By Ranger Steve Mueller
Chipmunks emerge from underground burrows in mid winter when conditions warm, the sun shines, water trickles, or warmth penetrates deep into their bodies.
During my naturalist career, we shared the best evidence-based scientific discoveries about hibernators, deep sleepers, and those that stay active all winter. Insects hibernate, diapause, or even stay active all winter but they are excluded from this discussion, as are birds that also have some hibernators. Those groups like reptiles and amphibians will merit their own nature niche adaptation stories.
Within the Class Mammalia, we taught Michigan has four groups with true hibernators, including some bats, the 13-lined ground squirrel, woodchuck, and jumping mice. Bears are deep sleepers but are not considered true hibernators. Chipmunks that periodically pop out of the ground during winter were reported as deep sleepers.
An authoritative book I depend on is Michigan Mammals by William Burt (1957). It referred to chipmunks as hibernators. Despite the rigorous scientific scrutiny used in making the text accurate, questions were raised regarding chipmunks’ winter behavior in regards to sleeping or hibernating. I was not greatly concerned with the issue and referred to the small striped mammals as deep sleepers.
I should have pursued the issue with more vigor but information seemed conflicting and I had other scientific controversies to address that seemed more pertinent and meaningful for society’s welfare. Things like climate change or animal species origins related to Earth’s biodiversity, for ecological sustainable conditions that people need, took precedence.
Recently my naturalist friend, Greg, spoke about chipmunk hibernation and I challenged the idea. It stimulated me to examine peer-reviewed research. New technology developments during recent decades make it easier to study winter sleep for various species. Small monitoring devices can be implanted in animals to monitor breathing, heart rate, and temperature on a 24-hour basis.
Studies supported chipmunks are true hibernators but there are still unknowns. Hibernators’ breathing and heart rate become extremely slow and body temperature drops to near freezing. Bears do not experience such dramatic reduction and are considered deep sleepers. Bear body temperature only drops from about 100 to 90 F. Respiration and heart rate slow but are not so reduced that it is difficult to arouse the bear.
Chipmunk heart rate slows from 350 beats per minute to about 4, temperature drops from 94 F to 40 F, and respiration changes from 60 to about 20 breaths per minute. It is difficult to arouse them. The adaptations merit the designation of true hibernation but other factors are not consistent with what is normally considered true hibernation.
Chipmunks awake periodically instead of remaining in deep torpor for months. The triggers causing them to periodically waken are unknown. They become active, eat cached food in burrows or even venture outside. Other true hibernators do not defecate or urinate for months, but chipmunks do.
I learned long ago that it is not either/or in nature. Most everything is on a gradation from one end of a continuum to another. It is not either hibernate or not hibernate. Different species demonstrate behaviors and adaptations along a continuum. Most might show a particular adaptation, such as hibernation, but all are experimenting through the process of natural selection and evolution for survival.
Natural history questions or topic suggestions can be directed to Ranger Steve (Mueller) at odybrook@chartermi.net – Ody Brook Nature Sanctuary, 13010 Northland Dr. Cedar Springs, MI 49319 or call 616-696-1753.
