Demography of Belding's Ground Squirrels

Sherman, Paul; Morton, Martin

For 11 consecutive field seasons (1969-1979) we studied the demography of a free-living population of Belding's ground squirrels (Spermophilus belding) at Tioga Pass, in the central Sierra Nevada of California. Our research involved repeatedly capturing and directly observing individually marked, known-age animals within and between years. Here we present information on age-specific survival and dispersal rates for both sexes and age-specific fecundity and reproductive value for females. Our data indicate that: (1) The 7-8 mo hibernation period (October-May) is a hazardous time; 66-70% of juveniles and 36-39% of adults disappear during the winter. (2) Males apparently live about half as long as females; males that survive their first winter typically remain in the population an additional 1.1 yr vs. 1.6 yr for females; while few males live > yr, some females live >11 yr. (3) The sex of ration of the adult population is female-biased, especially in the older age-classes. (4) Throughout the active season (summer), males disappear more rapidly than females; this disappearance is due both to dispersal and to mortality. (5) During intrasexual combat for mates, males injure each other severely; such injuries lead directly and indirectly to male mortality. (6) Litter size and reproductive value change with female age; they are lowest for young (0-1 yr) and old (5-9 yr) female and highest for 2-4 olds. (7) The net reproductive rate (R"0) for 1969-1975 was 1.36 daughters per dam, while the mean generation length (@?) was 2.26 yr. Sexual asymmetries in the demographic characteristics of S. beldingi appear to result from the animals' polygynous breeding system.

Demography of Belding's Ground Squirrels

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