Illustration by Adelaide Tyrol
Each fall day he appears with a skinny face and leaves with ballooned cheeks. Over and over, he fills his cheeks and runs away to empty them. Our eastern chipmunk, it seems, is living in a good neighborhood. Our bird feeders provide him with an endless supply of sunflower seeds.
Impossible to count as he gathers them, the seeds make me wonder how many he carries on each trip. University of Vermont biology professor Bernd Heinrich pondered the same question. While examining a road-killed specimen, he found that he could stuff 60 sunflower seeds in one cheek, about a heaping tablespoon.
Chipmunks can hoard up to 8 pounds of seeds for the winter. So how many trips would the chipmunk have to take to fill up his storehouse? I weighed 120 sunflower seeds on a kitchen scale. At 2 ounces a mouthful, it would take him just 64 trips.
Sometime in November, I noticed that our resident chipmunk was no longer making trips to the seed market. And now standing on frozen ground and a bit of snow, I imagine him lying curled asleep in his nesting chamber. The main tunnel is perhaps 20 to 30 feet long with several full granaries, sleeping quarters, and separate escape tunnels. But what is it actually doing down there in the dark for five months?
Eastern chipmunks are restless hibernators. They don’t just sleep away the winter months. Chipmunks live off their seed hoard. Unlike other mammals such as bats, chipmunks don’t lay on fat for winter. Instead, they rely on good food stored in the pantry. All winter long, chipmunks eat and chill out, eat and chill out. And not just “chill out” in the sense of kicking back. They really do chill out by falling into torpor for stretches of up to eight days. Torpor is characterized by reduced body temperature, oxygen consumption, heart rate, and breathing, which all lead to much lower energy use. It makes a mouthful of sunflower seeds go a long way.
Many animals enter torpor during times of resource scarcity. When things get tough, they shut the system down and wait for better times. But torpor can have serious physiological costs. It’s not easy on the body in the long run, but it does allow for short-term survival. Individuals that have good energy reserves may not enter torpor as much as those that may need to stretch their food stores a bit more.
Each arousal from torpor is also energetically costly. Arousals can account for 80 to 90 percent of total energy expenditure each winter. But they can’t be avoided: long bouts of torpor can depress the immune system, cause dehydration, memory loss, and damage to tissues through oxidation. As an animal cools down, the circulation of antioxidant enzymes and vitamins are slowed, resulting in oxidative damage to tissues over time.
Daniel Monroe and his colleagues from Sherbrook University in Quebec thought that chipmunks might be faced with a cost/benefit trade-off. They can benefit in the short-term by going into deep and prolonged torpor to allow for energy savings in lean times, but they risk long-term physiological damage to their bodies from staying in torpor for too long if conditions stay rough.
They set out to test this with free-ranging, wild chipmunks using miniature data loggers that measured skin temperature mounted on tiny collars that the chipmunks wore around their necks, a proxy for internal body temperature. Chipmunks in the summer had skin temperatures that averaged about 99 degrees F. In midwinter, they averaged 97 degrees F when not in torpor, in a chamber that was usually below 50 degrees F. Those on a natural diet spent a total of 104 days in torpor with an average skin temperature of just 48 degrees F, while those fed on a diet of black sunflower seeds and peanuts only spent 13 days in torpor with a skin temperature of 72 degrees F. Clear evidence that chipmunks could adjust the depth and duration of torpor according to the size and composition of their food cache.
They also found that male chipmunks are more responsive than females to food supplementation. Males probably use more food during the winter to ensure that they have maximum reproductive capacity for early spring mating. Females, meanwhile, maintain deeper, more prolonged torpor to conserve their food cache for pregnancy and lactation during the early spring before fresh food is available.
The chipmunk in my backyard feeding on sunflower seeds has a diet far higher in fatty acids than those in the woods eating acorns and beechnuts. And because nut crops wax and wane over the years, woodland chipmunks may also have a smaller hoard most years. My backyard chipmunk is guaranteed a steady supply of sunflower seeds that contain 30 to 50 percent fat. With the pile of seeds that our backyard friend has, he’s surely living the good life right now, underground.
Kent McFarland is a biologist with the Vermont Center for Ecostudies.