Fur color that differs from the typical shades of brown and gray occurs in eight species of marmots. Albinism generally is rare whereas melanism is more common. Melanism may persist in some populations at low frequencies averaging 16.1 in M. monax and in M. flaviventris for as long as 80 years. White (not albino) and "bluish" marmots generally are rare, but a population of white M. marmota persisted for at least 10 years. Four species are characterized as having pelages of "extreme colors"; M. caudata, red; M. vancouverensis, dark brown; M. caligata, white; M. baibacina, gray. Fur is involved in heat transfer between the marmot and its environment. Heat transfer depends on fur structure (fur depth, hair length, density, and diameter), on fur spectral properties (absorptivity, reflectivity), and on the thermal environment (temperature, wind speed, radiation). Heat transfer is highly sensitive to solar radiation. Metabolic rates calculated from the fur model corresponded closely with measured values at ambient temperatures <= 20 degrees C. Solar radiation can either provide heat that could reduce metabolism or thermally stress a marmot. M. flaviventris orients towards the sun when solar radiation is low and reduces exposure when it is high. Light fur reduces and dark fur color increases absorptivity, I hypothesize that fur color functions primarily in heat transfer. This hypothesis is supported by the occurrence of light-colored M. flaviventris, M. caudata aurea, and M. himalayana in warmer environments. Experiments to test this hypoythesis are suggested.
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