The men, now both retired from the University of Wisconsin-La Crosse Sociology and Archaeology Department, believe the cave images tell what might have happened.

The Effigy Mound people were among the first to use bows and arrows to more effectively hunt their primary food source, white-tailed deer. They ate well and the population flourished but eventually reached a critical mass that could not be supported by the dwindling number of deer.

The pregnant deer in the cave art indicate a late winter or early spring hunt, which would be very rare, Theler said.

“It’s suggestive of desperation, because native peoples are usually pretty careful about not overusing resources,” he said.

The theory clicks with other evidence uncovered in the Driftless Area, they said. Archaeologists found the Effigy Mound people showed less seasonal movement in the years leading up to their disappearance. People remained on the Mississippi River year-round, harvesting large numbers of mussels — an indication the population had become too large to relocate to more ideal inland wintering areas.

The pair have published a book and two papers on their theory, including in the national archaeology journal American Antiquity in 2006.

Research continues