Thursday, July 18, 2024

On a warm, 46-degree day in January 2022, a chipmunk ventures out of its burrow. (Photo: Tom Murray)

Chipmunks are a uniquely North American mammal. Of the 25 species of chipmunks that exist, 24 live in North America (one lives in Siberia). The eastern chipmunk (Tamias striatus) in our region is the largest chipmunk species, yet is one of the smallest members of the squirrel family. Chipmunks are one-third the size of gray squirrels, for example.

And unlike gray squirrels, which live in trees, eastern chipmunks live underground and spend as little time as possible outside the burrow looking for food. Many predators eat them, so these little critters are under constant threat when they are not hidden away in their burrows. They don’t waste time pausing to eat the food they find; they stuff it into their cheek pouches to take back to the safety of their burrows.

A chipmunk stuffs its cheek pouches with acorns in October. (Photo: Tom Murray)

Chipmunk cheek pouches extend to their shoulders and stretch when filled. Each pouch can hold about a heaping tablespoon of nuts and seeds or other food. After a chipmunk scurries to its burrow with a hoard of seeds, it uses the front paws to push the food out of its cheek pouches. 

Chipmunks eat buds, tubers, mushrooms, worms, snails, bugs, berries, bulbs and occasionally eggs or baby birds. In the fall, chipmunks gather and store for winter acorns and other nuts and seeds. In a good year, a 4-ounce chipmunk can stockpile in its burrow about 8 pounds of food. One chipmunk can gather and store up to 165 acorns a day! At this rate, it doesn’t take long to gather enough food for a winter. Chipmunks don’t stop, though. They gather food for as long as conditions are favorable.

A chipmunk peeks through a crevice in Watertown. (Photo: Ann Schlesinger)

Chipmunks dig elaborate burrows that can extend 30 feet from end to end. At the end of a long tunnel, the chipmunk digs a nest for sleeping. Other tunnels lead to other chambers – one for toileting, one for birthing and the rest for food storage.

Right about now, chipmunks are retreating underground. Unlike many animals, chipmunks do not store fat on the body for winter. Instead, they rely on their food stores to sustain them through the cold weather. Although they might be active in their burrows, we will probably not see them again until spring. If the winter is typical, chipmunks spend it in a kind of intermittent hibernation called torpor. Their body temperature, heart rate, breathing and even brain activity drops dramatically. Torpor reduces a chipmunk’s energy needs by 75 percent, helping its food stores last through the winter. Every few days to a week, a chipmunk wakes up and its metabolism returns to normal. It eats, urinates, defecates and returns to a state of torpor for a few more days.

In Coolidge Hill, Cambridge, a chipmunk forages for scarce spring food on May 16, 2021. (Photo: Richard George)

Torpor is not without its disadvantages. As an animal cools down, its circulation of antioxidant enzymes and vitamins slows. Long bouts of torpor can depress the immune system, cause dehydration, memory loss and tissue damage. It’s a delicate balance: Chipmunks want to conserve energy through torpor, but don’t want to harm themselves by remaining in torpor for too long.

In warm winters, chipmunks might not ever enter a state of torpor. If that leads to them finishing their food supplies before spring, chipmunks may leave their burrow on warm winter days to search for food. A female chipmunk, especially, needs to conserve her food supply. She needs her food stores to last not only through the winter but through spring mating, pregnancy and nursing, all of which occur before many plants and buds return. Trees with seeds that ripen in the early spring, such as red maple, are important food sources for chipmunks.

A chipmunk prepares for hibernation Nov. 11, 2020. (Photo: Tom Murray)

Chipmunks in Massachusetts usually mate twice per year – once in March or early April and again in June or early July. A month after mating, the female gives birth to four or five bumblebee-sized hairless babies. The mother nurses them for three weeks. Then the babies venture out of the burrow and begin foraging for themselves. After two months, the mother doesn’t allow the youngsters back into the burrow. This forces the youngsters to go off and dig their own burrows. Males may travel quite a distance, but females make a burrow near the mother. The females remain close by for the rest of their two- to three-year lives.

A chipmunk, viewed from above, shows off its five black stripes on a warm December day. (Photo: Tom Murray)

The diminutive chipmunk has many enemies. Hawks, owls, snakes, foxes, raccoons, coyotes and weasels all devour them. Domestic dogs and cats can be enemies, too. Therefore, chipmunks make alarm calls to alert other chipmunks of danger. They make a high-pitched chip sound (hence their name) when a predator approaches on the ground. This is the sound you most likely hear when you walk in the woods near a chipmunk. But chipmunks make a different sound, too: a lower-pitched chuck when a predator approaches from the air. Although chipmunks live alone, nearby chipmunks are often siblings or offspring, so it makes sense that chipmunks warn their neighbors of danger. 

A chipmunk pauses on a tree stump Nov. 19, 2018. (Photo: Richard George)

Scientists have discovered that small animals with fast metabolisms, such as chipmunks, perceive visual stimuli faster than larger animals with slower metabolisms, such as humans. This makes chipmunks quick to react and harder for predators to catch. In addition, a chipmunk’s olfactory bulb, which processes information related to smell, is larger than in other rodents. This suggests that chipmunks have a superb sense of smell and can detect trace amounts of odor molecules in the air. This sniffing prowess comes in handy when searching for acorns underneath leaves or snow, for example. Chipmunks have very poor night vision, so they are active only during the day. They take naps, too: chipmunks sleep about 15 hours per day. (Mammals that can hide while they sleep, such as chipmunks, bats and rodents, sleep longer than mammals that must remain alert while they sleep.)

Chipmunks are valuable to forest ecosystems. They eat various types of seeds and fungi. In their feces, they spread fungal spores that live around tree roots and help the roots absorb oxygen and nutrients critical to tree survival. The next time you notice a particularly healthy-looking tree, it may be thanks to the tiny chipmunk.

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Seen nearby

Yu-Chieh Jay Lee spotted this red fox Oct. 30 near the Medford/Tufts MBTA green line station. 

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Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.