Juvenile survival curves in a solitary ground squirrel with a prolonged hibernation: effects of individual characteristics, environment, and maternal investment.

Abstract

Juvenile survival is a key life-history influence on population dynamics and adaptive evolution. We analyzed the effects of individual characteristics, early environment, and maternal investment on juvenile survival in a large solitary hibernating rodent-yellow ground squirrel Spermophilus fulvus using Cox mixed-effects models. Only 48% of weaned pups survived to dispersal and 17% survived to hibernation. Early life expectancy was primarily determined by individual characteristics and, to a lesser extent, by the early environment. The strongest and positive predictor of juvenile survival was body mass which crucially affected mortality immediately after weaning. Males suffered higher mortality than females after the onset of dispersal; however, the overall difference between sexes was partly masked by high rates of mortality in the first days after emergence in both sexes. Later emerged juveniles had lower life expectancy than the earliest pups. The overall effect of local juvenile density was positive. Prolonged lactation did not enhance juvenile survival: Pups nursed longer survived shorter than the young nursed for a shorter period. Our findings support the hypothesis that females of S. fulvus cannot effectively regulate maternal expenditures to mitigate the effects of unfavorable conditions on their offspring. The strategy to deal with seasonal time constraints on life history in female S. fulvus suggests an early termination of maternal care at the cost of juvenile quality and survival. This female reproductive strategy corresponds to a "fast-solitary" life of folivorous desert-dwelling S. fulvus and other solitary ground squirrels with prolonged hibernation.