Arctic foxes create "nest eggs" each year to prepare for leaner times, according to a new study.
Like squirrels gathering nuts for the winter, the small foxes hoard bird eggs in case there's not enough of their favorite prey—the collard lemming—to go around in the spring.
The stored eggs can last for up to a year after being buried, thanks to the Arctic permafrost and natural preservatives inside the eggs.
"It appears as if cached eggs are used as a backup for unpredictable changes in lemming numbers," lead study author Gustaf Samelius of Grimsö Wildlife Research Station in Riddarhyttan, Sweden, said in an email.
"This is a neat adaptation in an environment where food abundance changes dramatically both among seasons and years."
Samelius added that the study is the first to show the extent to which the carnivores can depend on stored rations.
Other carnivores are known to store food, the researcher noted, but they generally cache for only a few days and base their diets more on fresh kills.
"Our results of about 50 percent of the [arctic fox's] diet coming from cached foods might be on the extreme end" compared with other meat-eaters, Samelius said.
Samelius's team based its findings on the behavior of arctic foxes living near Karrak Lake in the Queen Maud Gulf Bird Sanctuary in the territory of Nunavut, Canada, between 2000 and 2004.
Karrak Lake is the summer breeding ground for up to a million snow geese and Ross's geese. The birds' nests supply ample pickings for the region's foxes, which store between 2,000 and 3,000 eggs a year.
Over the course of their four-year study, which appeared in last month's issue of the Journal of Animal Ecology, the team found that the foxes stored similar numbers of eggs each year.
But the degree to which the mammals relied on their caches varied with changes in lemming abundance.
Collard lemming populations fluctuate dramatically over three- to five-year cycles, Samelius said, and the changes are largely unpredictable.
When lemming numbers were high, the stored eggs made up less than 28 percent of the foxes' springtime diet.
When the rodents were scarce, the eggs accounted for up to 74 percent of the mammals' food.
For the foxes, the eggs are a reliable backup system because they are abundant during goose nesting season and are well suited to long-term storage.
"Eggs are protected by the egg shell, several membranes, as well as chemical properties of the albumen [egg white], preventing microbial activity," Samelius said.
The cold conditions of the Canadian Arctic also extend the shelf life of stored eggs, according to the study team.
The new study monitored the arctic foxes' yearly diets using analysis of chemical markers called isotopes in samples of the animals' fur and blood.
Different levels of carbon and nitrogen isotopes in these samples reflected the different foods the mammals ate in the spring and fall of a given year.
Vincent Careau is an arctic-fox researcher at the Université de Sherbrooke in Quebec who was not involved with the study.
He said that the new research offers solid evidence for what many biologists had previously suspected.
"Many noted the presence of egg shells in fox scats while there was no goose around, meaning that the consumed egg was inevitably recovered from a cache," Careau said, "but these observations remained anecdotic until this study came out."
How the foxes manage to locate their egg caches after such a long time remains a mystery, Careau added.
But studies of closely related red foxes and observations of the arctic foxes suggest that the animals use "spatial memory of cache locations and exploratory digging," he said.
"We also observed foxes spending a lot of time moving eggs from cache to cache," Careau added.
"This re-caching behavior probably improves memory of cache locations, especially if they progressively bring their eggs into one basket."