Altitudinal Variation in the Life History of the Golden‐Mantled Ground Squirrel (Spermophilus Lateralis)
1979
Bronson, Michael T.
The life history of golden—mantled ground squirrels (Spermophilus lateralis) varied across a climatic gradient. I live— and snap—trapped at 4 and 5 elevations, respectively, in the northeastern Sierra Nevada of California for 3 yr. Higher elevation populations were characterized by (1) shorter aboveground active seasons of adults and juveniles, (2) later age at first reproduction in ♀ ♀, (3) greater ♀ survivorship, and (4) lower ovulation rates yielding smaller uterine litters. Analysis of covariance of litter size (N = 284) showed that litter size is partly explained, in decreasing order of importance, be elevation, body weight, and age of the mother. Neither loss of ova nor embryonic mortality contributed to elevational differences in live embryos per litter. Embryonic mortality decreased with altitude. Survivorship of young to 1 yr of age was low (15%—18%) and did not vary with altitude. The sex ratio was even among young—of—the—year (N = 351), but favored ♀ ♀ at 1 yr of age and older (N = 438). Based on tooth wear ages and uterine litter sizes, I constructed fecundity tables for 4 populations. The demographic variation may have arisen from several sources: (1) selection were early winters may have caused variable juvenile mortality (bet—hedging strategy), (2) selection where long winters may have enhanced adult survivorship and diminished the reproductive capability of younger ♀ ♀, or (3) phenotypic plasticity.
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