Cache in Hand

The ingenious ways wildlife prepare, store, and defend their food. By Sam Curtis

This story is featured in Montana Outdoors
November–December 2002

It’s fall. The first real feel of impending winter has snapped into my bones with a series of deep frosts, punctuated by piercing winds out of the north. These messengers of colder times goad me into thinking of the carrots still stuck in the garden and the chilly fruit clinging to our apple trees. It’s time to get off my duff and store that stuff in the root cellar.

Out on the back porch, I grope for boots that have hung by their laces all summer. I pull on one, but the other is filled with something that turns out to be dried dog food that some mouse has robbed and stashed away. It’s fall, and we’re all thinking the same thing.

When I go outside, a red squirrel peppers me with cones from the big fir near the path to the garden. The ground is littered with cones. Piles of them have grown large against the tree trunk over the last few days. I have to laugh at our kinship—the mouse, the squirrel, and me.

We’re food hoarders, all three of us. It’s our way of dealing with the problems of an intermittent and unpredictable supply of edibles. I can’t grow vegetables all year in Montana’s climate, so I harvest food when it’s ready and store much of it for later consumption. I freeze green beans, can berries, and keep carrots and apples in the root cellar. But my pantry preparations seem paltry compared to what insects, birds, and mammals do to prepare, store, and protect their food. Every storage technique

I practice has been mastered by animals, which also employ additional methods so ingenious they make me feel like a larder-stocking amateur. I’ve discovered some of these secrets by watching wildlife. But while rooting in the library recently, I came upon a mother lode of information in a book titled Foraging Behavior, by Stephen Vander Wall and Kimberly Smith. What they learned after reviewing decades of research on “cache-protecting behavior of food-hoarding animals,” the title of one fascinating chapter, made me rethink much of what I thought I had known about animals preparing for winter.

For animals, food hoarding is much more than pushing seeds into a pile and calling it done. Things can go wrong: Food can spoil, it can be stolen, or, if it’s a living edible such as a spider’s fly, it can even escape on its own. Caches of food can get buried by snow. Or their owners might simply lose the hoard for lack of remembering where it was stored.

With their lives at stake, animals have devised intricate ways to prepare, store, and defend their food against these and other possible losses. One major concern is rot. You know the story: A single raspberry in the box turns gray and fuzzy, and, if you don’t throw it out, soon all the others are growing beards too. Clark’s nutcrackers and gray squirrels can’t afford such waste. Nut-crackers visually check each seed they collect and toss out any that look rancid. Squirrels, on the other hand, sniff out nuts that smell wormy and toss them out so the worms won’t crawl into other stored nuts.

After inspecting seeds, nuts, and grasses, many animals go to great pains to dry these foods out to prevent spoilage. Woodpeckers turn stored nuts frequently to dry them evenly. After pikas harvest grasses and stack them in piles under rock overhangs, they fluff the hay often so it dries faster. Kangaroo rats dry seedpods under light dustings of soil during the heat of summer before moving them into subterranean caches in the fall. Harvester ants store grain in cool, dry underground tunnels and will even carry seeds that show signs of sprouting back up into the sunshine for further drying.

Red squirrels are especially ingenious, according to Vander Wall and Smith. “Red squirrels are known to collect, dry, and store mushrooms for winter use,” they write. “Mushrooms are carried into trees and placed in hollow cavities, on top of old bird nests, or lodged in foliage. These mushrooms dry rapidly and become tough or brittle and resistant to decay.” (Sounds a lot like the dried shiitakes my wife, Linda, just brought home from Costco.)

Red squirrels know that different foods require different processing techniques. The rodents’ staple diet consists of conifer cones, which, when they dry out, become less palatable. So squirrels often store cones in the cool, damp depths of their middens, producing piles of cone litter below the branches where they sit and eat. This moist refrigeration keeps cones edible months longer than they would last in the open air.

Honeybees have an even more high-tech method of processing food so it won’t spoil. Through their own body chemistry, they convert gathered nectar into honey, which, because of its high sugar concentration, contains antibacterial properties that greatly extend its shelf life. Food storage becomes an even more complicated problem for carnivores. Once prey is dead, it decomposes quickly. But if prey is captured and not killed, it might escape.

Predators such as grizzly bears and gray wolves must gorge on large kills, such as elk and moose, over relatively short periods so the meat doesn’t spoil and go to waste. Other predators have a few more options. Spiders, for example, wrap live prey in silk to keep it secure and alive until it’s time to dine. One species of wasp keeps food fresh for its larvae by paralyzing flies with a sting and sealing as many as a dozen of these living but incapacitated edibles into each larval cell for the maturing wasps to eat. And there’s a type of ant that marches into rival colonies, stings the larval ants, and then drags them back to store them in a state of metabolic limbo for as long as two months before devouring the unfortunate creatures.

Larger creatures have similar, no-nonsense means of food preservation. Using a toxin in their saliva that they inject with a bite, shrews keep mice and earthworms alive but comatose for up to five days. Moles will maim but not kill worms so the creatures can’t escape but will remain fresh for eating later. Likewise, crows along the northwestern Pacific coast pluck the legs off crabs before caching them under moss and grass for consumption after a day or two.

In a cold climate, such as that in Montana, fresh food turns into frozen dinners. Saw-whet and great horned owls kill and stockpile mice in winter. When it comes time to take one out of the freezer to eat, they sit on it, as if they were incubating an egg, until it is thawed enough to tear apart and eat.

None of this varied food preparation does an animal any good, however, if another one raids its pantry. And there are lots of robbers in the animal world; even mates steal food from one another. To prevent pantry pilfering, some birds and animals go to great lengths to hide their food in trees, under grass, and even under ground. And they try to do this when potential robbers aren’t looking.

Red-headed woodpeckers, for example, poke seeds in holes in trees and seal them over with wood splinters. A Clark’s nutcracker will bury thousands of seeds on high ridges where winter snows later blow in and keep the cache deeply buried, even from rodents, until early spring. Then, as the warm sun melts the snowbanks, the nutcracker feeds greedily on newly exposed seeds it hid months earlier.

Another way animals foil would-be robbers is to move food to a spot where the thief is unlikely to venture. Blue jays often transport acorns from the woods where they are found, and where squirrels are likely to steal them, out into open meadows where the furred thieves rarely venture.
Hiding and camouflaging food stores is typical of birds and other scatter-hoarders—animals that cache small amounts of food at many locations scattered about their range. These creatures must rely on their mobility and excellent spatial memory to retrieve stored food.

Larder-hoarders, on the other hand, store all their food at one or two closely spaced spots so they don’t have to move far to reach it. Since large quantities of food are difficult to hide, however, these animals must rely on aggressive defense of their larders to protect them from robbers. So, squirrels scold; birds dive-bomb; bees sting. But a real threat comes when a potential robber gets between a large carnivore, say a mountain lion or a grizzly bear, and its meat cache. Thomas McNamee gives you a feel for the protective sense of a sow grizzly guarding her kill in his classic treatise The Grizzly Bear:

“She excavates a shallow elk-sized hole and maneuvers the carcass in. She shovels dirt, duff, mud, snow, sticks, rocks, whatever is handy, to cover her property and warn off any interlopers; this covering will be known to all as the signature of a grizzly bear, to be gainsaid only at great peril. She even drags a good-sized tree trunk out of a logjam downstream and heaves that over too.

“Curled up here, concealed to perfection within defensive-charging distance of her meat cache, the bear and her young may sleep their breakfast off in watchful security.”

Grizzlies rarely have their pantries robbed. But there are more subtle ways for stored food to disappear. Seed-caching animals always face the possibility of having seeds germinate and get consumed by the plants that grow from them. Birds try to prevent this by stashing seeds where conditions are poor for germination: off the ground, in leaves, and under flaps of bark. Rodents haul seeds deep underground, where darkness and coolness discourage growth. Some squirrels and chipmunks even go so far as to chew off embryos and shoots from nuts before they store them, in order to literally nip germination in the bud.

Food may also disappear by falling or being blown from where it was cached. Birds are particularly adept at fastening seeds down to prevent them from moving. They wedge seeds in cracks and behind bark. They use filaments from spider webs, cocoons, and fibrous plants to hold seeds in place. And they use their own saliva to glue seeds down. Raptors will even use blood and other body fluids of prey to securely paste it onto trees or rocks to await later consumption.

Though wind, wetness, and thievery pose problems to nature’s hoarding wildlife, the biggest threat comes from winter. If a food cache gets buried under snow or frozen into ice, it will be useless to hoarders just as surely as if it had spoiled, blown away, or been stolen. Which is why Vander Wall and Smith give a tip of the hoarding hat to the species that appears to have mastered food storage during even the worst winter weather. “Beavers store tree branches and logs during late summer and fall and feed upon young wood, bark, and cambium in winter,” they write. “They bury food in bottom sediment and construct floating caches.

A floating cache is started by constructing a raft of low preference species such as alder and non-food items such as peeled aspen logs. High preference food species such as aspen and willow stems are placed beneath the raft. The raft gradually becomes waterlogged, causing the cache below it to sink. When beaver ponds freeze in winter, rafts containing low preference and non-food items become locked in ice, but preferred food species are accessible, lodged below the raft.”

Beavers can build a submersible refrigerator able to feed an entire family unit throughout the winter. And here I was patting myself on the back for getting a few carrots down to the root cellar.Bear bullet

Sam Curtis is a freelance writer from Bozeman, Montana