The family cat may sit on your lap for hours, purring in domestic bliss as it savors human companionship, but underneath that friendly fur beats the heart of a lion.
An international team of scientists led by geneticist Wes Warren of the Washington University School of Medicine in St. Louis has completed the first comparative analysis of the complete genomes of wild and domesticated animals, including some of our favorite friends: dogs and cats.
They found some differences, caused by genetic changes driven by humans through selective breeding programs, but the differences between cats and their wild kinfolk is not all that great, Warren said in a telephone interview. It is not nearly as great, for example, as the difference between dogs and wolves.
Dogs have been domesticated for about twice as long as cats, and despite human efforts to push the domesticated cat further away from wild varieties, "the cat has still maintained more of its wild cat genome, as in behavior characteristics, than the dog," he said, so cats should be considered "semi-domesticated."
"Many cat owners will vouch for this," he added. "If you release your cat at night it will go out and hunt and be as efficient as a wild cat at catching mice and birds and many other critters."
Other scientists agree. Researchers at the University of Illinois at Urbana-Champaign attached radio collars equipped with tilt and vibration sensors to snoop on the lives of 42 cats over a two-year period. The subjects included both feral and domesticated cats, and the study concluded that there's a lot going on out there under the cover of darkness.
The domesticated cats tended to stay closer to home than the wild variety, but they strayed farther than their owners would have expected, the researchers noted.
Other studies have shown that domestic cats routinely mate with feral cats, but the Illinois study revealed that it's not exactly a blissful relationship.
The leading causes of death among the feral cats were fights with other cats and disease, so when the family kitten goes out to play it may come back with scars and diseases that can be passed on to other cats.
The researchers found one feral cat that would wait behind a house each night for a pet cat to emerge, and then chase it out of its own backyard. Another pet got chased out of a dairy barn.
That kind of shaky co-existence undoubtedly dates back to when the first cat joined ranks with humans, whenever that was. The best evidence so far of the first attempts at domestication comes from the archaeological discovery in Cyprus a decade ago of the bones of a cat and a human buried together, suggesting a real friendship between felines and humans.
Some scientists, including researchers at the University of California, Davis, who studied 11,000 cats, or their remains, believe the domestication process really began in a Middle East region called the "Fertile Crescent." That area is also known as the "cradle of civilization" because rich natural resources there gave birth to the agricultural era.
Cats were welcomed by the nascent farmers around 5,000 years ago because, as we all know, they are great mousers. And mice, like other rodents, can destroy a crop by devouring the grains and seeds.
So the cat, like the dog much earlier, was welcomed into the human family because it could be useful.
Like all domesticated animals from elephants to foxes, the most valuable trait in any potential pet is tameness, and in the beginning that was the primary target of all breeders. Find the animals that were most pleasant to be around, and help them breed more quickly at the expense of the wilder varieties.
Over the years that has changed significantly. Breeders today carry out natural selection their own way, so it really isn't evolution, its human -- not natural -- selection. And the most cherished features among breeders for cats today? Fur and color, Warren said.
That may not be much help to scientists, but he said there are some changes in the cat genome through the domestication process that could help reach what he called the "holy grail" among psychiatric researchers.
"We are trying to understand genes that influence behavior," he added.
By comparing the genomes of cats and dogs and humans and other domesticated animals, he and his coworkers, who published their latest findings in the Proceedings of the National Academy of Sciences, hope to use similarities in the DNA to learn more about which genes are involved in which behavioral disorders and diseases.
Cats, for instance, can also suffer from a feline version of Alzheimer's disease, as well as other illnesses that afflict humans.
It is the differences among species that can be most enlightening. Cats have extraordinary hearing and vision, even at night, and dogs have an amazing sense of smell, as all pet owners probably know.
"We are comparing sensory systems, like visual, hearing and sense of smell," Warren said. "When you compare cats to humans, changes in genes show up, and that makes sense because they have a much higher ability in seeing and hearing than humans do.
"And when you compare dogs to cats you see an evolutionary tradeoff," he said. "Dogs are well known for their sense of smell, and cats not so much."
Sensory receptors are critical for the animal's ability to pick up odors, so it stands to reason that dogs should have more receptors for smell of a wide range of odors than cats, and that's just what the researchers found.
Conversely, cats have more receptors for chemicals called "pheromones," that tell when a female cat is ready to mate, since cats (except lions) tend to be solitary animals and need a little help finding true love.
Incidentally, one cat played a major role in the research by Warren's team as well as earlier work led by Cold Spring Harbor Laboratory.
Both teams focused on Cinnamon, an Abyssinian cat whose lineage reaches back over generations to Sweden and is thus clearly domesticated.
Like some humans, that breed is vulnerable to degeneration of the retina, leading to blindness, and Cinnamon has helped many scientists understand a little more about that disease. But she's no longer available.
"She succumbed to old age and passed," Warren said. "But we still have blood samples and DNA in the freezer," so she lives on.