Lean times and fat times recorded in teeth.

Cementum, a bone-like tissue that grows in bands and anchors the teeth in the mouth, can reveal crucial details about an animal's life and death. It not only records the age and season of death of mammals, but also can be used to reconstruct diet and growth rate over the animal's lifespan, says Daniel E. Lieberman, an anthropologist at Harvard University.

Implications of the discovery span the fields of archaeology, anthropology, and bone biology, says Farish A. Jenkins Jr., an anatomist at Harvard. Lieberman published his findings in the Aug. 27 SCIENCE.

"Cementum is like tree rings in that it continues to grow throughout life," preserving a record of the seasons, Lieberman says. But while scientists have used cementum as an indicator of age for many years, they did not understand what causes the bands to form.

To get to the root of the matter, Lieberman put six goats on strict diets. He varied their cuisine every four months for one year, giving different goats food of varying hardness and nutritive value. He found that two phenomena explain the banding patterns: nutrition and the strain that chewing places on teeth.

A complex array of collagen fibers, extruded by the gums, holds teeth in place. The fibers become mineralized, forming cementum. Lieberman discovered that when an animal chews tough food, the collagen fibers orient more vertically to keep the teeth from sinking. On a softer diet, the fibers orient horizontally. When viewed under a polarized-light microscope, vertical bands appear opaque, horizontal bands translucent.

In most mammals, therefore, opaque bands indicate hard food, lean times, and winter, whereas translucent bands indicate the seasonal abundance of summer's often softer, easier-to-chew food.

Furthermore, cementum growth slows when animals are nutritionally deprived. These slow-growing bands, however, are highly mineralized and thus strong.

Lieberman is using the bands to answer questions about the behavior of our human ancestors. One ongoing study examines what prompted hominids to begin eating meat -- a high-risk activity given the dangers of the hunt. Many anthropologists propose that necessity drove protohumans to become carnivorous during dry seasons, when other food was scarce. Cementum of animal teeth found at archaeological sites should record the season of the animal's death, thus bolstering or refuting this theory. In a separate study, Lieberman is analyzing differences in diet between the line of hominids that evolved into modern humans and their cousins, who turned out to be an evolutionary dead end.

The findings also provide a means of studying bone growth. Lieberman's experiments demonstrate a direct relationship between force (from the strain of chewing) and the shape of the tooth, says William J. Landis, a cellular biologist at Harvard Medical School in Boston. This relationship between force and bone structure may provide insight into the mechanisms involved in osteoporosis, care of bedridden patients, and bone depletion in astronauts living in zero gravity, Landis says.