Marmota monax - Woodchuck

Description:

The Woodchuck is the largest squirrel species occurring in Wisconsin (Kurta 1995).  Adults range from 16.5”to 32.25” in length and weigh from 4.5 to 14 pounds.  Adults are 3.5” to 10” tall, and have long bushy tails ranging from 3.9” to 6.3” long.  Woodchucks have small ears and short powerful legs adapted for digging.  Coat color ranges from a grizzled brown to red or black across the dorsum, neck, and head (Kwiecinski 1998).  The head is devoid of white markings except around the nose.  The forelegs are covered in deep red or hazel hairs, which fade to black on the foot.  Tail color is highly variable, ranging from dark brown or black to pink or cinnamon.  Wooly under-fur covers the animal’s flanks and back.  White-tipped guard hairs cover the entire body, and result in a somewhat frosted appearance (Kurta 1995).

Distribution:

The Woodchuck is the widest ranging of all North American marmots, and occurs throughout the entire state of Wisconsin (Kwiecinski 1998).  The Woodchucks range extends north across southern Canada and Nova Scotia up into southern Alaska, and south into Georgia, Alabama, Louisiana, and Arkansas (Knopf 1993).  Their range extends from eastern North Dakota east to the Atlantic coastline.  Nine subspecies occur across the Woodchucks broad geographical range.  Marmota monax rufescens is the only subspecies occurring in Wisconsin (Kwiecinski 1998). 

Ontogeny and Reproduction:

Males emerge from winter hibernation 1 to 3 weeks before females, and begin to fight amongst one another for mating rights.  Females are monoestrous, and come into estrus just after emerging from hibernation.  Males are promiscuous and quickly seek out the dens of receptive females, in which they stay for a brief period in order to copulate (Knopf 1993).

Copulation usually takes place at the burrow entrance or in grassy or bushy areas nearby.  Pregnancy rates average 76 percent, with some yearling females breeding.  The gestation period is 32 days, with a litter of 1 to 8 young born in April or May (Kwiecinski 1998).

Newborns weigh 26 grams, and are nearly naked except for a fine layer of vibrissae covering the head, chin, and muzzle.  The rest of their bodies are covered in a red, wrinkly skin.  Young are born with their external auditory meatus closed and their pinnae erect.  They are blind, with their eyes appearing as black spots on their faces.  Their forelegs are better developed than their hind legs at birth, and they are toothless.  The sex ratio of newborns is 1:1 (Kwiecinski 1998).

Young Woodchucks develop quickly with skin pigmentation occurring at one week of age, a coating of black hair occurring at two weeks of age, crawling beginning at three weeks of age, eye opening and solid food consumption occurring at four weeks of age, and whistling and tooth chattering vocalizations and forays out of the burrow beginning at 5 to 6 weeks of age (Snyder 1960).  Wandering young are carried back to the burrow in their mother’s mouth.

Females nurse while standing on all four legs.  Lactation stops after approximately 44 days, with young Woodchucks being completely weaned by six weeks of age (Kwiecinski 1998).  After weaning, the young quickly disperse from the mother’s burrow.

Burrow flooding and incisor malocclusions are the most common causes of young Woodchuck death (Snyder 1960).

Ecology and Behavior:

Woodchucks can be found in a wide variety of habitats, but prefer open woodlands, or lightly wooded fields.  Pastures, meadows, fencerows, and stream banks are also used extensively by Woodchucks (Knopf 1993).  Woodchucks are adaptable to human disturbance, and can often be found near human dwellings where the soil has been disturbed facilitating burrow digging.  Woodchucks prefer wooded areas for hibernation, and open fields or pastures for breeding and foraging (Kwiecinski 1998).

Woodchucks dig extensive burrow systems called hibernacula, and may utilize several each year.  Burrow excavation is done primarily through the use of the powerful forelimbs, with stones and large rocks being pushed aside with the animals head.  Burrows are usually dug in loose, sandy soil, and can be from three to six feet deep and up to 33 feet long.  The burrow entrance is eight to twelve inches across, and is clearly defined by a mound of excavated earth.  The burrow consists of a long shaft terminating in an enlarged nesting chamber lined with dead grass and dry leaves (Kwiecinski 1998).  Several side chambers lead off of the main shaft and serve as holding chambers for feces.  Fecal matter deposited in these chambers is often covered by a thin layer of earth.  A plunge or escape hole is also constructed off of the main shaft.  This hole is much smaller than the burrow entrance, and is not marked by a loose earth.  The plunge obscured from view by a light layer of vegetation or brush, and drops some two feet straight down into the burrow system.  The plunge allows the Woodchuck a quick escape if threatened outside its burrow (Kwiecinski 1998).

Woodchucks are commonly preyed on by humans, dogs, red fox, grey fox, black bear, lynx, mink, bobcats, hawks, owls, weasels, and large rattlesnakes (Kurta 1995).  Automobile collisions also kill many Woodchucks each year (Knopf 1993).  If threatened or pursued by predators, Woodchucks will whistle loudly and retreat to the safety of their burrows.  If they are unable to successful retreat to their burrows, they will often climb trees in order to escape (Kwiecinski 1998).

Woodchucks have been known to share their burrows with several other animal species during their long hibernation period.  Cottontail rabbits, opossums, raccoons, and skunks have all been found in conjunction with hibernating Woodchucks (Kwiecinski 1998).  A variety of animals also inhabit vacant or abandoned burrows.  River Otters, domestic cats, chipmunks, meadow voles, short-tailed shrews, pine voles, weasels, snakes, and birds all use old Woodchuck burrows for shelter (Knopf 1993).     

Woodchucks are generalists, and have a broad catholic diet.  The diet primarily consists of a variety of plants and grasses such as: alfalfa, wild lettuce, sweet clover, red clover, white clover, plantain, dandelion, daisy, goldenrod, hackberry, Norway maple, and red mulberry (Kwiecinski 1998).  The early spring diet of Woodchucks is comprised mostly of the bark, buds, and twigs of black cherry, sumac, and dogwood trees (Knopf 1993).  Woodchucks also take small invertebrates such as: June bugs, snails, and grasshoppers.  Woodchucks can be pests to farmers and gardeners as they will actively consume carrots, beans, corn, soybeans, and celery (Knopf 1993).  Feeding primarily takes place in the afternoon for adult animals, with younger fast-growing individuals feeding heavily throughout the day.  Due the poor ability of grasses and herbaceous plants to keep over long periods of time, Woodchucks do not cache their food, but instead put on a dense layer of fat before going into true hibernation until the onset of early spring (Kwiecinski 1998).

Hibernation begins in early October and lasts until March or April and is characterized by a series of “test drops” in body temperature of 2-4oC.  Woodchucks often choose wooded, well-drained locations near hedgerows, steep inclines, or haystacks for their hibernation burrows.  The beginning of hibernation is characterized by a sharp decline to body temperature to a low of 2oC with most animals reaching an average body temperature of 6.8-8.4 oC (Zervanos 2003).  Heart rate then quickly falls to 4 to 10 beats per minute.  Breathing rate falls to one breath every six minutes.  Blood composition also changes during this time, with an increase in blood oxygen, carbon dioxide, magnesium, potassium, and hematocrit (Kwiecinski 1998).  Blood glucose level also decreases.

Hibernation consists of a regular cycle of four to ten days of torpor followed by one to five days of arousal.  Torpor periods become increasingly longer with time and colder temperatures.  During the periods of activity, Woodchucks eat nothing, but rely on their fat reserves for sustenance and survival (Knopf 1993).

Termination of hibernation is characterized by an initial increase in heart rate to 220 beats per minute.  Heart rate then begins to fall as the Woodchucks body temperature begins to rise.  When body temperature reaches 37oC, heart rate settles at 130 beats per minute.  After hibernation ends, Woodchucks will have lost 30 percent of their total body fat (Kwiecinski 1998).

Sexual differences in hibernation have been identified.  Males have shorter hibernation periods lasting an average of 104.8 days, while females have longer hibernation periods lasting an average of 121.8 days (Zervanos 2003).  This is most likely tied to the early aggressive behavior of male woodchucks seeking to establish mating territories.  Males also maintained a higher average body temperature than females during hibernation, resulting in a higher energetic cost of hibernation in males.  Males use 38% more energy than females in hibernation (Zervanos 2003).  This fact is reaffirmed by male woodchucks putting on a higher percentage of fat before hibernation begins.   

Remarks:

Groundhog Day is celebrated on February 2nd, and is named after the Woodchuck.  Groundhog Day has its roots in the forth century event honoring the Virgin Mary.  This Catholic holy day was called Candlemas Day (Kwiecinski 1998).  This day also signified the half way point of winter, and was celebrated in agricultural fairs in Europe.  On this day hedgehogs and badgers would emerge from their burrows to predict the coming of spring.  Clear cold days meant winter would continue, and over cast days meant spring was near (Kwiecinski 1998).  Colonists in what would be the United States didn’t have badgers or hedgehogs, but instead relied on the woodchuck.

Literature Cited:

Craven, S. R., and S. Keith.  1996.  Woodchucks: their ecology and control.  University of Wisconsin Press, Madison, Wisconsin, USA.

Ferron, J.  1996.  How do woodchucks (Marmota monax) cope with harsh winter conditions? Journal of Mammalogy, 77:412-416.

Knopf, A.A.  1993.  The Audobon Society Field Guide to North American Mammals.  Ninth ed., Chanticleer Press Inc., New York, USA.

Kurta, A.  1995.  Mammals of the Great Lakes Region.  Revised ed., University of Michigan Press, USA.

Kwiecinski, G.G.  1998.  Mammalian Species Account No. 591: Marmota monax.  The American Society of Mammalogists.

Lyman, C.P.  1958.  Oxygen consumption, body temperature, and heart rate of woodchucks entering hibernation.  American Journal of Physiology, 194:83-91.

Snyder, R. L., and J. J. Christian.  1960.  Reproductive cycle and litter size of the woodchuck.  Ecology 41:647-656.

Snyder, R. L., D. E. Davis, and J. J. Christian.  1961.  Seasonal changes in the weights of woodchucks.  Journal of Mammalogy 42:297-312.

Zervanos, S. M., and C. M. Salsbury.  2003.  Seasonal Body Temperature Fluctuations and Energetic Strategies in Free-ranging Eastern Woodchucks.  Journal of Mammalogy 84: 299-310.

Reference written by Josh Spangler, Biol 378: Edited by Chris Yahnke. Page last updated 4-27-04.