Demography of Belding's Ground Squirrels

Abstract

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 (R0) 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.