Oldest Rodent Cache Found—Filled With Fossil Nuts

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This and other evidence, such as the width of the tunnels, hinted at the type and size of the burrow's former occupant or occupants.

"Burrowing animals make tunnels only big enough to squeeze through. It would otherwise be a waste of time and much effort," said Gee.

Hamster and ground squirrel remains were known from neighboring sites of a similar age, making those species likely contenders.

"We narrowed the possibilities down to a hamster because hoarding is what hamsters do best," she said. For example, the black-bellied hamster (Cricetus cricetus) that lives in central Europe today is known to store up to 90 kilograms (200 pounds) of grain, peas, and potatoes within a year. In contrast, ground squirrels eat excessively before the months get cooler in order to build up fat reserves for winter hibernation.

Climate Enigma

That left one final mystery, said Gee. Hamsters and other rodents today usually build food stores like this to see them through winter months with scarce food resources. However, research on animals and climate in the Miocene of Central Europe indicates that conditions would have been warm and humid at this time.

The burrow system was found in what would have been beach dunes on the coast of the ancient North Sea, where resources may have been scarcer than in nearby forests. Otherwise, "there could have been a wet and a dry season just as severe," as a winter in terms of resource scarcity, suggested Hooker.

"Food hoarding takes a lot of energy…and must give the food hoarder some sort of advantage to make it worthwhile," said Gee. It is also possible that an annual cold season was just developing, but has not yet been reflected in other evidence, she said. "This remains an unsolved, but very interesting, puzzling aspect."

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