No Nuts, No Problem: Squirrels Harvest Maple Syrup
for National Geographic News
|February 18, 2005|
Though daylight lingers longer this time of year, winter's grip
remains strong, and many critters' food stores are running low. How do
"There's always some mechanism that allows animals to make it through the winter as a species," said John Serrao, a naturalist in Pennsylvania's Pocono Mountains.
One of the more crafty mechanisms, Serrao notes, is the maple syrup harvesting of the red squirrel (Tamiasciurus hudsonicus). The tree-dwelling rodent is found throughout the northern United States and parts of Canada.
When its stores of pinecones and nuts gathered during the summer and fall run low, the squirrel scores the bark of sugar maple trees with its sharp teeth, allowing the sap to drain.
Once most of the sap's water has evaporated in the winter sunlight and left a sugary residue on the bark, the squirrel returns to lick it. The sugar provides an energy boost in an otherwise lean time of year, Serrao said.
"Getting the sugar from the trees is a great strategy," said Bernd Heinrich, a naturalist at the University of Vermont in Burlington and the author of several nature books, including Winter World: The Ingenuity of Animal Survival.
Heinrich is credited with describing the red squirrels' maple syrup harvest for science. He says, however, that the discovery of the survival strategy dates back to an Iroquois Indian myth: A youth notices a squirrel licking the sap and decides to try some too.
In Winter World, Heinrich notes that only a tiny percentage of the world's animals store food for the winter. The reason, he says, is that most food is perishable, and despite the freezer effect of winter snow pack, a warm spell will ruin any perishable item.
"You can't rely on stored food unless there are unusual circumstances," Heinrich said. For example, Heinrich's work with ravens shows that the carnivorous birds will store bits of meat only for the short term. Spoilage is an issue. But more important, the keen sense of smell of foxes, shrews, and coyotes means any meat left unguarded is fair game.
Like the red squirrel, a few other animals depend on stored food for their winter survival. Among them are the pika, a relative of the hare found in the Rocky Mountains. The small mammal collects and dries grasses, which it stores under rocks.
Beavers also pack a winter larder, storing branches in chilly, underwater food caches. Birds such as chickadees and nutcrackers have ample caches of seeds.
"It's nice to get models and think about why some [animals store food] and others don't," Heinrich said. "Bears don't have to. They get so fat and lazy, they turn on the hibernation hormone and their appetite is suppressed and they're fine."
Harsh winter weather can be a hardship for any creature. But naturalists say the amount of mastnuts, acorns, and the likethat drops on the forest floor in the fall is a key indicator of how well food-storing animals will survive come winter.
The abundance of mast is cyclical and related to reproduction strategies, according to Heinrich. If only one tree produced a bunch of nuts one year, critters would eat them all, and the tree would stand little chance at producing offspring.
Therefore trees in any given region collectively alternate between years of producing a lot of mast or very little. In years when they produce a lot, there is sufficient food for all the critters, and plenty of the nuts and acorns will be left over to take root.
"But sometimes there's not enough mast, and no matter what [the animals] store, it's insufficient to get them through winter," Serrao, the Pennsylvania naturalist, said. "They have to look for other means of energy, and that's probably when the red squirrels do more of the sugar scraping."
Hibernating black bears also depend on mast in the autumn to fatten up for their winters' slumber. When there's not much mast around, they go into hibernation early instead of wasting their energy looking for nuts and acorns that aren't there, Serrao said.
While a few animals will starve to death when food supplies are low, the species will survive. "Winter may be one of the chief conditions that limit the populations of some of the animals," Serrao said. "But never, except in extreme circumstances like a heavy winter snow, never do they have extensive mortality."
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