Tusk analyses suggest weaning took years.

Tusks of juvenile mammoths carry a chemical record of the animals' environment and behaviors, including how quickly they became weaned from their mothers, scientists have found.

As the tusks of modern elephants do, a mammoth's tusks grew continuously from birth. Features in the ancient ivory, analogous to the growth rings of a tree, chronicle years, weeks, and even individual days in a mammoth's life, says Adam N. Rountrey of the University of Michigan in Ann Arbor. The proportions of nitrogen and carbon isotopes present in tusk material reflect what an animal ate and the climate in which it lived, Rountrey notes.

In their study, he and his colleagues analyzed a 39-centimeter-long fragment of the left tusk of a juvenile mammoth that had lived on an island off northeastern Siberia. The scientists estimate that the piece represented about 4.5 years of growth in an animal that was about 6 years old when it died.

The team mapped how the proportions of carbon and nitrogen isotopes varied in the tusk's collagen, a protein constituent of ivory. They found that the collagen that was deposited when the mammoth was 2 years old had higher-than-average concentrations of nitrogen 15 but lower-than-average concentrations of carbon-13--signs that the animal was then deriving much of its nourishment from its mother's milk, not vegetation.

As the young mammoth grew, the proportions of the nitrogen and carbon isotopes in newly deposited tusk material came to more closely resemble those of an adult, grass-eating mammoth, says Rountrey.

Because by the time that the mammoth died, its isotopic proportions were near adult values, Rountrey and his colleagues suggest that mammoths took at least 6 years to fully wean their young. Using the analysis technique on other mammoth fossils may enable paleontologists to better estimate the age at which those individuals died, says Rountrey.