Evolution of Tabby Cat
Mapped in Brain Study

By comparing the brains of Spanish wildcats and American domestic tabby cats, a researcher in Tennessee has discovered a biological mechanism that may explain how members of a species may adapt their brains to undego rapid evolutionary changes.

The finding is that wildcats and domestic cats development the same number of brain cells as fetuses, but in each species different sets of neurons are killed off just before birth. The result is presumably that each cat's brain is better adapted to fit its environment.

Thus the wildcat retains nerves cells that mediate excellent color vision and enable it to hunt in the bright Iberian sunlight, said the researcher, Dr. Robert Williams, as assistant professor of anatomy and neurobiology at the University of Tennessee College of Medicine in Memphis. [This is pure speculation: Color vision has never been tested in wildcats, and it is not known whether they hunt at day.]

But the domest cat, which is nocturnal discards most neurons for color vision and instead nurtures cells that sense motion and objects in dim light. Other important brain areas are also sculptured differently in the two cats, Dr. Williams said. [The difference between domestic cats and wildcats are primarily quantitative and not qualitative as implied in the preceeding paragraphs. There do appear to be differences in relative abundance of cells in retinas of wildcats and domestic cats, and some of these differences may arise from variation in the severity of cell death. However, it is not unlikely that there is a great deal of variation among domestic cats themselves.]

The immense evoutionary advantage of adapting to different environments by killing off selected brains cells before birth, Dr. Williams said, is that the animal retains the ability to re-evolve traits rapidly should the world change abruptly. Thus, the domestic cat has the latent capacity to redevelop rich color vision should it ever need to switch to daytime hunting, he said. [Lineages can evolve, lose, and re-evolve traits. See Christoper Wills' book The Wisdom of the Gene]

Compared to closely related wildcats, the domestic cat las lost 30 to 50 percent of its brains cells in adapting to the lap of luxury, Dr. Williams noted, although this does not mean pussycats are stupider that wildcats. Each animal has an intelligence honed for making a living in its chosen niche, he said.

Dr. Williams's work, described as the first experimental study linking cell death with brain evolution, appears in the January issue of The Journal of Neuroscience. His co-authors are Dr. Carment Cavada and Dr. Fernando-Reinoso-Suarez at the University of Madrid.

Dr. Leo Chalupa, a professor of neuroscience at the University of California at Davis and Dr. Williams's thesis adviser in the mid-1980's, said his former student "really deserves a lot of credit for being innovative and clever." He added, "Very few people in neuroscience take an evolutionary perspective, and this is a very important finding."

Dr. Harvey Karten, an expert on evolutionary biology at U.C. San Diego, was more cautious. "To say that selective fetal cell death is a mechanism for rapid evolution is an interesting idea," he said. Dr. Williams "demonstrates the loss of color vision in the domestic cat in just 20,000 years, which is extremely rapid," Dr Karten said. However, he said, "to say cell death in the mechanism through which evolution expresses itself goes beyond current data." [Cell death is just one of many traits that can be modulated to alter the size of neuronal populations. I agree with Dr. Karten, the at the data are not yet adequate.]

Dr. Murray Sherman, an expert on the cat visual system at the State University of New York at Stoney Brook, said it was not unusual for closely related species and even subgroups of the same species to have very different visual systems. "the real surprise is the speed at which the change occurred in the cat," he said. "The idea that a part of the brain can evolve so differently in so short a time makes one wonder about different human races. Are we different in subtle ways?" [There are very prominent differences in brain structure between individual humans. For example the area of the visual cortex varies three-fold among humans. Against this huge variation among normal humans, that between racial averages is likely to be comparatively small (see S. J. Gould's The Mismeasure of Man). If you are interested in the genetic basis of variation in brain structure refer to recent papers by R. Williams and colleagues.]

In mammals, up to half of all fetal brain cells are killed off before birth, Dr. Williams said in a telephone interview. It [cell death] happens in every brain structure examined. For example, an adult human has 1.2 million to 1.5 million ganglion cells in each eye, while a third-trimester human fetus has 2.5 million such cells in each eye.

But cats exhibit an even greater level of cell death in the developing eye, Dr. Williams said. An adult has 150,000 ganglion cells while an unborn kitten has 900,000 cells per eye. "The domestica cat loses four out of five ganlgion cells produced befor birth," he said. "Why?"

The fossil record shows that the domestic cat shrank in size over the last 20,000 years, Dr. Williams noted. If is half as large as the wildcatts from which it descended, so perhaps its brain shrank as well.

But how? A brain can shrink by making few cells, Dr. Williams said, or by packing the same number of cells more densely or by killing off more cells before birth.

Living Fossils in Iberia

It occurred to him that the answer might emerge in comparing the brains of wildcats and modern domestic cats. By a fluke of nature, a wildcat species--Felis silvestris tartessia--has survived unchanged for the past 20,000 years in the mountains of Spain, Dr. Williams said. The Gulf Stream protected the Iberian peninsula from the ravages of ice ages and rapid wamring so that the wildcat's environment has remained amazingly stage, he said. As living fossils of the Pleistocene, the tabby-like cats successfully evade human contact while their cousins in Europe and Africa evolved into domestic species.

Several years ago, gam wardens captured two of the solitary wildcats, Dr. Willimas said. Although the animals ususally weigh 14 pounds, these were skinny and so badly injured that they could not be released into the wild. After attempts to breed them produced no kittens, the cats were sacrificed for scientific study.

Dr. Williams compared them with a domestic male tabby cat that weighed nearly 20 pounds. While most domestic cats weigh about seven pounds, a large cat was used to rule out the effect of body weight on brain size.

Surprisingly, the brain of the fat tabby weighed 28 grmans, but that of the male wildcat, thin as it was, weight 37 grmans, or nearly a third more. Moreover, the domestic cat's skul was twice as thick as the wildcat's.

Through the retinas of the two cats were the same size, that of the wildcat packed 100,000 cones--cells that specialize in color vision--onto each square millimeter, compared with the mere 35,000 cones of the domestic cat. The wildcat had [nearly] twice as many ganglion cells that connect the eye to the brain and a third more cells in the brain's first visual relay station behind the optic nerve.

But cells that facilitate black and white vision, motion detection, object recognition and perception in dim light remain the same in both cat species, Dr. Williams said. The part of the brain that helps a cat catch a mouse in the dark is intact.

Dr. Williams concludes that the domestic cat lost color vision over the past 20,000 years, which in evolutionary terms is extremely rapid. It is not likely that the Spanish wildcat gained color vision in that period of time, he said, since the trait is extremely complex. A more likely explanantion, in his view, is that domestic cats pruned away most cells for color vision before birth because they had less need for them.

The rest of the domestic cat brain has undergone similar changes, Dr. Williams said.

Unbeknownst to the gamekeepers [or the scientists involved], Dr. Williams said, the female wildcat was pregnant with a single kitten. Upon examination, its brain contained a similar number of neurons as a developing domestic kitten of the same gestational age.

Thus programmed cell death in the developing brain may explain why animals that live in stabel environmetns undergo little evolutionary change while animals under enrionmental pressure can change extremely rapidly, gaining or losing traits with astonishing speed. Such changes do not occur in one generation, Dr. Williams said, but take thousands of years.

Domestic cats probably got smaller in response to climate shifts and became nocturnal in response to hanging around villages filled with hostile humans, Dr. Williams suggested. With the exception of ancient Egyptians and a minority of modern humans, he said, people have thrown rocks at, hung or tortured cats more than they have cuddled them. For a cat in human proximity, it makes sense to hunt or steal food at night when people are asleep.

Domestication may have altered the size of the domestic cat's amygdala, a brain cetner that controls aggression and docility, Dr. Williams said. he plans further studies to look at the amygdala and other brains regions of the two cats.

But why the domestic cat's skull is thicker remains a mystery. Although there is a fold saying that "the thicker the skull, the dumber the animal," there is no evidence domestic cats are stupider that their wild brethren, said Dr. John Gittleman, a zoology profesor at the Univesity of Tennessee and expert on carnivore evolution. There are many kinds of intelligence, he said, and the nubmer of brains cells is less important than the way in which they are connected.