EFFECTS OF LITTER SIZE AND POPULATION-DYNAMICS ON JUVENILE AND MATERNAL SURVIVAL IN COLUMBIAN GROUND-SQUIRRELS

(1) The relationships between litter size and juvenile and maternal survival were investigated for 9 years in a marked population of Columbian ground squirrels (Spermophilus columbianus Ord.) in south-western Alberta, Canada. (2) A population increase during the first 4 years was characterized by larger litter size, greater adult female body weight and greater survival than in the last 4 years, during which the population decreased. (3) During the increase phase, modal litter size at emergence from the natal burrow was four, and the number of juveniles surviving to the following spring increased with litter size. During the decrease, modal litter size was three, and juveniles in large litters experienced high mortality. (4) Maternal survival was independent of litter size during the increase phase, but declined for litters larger than three in the decrease phase. The fitness costs of different litter sizes seem to vary according to population dynamics, possibly as a result of changes in resource availability.