Unusual mortality in a yellow-bellied marmot population [Mortalité inhabituelle dans une population de marmottes à ventre jaune].

Résumé

The winter of 1991-92 apparently reduced survivorship and reproduction of yellow-bellied marmots (Marmota flaviventris) in our study area in the East River Valley, Gunnison Country, Colorado, at 2900m. I divided the study area into two groups, lower valley where winter conditions are milder and upper valley where winter conditions are more severe. In the lower valley the rate of survivorship of adult females was 0.72 for the three previous winters and 0.75 for 1991-92. For the upper valley, the rate of survivorship of adult females was 0.76 for the thee previous winter and 0.42 for 1991-92. Likewise, survivorship of young to yearlings in the lower valley was 0.67 for the three previous winters and 0.73 for 1991-92. For the upper valley, juvenille survivorship was 0.61 for the three previous winters and 0.26 for 1991-92. the percentage of adult females reproducing in the lower valley was 47.4% for the three previous years and 91.5% for 1992. For the upper valley in average of 69% of the adult females produced litters in the three previous years whereas only 30% reproduced in 1992. In the lower valley of five adult females that reproduced in 1991 and survived to 1992, three successfully reproduced. Six females that did not reproduce in 1991, did so in 1992. In the upper valley, no female that reproduced in 1991 successfully reproduced in 1992 and five of the nine1991 reproductive females did not survive. The three females that weaned litters were all three years old and reproduced for the first time. Young in 1991 in the upper valley reached only 65.7% of the body mass on September 15 that young averaged the previous two years whereas young in the lower valley achieved 88% of the body mass. Similarly, reproductive females in the upper valley weighed less on September 15 in 1991 than they did in either of the two previous years. Conversely, reproductive females in the lower valley weighed more in 1991 than in either1989 or 1990. In 1991 the non-reproductive females (2-years-old) in the upper valley weighed 3.58 kg on September 15 whereas the average mass of the reproductive females was 3.29 kg. Thus, both survivorship in the Winter of 1991-92 and reproduction in the summer of 1992 are related to the mass of animals just prior to imergence into hibernation.