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Grand Teton National Park Report

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Several microtine species exhibit multiannual fluctuations in population density. These fluctuations have been described as occurring with sufficient regularity to be called "cycles". This phenomenon has been known since antiquity. However, despite the extensive work that has dealt with this problem, the facts underlying these fluctations remain largely unknown. Preliminary field observations of populations of montane voles (Microtus montanus ) in Grand Teton National Park were made in 1961, and from 1966 through 1968 (Pinter, unpublished observations). At about the same time laboratory studies began to reveal the remarkable sensitivity of the reproductive system of these rodents to environmental variables . However, little was known to what degree environmental factors and reproductive responses of these rodents contributed to the cyclicity of their population density. Consequently, in 1969 a long term study was initiated. The purpose of the study was essentially fourfold. First, to characterize the environmental variables that might affect Microtus in different seasons of the year. Second, to record the growth, maturation, and reproductive activity of Microtus montanus under natural conditions. Third, to determine the maturational as well as the seasonal pelage changes of these rodents. Fourth, the data resulting from the execution of the first three objectives would be correlated in an attempt to determine the causes underlying the multiannual fluctations in population density of these microtine rodents.