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Fall 2012

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

Chipmunk, red fox, white-tailed deer, acorns

November 13, 2012

The future that acorns predict

Eastern chipmunks are winding down a year of obsessive hoarding. For the past couple of months, they have been jamming their cheeks full of acorns to deposit in underground wintering burrows.

They stockpile their loot in galleries at the end of long entrance tunnels near larger nesting cavities padded with crushed leaves. The rodents will spend most of the winter sleeping, waking from their torpor every few weeks to feed from the larders. They may even venture out on warm days to nab a few sunflower seeds from underneath bird feeders.

Older, mature chipmunks tend to occupy the burrows near white oak trees, which produce relatively sweet acorns. Young, inexperienced chipmunks seem to inherit the second-rate territories underneath red oaks, which yield bitter acorns.

This year's profuse acorn crop is much less likely to predict the severity of the upcoming winter than it is to foretell an acorn-fueled boost to next year's chipmunk population.

Scientists can see even further, correlating this year's acorn bonanza with an increased threat of Lyme disease almost two years from now.

Although people may first think of deer ticks and deer when they hear "Lyme disease," it's chipmunks and white-footed mice that are the primary reservoirs of the corkscrew-shape bacterium that causes the disease.

Next year's flood of chipmunks can become infested with tick larvae, which would feed on their hosts through next summer, fall and winter, dropping off as tick nymphs in the warm-weather months of 2014.

Those nymphs can latch onto a human and, after feeding for a couple of days, possibly infect their two-legged host with the bacterium.

Most often, the tick nymphs climb aboard a deer, where they mature and mate. Blood-bloated, females drop off the deer to lay eggs that hatch into larvae questing for a small rodent.

One might conclude that curbing the swollen population of deer might reduce the threat of Lyme disease. Not the case, according to a study published this year by the National Academy of Sciences, which found no significant correlation between deer density and human cases of Lyme disease.

What the study did discover: Areas with high densities of foxes have significantly fewer incidences of Lyme disease. Foxes are very efficient hunters of small rodents; they even store their catches to snack on later.

But foxes don't thrive in the presence of the robust coyotes that cross-bred with wolves as they expanded their range into the northeastern United States. These "coy-wolves" are known to kill and eat foxes — and they are much less adept at catching mice and chipmunks.

Sources: Proceedings of the National Academy of Sciences, plosbiology.org, Smithsonian Contributions to Zoology, University of Michigan, Penn State University