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egg-eating foxes

Blogging on Peer-Reviewed ResearchAnimals may put food away for a rainy day - or at least, for a time when supplies are in short supply. Squirrels do it, storing nuts in hollow trees or holes dug in the leaf litter. How many they find later is another matter! But I didn't know that foxes are also into caching excess food.

As  I said, it's known that animals will cache food in times of plenty. But Samelius & his colleagues (2008) note that the extent to which animals use cached foods and how this varies in relation to fluctuations in other foods is poorly understood in most animals. They studied caching behaviour in Arctic foxes (Alopex lagopus), which live in an environment where there is enormous seasonal variation in food availability. For example, many geese species migrate to the Arctic to breed, so that during the nesting period there's an oversupply of eggs. The foxes are known to eat & to store these eggs, but less well-understood is the relationship between this behaviour & the availability of other sources of food.

    An Arctic fox taking an egg from a snow goose nest.

Samelius et al. were interested in the pattern of cached-egg use by the foxes - was it related to the availability of alternative food sources, such as lemmings? And how long did the foxes rely on their egg caches?

Collared lemmings (Dicrostonyx groenlandicus) are another food source for the foxes, but the size of the lemming population can vary considerably from year to year. (They don't actually jump off cliffs when their population size gets too high, but they do migrate en masse when local food supplie are low.) Thus the supply of lemmings is unpredictable compared to the supply of eggs, which is both 'predictable and seasonally superabundant' (Samelius et al. 2008). The team predicted that eggs would make up a greater proportion of the foxes' diets in years when lemmings were in short supply. (This was a long-term study, extending from 2000 to 2004.)

The team found that the foxes in their study area were very reliant on eggs that they cached during the goose nesting period in summer - and that this use varied dependent on the abundance of collared lemmings. In fact, the foxes ate lemmings as & when they could catch them, but when lemming numbers dropped the foxes turned to their stored eggs. In other words, there was an inverse relationship between lemming numbers and egg-eating. (Similar numbers of eggs were cached every year.) The foxes also ate eggs almost a year after hiding them - a long storage time made possible by the often frigid ambient temperatures.

Samelius & his colleagues concluded that Arctic foxes switching to cached eggs in year when collared lemmings were scarce may explain why animals often store more foods than needed. Specifically, arctic foxes may cache eggs independently of small mammal abundance to compensate for unpredictable changes in future lemming abundance.

This is not to say that foxes do this with forethought: the caching behaviour may be evolutionarily hard-wired, & we'd need studies looking at learning associated with caching to sort this one out. More food for thought ;-)

G. Samelius, R.T. Alisauskas, K.A. Hobson, & S. Lariviere (2007) Prolonging the arctic pulse: long-term exploitation of cached eggs by arctic foxes when lemmings are scarce. Journal of Animal Ecology 76:873-880

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