is falling. With the legs pointed downward, the cat then spreads its legs so that its body forms a sort of parachute that increases air resistance. With its limbs flexed, the cat also cushions the force of impact by landing on all four legs. The force of the impact is distributed through the muscles and joints.

Whitney and Mehlhaff believe that the parachute effect comes into play mainly above four stories, at the point where the cat has reached its greatest rate of descent. Of the 115 cats the researchers studied, only 1 of 13 cats that fell nine or more stories sustained a bone fracture, whereas most of the cats that fell from lower stories suffered some type of broken bone.

Cats may be able to escape injuries from falls, but many suffer from serious diseases, such as feline leukemia; infectious peritonitis, an inflammation of the membrane that lines the walls of the abdomen; and panleukopenia (cat distemper). Since the mid-1980's, veterinary researchers have made great strides in understanding two other major diseases: a type of heart disease called dilated cardiomyopathy and a type of AIDS that strikes only cats.

For years, dilated cardiomyopathy was recognized as a significant cause of death of pet cats in the United States. This disease is characterized by enlargement and weakening of the heart to the point where it is incapable of pumping blood. Until 1987, the cause of the disease was unknown, and veterinarians could offer no cure. In 1987, veterinarians Paul Pion and Mark Kittleson and their colleagues at the University of California, Davis, School of Veterinary Medicine reported that cats suffering from dilated cardiomyopathy had a deficiency of an amino acid called taurine. (Amino acids are the building blocks of protein.) Giving the cats taurine supplements reversed the disease and led to a full recovery.

Before the California study, cat specialists had thought that taurine deficiencies occurred only in cats fed commercial dog food or home-cooked food that was nutritionally unbalanced. But the California veterinarians discovered that cats fed exclusively with some types of commercial cat food could develop the heart disease. Since the 1987 study, manufacturers of the cat foods have added more taurine to their products, and there has been a dramatic decrease in the number of cases of the disease.

Another important discovery was the recognition of feline AIDS in 1988 by veterinarian Neils Pedersen and his colleagues at the University of California, Davis. The disease is caused by a virus called the feline immunodeficiency virus (FIV). FIV is usually transmitted during a fight through the saliva of an infected cat that bites another cat, puncturing the skin. Because cat fights generally occur outdoors, feline AIDS is found mainly in cats that are allowed to roam freely or in multiple-cat households that adopt wild or homeless cats. In many areas of the United States where there is a large population of freely roaming cats, about 5 per cent of the feline population is infected. Since the disease was first diagnosed in 1988, veterinarians have found feline AIDS in cats throughout the United States, Canada, South Africa, Australia, New Zealand, and Japan.

The virus and the disease pattern are similar to that of human AIDS, but cats do not get the human virus, and people are not susceptible to the cat virus. Veterinarians had observed the symptoms of FIV infection for many years, but investigators did not recognize the disorder as a type of AIDS until the symptoms of human AIDS were recognized as a disease.

Like human AIDS, feline AIDS attacks the body's disease-fighting immune system. The feline disorder has two stages. The first stage begins when a cat is infected through a bite. About four weeks after the initial infection, the cat becomes ill with a fever but usually recovers. The second stage begins about two to four years after infection and is marked by the appearance of infections in the mouth and respiratory system and on the skin. These infections are called opportunistic because they afflict only cats whose immune systems are too weak to fight off the infection-causing microbes. Death from these AIDS-related infections is inevitable.

According to veterinarians, cat owners can best protect their cats from FIV infection by not allowing the cats to run free. If a cat is kept indoors even with an infected animal, the likelihood of transmission is small because indoor cats rarely bite each other.

Another major focus of research is cat behavior. Many people think of cat behavior merely as grooming, scratching, and sleeping. However, just because cats are not often seen performing tricks does not mean that cats do not easily learn.

Animal behaviorists say the intelligence or learning ability of cats is no greater or less than that of dogs. But comparing different species is difficult, because tests of their learning abilities must take into account differences in the animals' sensory and motor capabilities and in their inherited behavioral predispositions. For example, cats being tested for learning ability will not respond to the reward of petting as consistently as dogs will.

Nevertheless, as early as the 1920's, researchers found that cats can learn complex tasks, especially if the reward is food. And in highly structured tests of learning ability, cats often outperformed dogs in the ability to master conceptual problems. In the 1950's, animal behaviorist J. M. Warren at Pennsylvania State University at University Park described the cat's ability to master "oddity learning" in which the animal is shown three objects and is rewarded for selecting the one that is most unlike the other two. In the test, cats learned to paw a square block rather than two round blocks presented at the same time, because food was hidden beneath the square block. In similar tests, the cat chose the different object when presented with one round block and two squares.

Such tests require the ability to understand concepts, in this case, that of similarity and dissimilarity. Researchers have found that some cats do as well with this type of conceptual learning as monkeys. And, aside from monkeys and other primates, cats are among the most adept at learning by observing the successes and failures of other animals attempting to complete tasks to obtain a reward.