A new theory links the extinction of mammoths and mastodons to the disappearance of salt licks in Michigan at the end of the last Ice Age.
The idea was put forward by three Michigan researchers, who observed that the state's many salt deposits are situated in the same region in which fossils of the extinct elephants have been found. Both are confined to the southern half of the state's lower peninsula.
Salt deposits are common in southern Michigan, which supplies about one-quarter of the salt used in the country. Much of it comes from a large salt mine under Detroit. The same region has 203 known mammoth or mastodon sites.
The giant mammals died out between 10,000 and 12,000 years ago, just as Ice Age glaciers were retreating not far to the north of the salt-fossil zone.
The theory assumes that the ancient animals resembled their modern relatives and craved salt. Modern elephants travel long distances in Africa for salt, some even venturing deep into dark caves to gouge out salt deposits with their tusks.
If mastodons and mammoths did the same, the researchers speculate, they may have trekked from faraway saltless parts of the Midwest, braving the harsh climate of Michigan, then a virtual tundra near the edge of the glacier, to eat salt that was then exposed on the surface.
But when the glaciers melted -- at about the same time the huge beasts died out -- the outwash and its huge load of sediment buried the salt layers.
The researchers do not claim that salt deprivation alone finished the mammoths and mastodons. It is known that the climate was changing and that human beings were in Michigan then, hunting the animals. Human artifacts of that period also cover the same area.
The theory, published in the winter 1988 issue of National Geographic Research, was devised by J. Alan Holman, a zoologist at Michigan State University; Laura M. Abraczinskas, a graduate student; and David B. Westjohn of the U.S. Geological Survey's Lansing office.