Sex and violence in the ice age world.

Mammoths and mastodons, those shaggy relatives of modern elephants, thrived through the ice ages only to disappear abruptly at the end of the Pleistocene epoch, 10,000 years ago. One paleontologist is tracing the cause of their demise by looking into the mouths of the ancient beasts.

Some researchers think that climate change drove the Pleistocene extinctions. They note that great elephants and many other North American mammals died out just as the ice age ended. Others assign blame to human hunters, who first migrated to North America around 11,500 years ago.

Daniel C. Fisher of the University of Michigan in Ann Arbor is testing the scenarios by studying annual growth rings in the fossilized tusks of mammoths and mastodons. Using living elephants as an analog, he determined how long it took the extinct animals to reach sexual maturity by looking for a distinctive span of depressed tusk growth during adolescence.

The timing of maturation is important because it hints at the kinds of stresses affecting the ancient elephant kin. Many modern animals take longer to reach sexual maturity when they encounter harsh climatic conditions or food shortages. In contrast, they mature more quickly if threatened by predators.

Fisher has completed an analysis of eight specimens of mammoths and mastodons, all of which lived in the eastern Great Lakes region. Judging from this limited group, he sees signs of accelerating sexual maturation between 12,000 and 10,000 years ago-a trend consistent with the hunting hypothesis, at least for this region. "I applaud the method," says S. David Webb of the University of Florida in Gainesville. He intends to use Fisher's technique to see whether Florida specimens show a similar pattern.