Abstract
1. The social environment has potent effects on individual phenotype and fitness in group-livingspecies.2. We asked whether the presence of kin might act on energy allocation, a central aspect oflife-history variation.3. Using a 22-year data set on reproductive and somatic allocations in Columbian groundsquirrels (Urocitellus columbianus), we tested the effects of co-breeding and non-breeding kinon the fitness and energy allocation balance between reproduction and personal body conditionof individual females.4. Greater numbers of co-breeding kin had a positive effect on the number of offspringweaned, through the mechanism of altering energy allocation patterns. On average, femaleswith higher numbers of co-breeding kin did not increase energy income but biased energyallocation towards reproduction.5. Co-breeding female kin ground squirrels maintain close nest burrows, likely providing asocial buffer against territorial invasions from non-kin ground squirrels. Lower aggressiveness,lower risks of infanticide from female kin and greater protection of territorial boundariesmay allow individual females to derive net fitness benefits via their energy allocationstrategies.6. We demonstrated the importance of kin effects on a fundamental life-history trade-off.