Baffling the Bandits
Eastern gray squirrels rely on deception and other strategies to protect their food stashes from pilfering by members of their own--and other--species
As every backyard bird-watcher knows, squirrels have a knack for thieving. Even the most elaborate devices meant to keep squirrels out of bird feeders can't foil these uninvited dinner guests for long. Now researchers are discovering that the need to guard against theft shapes the eastern gray squirrel's own feeding strategies in complex, often surprising ways.
Many species of squirrel, including the red squirrel, are "larder hoarders." Each individual stores its food in one central area that it defends aggressively against invaders. But gray squirrels are "scatter hoarders," collecting and burying one nut at a time throughout home ranges up to 7 acres in size.
It's impossible for them to keep an eye on all their caches at once. Other gray squirrels, as well as other animals such as blue jays, take advantage of these widely dispersed stashes to score a free meal. By some estimates, certain squirrels may lose as much as 25 percent of their cached food to thieving members of their own species. That amounts to a lot of nuts--mature squirrels eat as much as two pounds of nuts weekly and hide thousands throughout the year, particularly in late summer and early fall.
To keep their caches safe from burglars, gray squirrels use a variety of anti-pilfering techniques. Michael Steele, a professor of biology at Wilkes University in Pennsylvania, first observed one of these techniques in action about 25 years ago while conducting an experiment with a group of undergraduate assistants.
"I sent the students out to recover seeds after we'd watched squirrels burying them," he says, "and noticed that [the students] were having trouble finding some of the seeds."
After monitoring the animals' behavior more closely, Steele realized that a squirrel would occasionally engage in what he and his colleagues call "deceptive caching." Rather than simply burying a nut, the squirrel would hold the tidbit in its teeth, dig a hole in the ground and vigorously cover it up again, all without depositing the nut in the hole. It would sometimes repeat this behavior several times before actually hiding the nut, leaving behind a trail of empty cache sites.
At Central Connecticut State University, professor of biology Sylvia Halkin came to a similar conclusion after she and her students observed that gray squirrels sometimes buried a food item, moved away, stopped and then pulled leaves or soil over another spot where nothing was buried. The squirrels only sometimes dug a hole at this other location before performing covering behavior. In contrast to the Pennsylvania squirrels, the Connecticut squirrels usually covered the additional, empty locations after, rather than before, they actually buried a nut.
"Since the deceptive behavior of the two groups of squirrels takes somewhat different forms," Halkin says, "it raises the question of how anti-pilfering behavior develops. Is it learned from other squirrels, or by trial and error? Are there genetic predispositions? Do local environmental factors play a role? There's a lot more for us to learn."
Why would squirrels expend time and energy covering fake storage sites rather than concentrating on locating and burying as much food as possible?
Steele assigned observers to watch closely as squirrels cached a supply of acorns, then attempt to retrieve the caches after the squirrels had moved on. The observers were able to locate the acorns only one time in ten when the squirrels engaged in deceptive caching. Even when the researchers were able to track down the cleverly concealed nuts, the task took more than twice as long as it did when the squirrels had simply cached their food. Confronted with a string of fruitless thieving attempts, a would-be pilferer might well give up the hunt and move on to less well-concealed targets.
In subsequent research, Halkin and her students found that if they acted as thieves and tried to dig up squirrel caches, the animals used a greater variety of anti-pilfering behaviors, including deceptive caching and caching food in spots difficult for pilferers to locate or access, such as under bushes and in muddy areas. A squirrel might even retrieve a carelessly hidden morsel and rebury it in a more suitable spot.
"One squirrel I was observing last winter saw me digging up one of its caches," Halkin says. "It had buried another nut under the snow, but dirt mixed into the snow made it obvious where the nut was hidden. As I watched, the squirrel came back, dug up the nut and carried it away to hide it under a bush."
There may be another reason for squirrels to revisit previously hidden caches. "We know from one of our study areas where squirrel densities are high, that squirrels keep a cache in its original location for only about three days regardless of the season," Steele says. "Digging up and relocating their caches may help 'recharge' their memories of where their food is located."
Whether tucking acorns into hard-to-reach crannies or engaging in elaborate food caching theatrics, gray squirrels demonstrate remarkable flexibility in coping with potential cache thieves. Understanding the role pilfering plays in these animals' lives may earn them a little respect--however grudging--from even the most committed advocate of "squirrel-proof" bird feeders.
Molly Newman writes about science and photography from her home in Portland, Oregon.
One advantage scientists have when studying eastern gray squirrels: The animals are undaunted by humans' presence, allowing researchers to observe their behavior without resorting to wildlife blinds or hidden video cameras. Unfortunately, this ability to acclimate to human-dominated settings has helped make eastern gray squirrels an invasive species throughout much of the United States and Western Europe. Able to adapt to a variety of habitats and diets, they're outcompeting native species in many areas.
In the northwestern United States, Douglas' squirrels and western gray squirrels rely on diverse forest ecosystems for food and shelter. These species have suffered as roads and subdivisions have replaced trees and meadows. Eastern gray squirrels, meanwhile, thrive in densely populated urban areas. They've become familiar neighborhood residents--and sometimes pests. None too fussy about their nesting sites, they make their homes in attics, chimneys and even the occasional barbecue grill. They raid bird feeders and garbage cans, making meals of everything from sunflower seed to stale potato chips.
Across the Atlantic, eastern gray squirrels' rivalry with England's native red squirrels (best known to Americans as the titular character in the Beatrix Potter story "Squirrel Nutkin") has made them less popular still. Gray squirrels thrive on a diet of freshly fallen acorns, while red squirrels can't handle the high levels of tannins provided by an all-acorn diet and depend instead on a variety of nuts, seeds and fungi to survive. While red squirrels are confined to forested areas, gray squirrels are unafraid to cross open stretches of ground. This boldness has allowed them to colonize most of England and southern Scotland. Wherever gray squirrels go, red squirrel populations typically disappear within 20 years. The squirrel pox virus, which two-thirds of gray squirrels carry without visible effect, is devastating to red squirrels, typically killing them within two weeks of the time of infection.
Though gray squirrels were introduced to England as a result of the Victorian passion for "civilized nature," their brassy habits have earned them stern disapproval in recent years. During a House of Lords session in 2006, Lady Saltoun of Abernethy summed up the current British attitude toward these American invaders and their native counterparts: "Red squirrels are rather like quiet, well-behaved people who do not make a nuisance or an exhibition of themselves or commit crimes and so do not get themselves into the papers in the vulgar way gray squirrels do."--Molly Newman