Increasing returns in the life history of Columbian ground squirrels

Abstract

1. We examined positive associations and trade‐offs of maternal and reproductive traits in a population of Columbian ground squirrels, Spermophilus columbianus. 2. Structural size, body condition, mother's personal allocation to body mass during reproduction, and timing of littering were estimated for live‐trapped reproductive females that were observed during an 8‐year period, and were compared to litter mass, litter size, and average pup mass using path analyses. 3. Mothers exhibited age‐structured traits that influenced reproductive patterns. Yearling mothers were significantly smaller, bred later, and had smaller litters than older females. Mothers that gained more body mass during reproduction and older mothers in good body condition that were structurally large had larger litters. 4. Early seasonal timing of littering was an important positive influence on successful reproduction by older mothers only in early breeding seasons and in years when conditions for reproduction were good for all females. 5. The number of offspring that survived to 1 year of age was most strongly associated with litter mass and litter size; date of breeding was of secondary influence, with earlier litters exhibiting greater success. 6. In general, mothers that gained the most in body mass during reproduction were concurrently more successful in weaning large litters (perhaps due to better quality of foraging habitat). 7. In addition to expected reproductive trade‐offs, reproduction by Columbian ground squirrels exhibited positive associations of life‐history traits that may reflect evolutionary increasing returns.