Waiting on a mule deer crossing…
Explores the ecology and productivity of high-elevation herbaceous meadows alongside physiological studies of alpine mammals, particularly energy regulation and hibernation in marmots.
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The subalpine meadows and rocky talus slopes around the Rocky Mountain Biological Laboratory in Gothic, Colorado, host one of the most intensively studied mammal communities in North America. At the center of that community is the yellow-bellied marmot (Marmota flaviventris), a large ground squirrel that spends roughly eight months of each year in hibernation beneath the snow and the remaining four months feeding, breeding, and dispersing across high-elevation meadows. Understanding how marmots and their neighbors — including bushy-tailed woodrats and ground squirrels — survive the long mountain winter, and how the meadow plants they depend on grow and reproduce during a short summer, is central to predicting how Gunnison Basin ecosystems will respond to a changing climate.
Hibernation is the key concept that ties much of this research together. Hibernating mammals do not simply sleep through winter; they enter a regulated state in which body temperature, heart rate, and metabolism drop dramatically, and the animal lives off stored body fat for months without eating or drinking. Two related ideas are important. The first is lipid metabolism — the way animals store, mobilize, and burn fats. Marmots accumulate large reserves of white adipose tissue (body fat) during summer feeding and gradually break it down during winter through a process called lipolysis. The second is seasonal physiological rhythms, the annual cycle of hormones, enzymes, and behaviors that prepares an animal for hibernation, arousal, and the subsequent feeding season.
These physiological questions cannot be separated from the meadows themselves. Subalpine herbaceous meadows produce most of their aboveground plant growth in a brief window between snowmelt and late summer, and that pulse of productivity is what fuels the marmot's annual fat-gain cycle. Disturbances to meadow hydrology — including those caused by unpaved roads, snowmelt timing, and erosion — can alter the timing and amount of forage available. Together, mammal physiology and meadow ecology form a coupled system in which what happens belowground in a hibernaculum is tightly linked to what happens aboveground in the surrounding plant community.
Early work at RMBL combined field natural history with emerging biotelemetry and biochemistry. Armitage and Downhower documented basic behaviors of marmot life history, including how animals retreat into burrows for hibernation (Armitage & Downhower, 1970) and how yearlings disperse from their natal colonies (Downhower & Armitage, 1981). Parallel methodological advances allowed researchers to track free-ranging animals and record their body temperatures continuously, opening a window into hibernation physiology under natural conditions (Shirer & Downhower, 1969); (Downhower & Pauley, 1970). Andersen's early work on female reproductive strategies established marmots as a model for socio-ecological studies of mountain mammals (Andersen, 1975).
On the plant side, Andersen and colleagues quantified aboveground productivity and floristic composition of a high subalpine herbaceous meadow, providing one of the first detailed accounts of how much biomass these meadows produce and which species dominate (Andersen et al., 1979). Together, these studies built the empirical foundation — animals, plants, and methods — on which decades of subsequent physiological and ecological work have been built.
A major thread of findings concerns how marmots manage body fat across the year. Florant and colleagues showed that hibernating marmots have elevated levels of free fatty acids in their blood, indicating active breakdown of stored fat, and that only three fatty acids — palmitic, oleic, and linoleic — make up more than 95 percent of the fat stored in their adipose tissue (Florant et al., 1990). Across the hibernation season, the proportion of saturated fats in storage tissue declined, suggesting that marmots preferentially burn saturated fats during winter while conserving certain essential fatty acids for use after arousal in spring (Florant et al., 1990). Wilson and colleagues extended this picture at the molecular level, showing that the genes encoding two key fat-handling enzymes — one that helps store fat and one that helps break it down — are switched on at different times of year, with the storage enzyme abundant during summer mass gain and the breakdown enzyme abundant during winter fasting (Wilson et al., 1992).
A second thread concerns the hormonal and neural control of the annual cycle. Florant and colleagues demonstrated that the normal daily rhythm of melatonin, the hormone that signals night length, disappears during deep hibernation because the cold pineal gland can no longer generate it, but returns immediately upon arousal (Florant et al., 1984). Follow-up work showed that even bright surges of stress hormones during arousal do not override this light-driven system (Florant, 1989). During the summer feeding season, insulin delivered directly to the brain reduced food intake and body weight, suggesting that brain insulin helps regulate how aggressively marmots fatten for the coming winter (Florant et al., 1991). Comparative biochemistry on related species, including hemoglobin structure in arctic ground squirrels (Duffy et al., 1987) and pancreatic hormone profiles in golden-mantled ground squirrels (Bauman et al., 1987), and hematology of high-elevation bushy-tailed woodrats (Frase, 2002), placed the marmot story within a broader picture of how mountain mammals cope with cold, low oxygen, and seasonal fasting.
A third thread links these animals to their habitat. Roads and other surface disturbances strongly affect mountain hydrology by altering infiltration, runoff, and the timing of water delivery to meadows and wetlands, with culverts and drainage structures playing an outsized role in routing snowmelt (Andersen, 2007). Because relatively small changes in water availability can produce large changes in meadow and streamside vegetation, these hydrologic effects ripple into the food base on which marmots and other meadow herbivores depend (Andersen, 2007).
The temporal trajectory of work in this area is striking. The bulk of foundational physiology and meadow ecology was published before 1990, with a second wave through the 1990s refining the molecular and hormonal picture of marmot hibernation. More recent contributions, such as the 2002 work on high-elevation woodrat hematology (Frase, 2002) and the 2007 synthesis of road impacts on mountain hydrology (Andersen, 2007), signal a shift from single-species physiology toward landscape-scale questions: how human infrastructure, changing snowpack, and altered water tables reshape the meadows that sustain these animals.
Emerging questions center on integration. How do the molecular signals that govern fat storage and mobilization respond to warmer winters and earlier snowmelt? Do disturbances to meadow hydrology change the quality and timing of the forage that marmots use to build their winter fat reserves? Methods originally developed for single animals — radio-telemetry, hormone assays, and tissue biochemistry — are increasingly being combined with long-term meadow monitoring and hydrologic assessment to address these coupled questions.
Several important gaps remain. It is still unclear how flexible marmot hibernation physiology is in the face of shorter, warmer winters, and whether the fatty acid composition of summer forage in subalpine meadows is shifting in ways that affect the fats marmots can store. The hydrologic effects of roads and other disturbances on semiarid, low-relief mountain landscapes remain poorly quantified (Andersen, 2007), and the consequences for meadow plant communities and the herbivores that depend on them are essentially unknown. Linking the molecular biology of hibernation to long-term demographic data from marmot colonies, and to the productivity and hydrology of the meadows around them, is likely to be the most productive direction for the next decade of work in the Gunnison Basin.
Andersen, D. C. (1975). Socio-ecology of Marmota: reproductive strategies of female yellow-bellied marmots (M. flaviventris). →
Andersen, D. C. (2007). Road impacts on the Baca National Wildlife Refuge, Colorado, with emphasis on effects to surface- and shallow ground-water hydrology - A literature review. →
Andersen, D. C., et al. (1979). Aboveground productivity and floristic structure of a high subalpine herbaceous meadow. Arctic and Alpine Research. →
Armitage, K. B., Downhower, J. F. (1970). Interment behavior in the yellow-bellied marmot (Marmota flaviventris). Journal of Mammalogy. →
Bauman, W. A., et al. (1987). Pancreatic hormones in the nonhibernating and hibernating golden-mantled ground squirrel. Comparative Biochemistry and Physiology. →
Downhower, J. F., Armitage, K. B. (1981). Dispersal of yearling yellow bellied marmots Marmota flaviventris. Animal Behaviour. →
Downhower, J. F., Pauley, J. (1970). Automated recordings of body temperature from free-ranging yellow-bellied marmots. Journal of Wildlife Management. →
Duffy, L. K., et al. (1987). The primary structure of the hemoglobin chain of the arctic ground squirrel. Comparative Biochemistry and Physiology. →
Florant, G. L. (1989). Melatonin profile in marmots: the influence of catecholamines, hibernation, and light. Journal of Pineal Research. →
Florant, G. L., et al. (1984). Plasma melatonin concentrations in hibernating marmots: absence of a plasma melatonin rhythm. American Journal of Physiology. →
Florant, G. L., et al. (1990). Plasma and white adipose tissue lipid composition in marmots. American Journal of Physiology. →
Florant, G. L., et al. (1991). Intraventricular insulin reduces food intake and body weight of marmots during the summer feeding period. Physiology and Behavior. →
Frase, B. A. (2002). Hematological parameters of high-elevation bushy-tailed woodrats. The Southwestern Naturalist. →
Shirer, H. W., Downhower, J. F. (1969). Radio tracking of dispersing yellow-bellied marmots. Transactions of the Kansas Academy of Science. →
Wilson, B. E., et al. (1992). Seasonal changes in hormone-sensitive and lipoprotein lipase mRNA concentrations in marmot white adipose tissue. American Journal of Physiology. →