Maternal effects in Columbian ground squirrels

Abstract

The phenotype of an organism is the physical expression of its genotype and is a result of both the genetic makeup of an organism and the environment it experiences. An individual’s phenotype can also be affected by the phenotype of its mother. A maternal effect occurs when the phenotype of the mother influences the phenotype of her offspring, independent of the offspring’s genotype or non-maternal aspects of its environment. In this study, maternal effects on pup growth and size in Columbian ground squirrels were assessed. A cross-fostering study was used to determine the contributions of maternal effects and direct genetic effects to variation in pup traits. Maternal effects were responsible for a large proportion of the variation in offspring growth rate and weaning weight, as indicated by the high associations in these traits between unrelated littermates (growth rate: R2 = 0.58; weight: R2 = 0.64). Heritability estimates for both offspring traits were zero, suggesting that maternal effects are a more important determinant of offspring phenotype than are genetic effects. Relationships between maternal phenotype and offspring growth rate and weaning weight and the influences of these traits on survival of pups over-winter were also examined. Litter size and maternal traits (body condition, timing of reproduction, structural size, and change in mass during reproduction) explained 69% of the variation in offspring weaning weight and 72% of the variation in offspring growth rate. Only maternal body condition and litter size had significant effects on offspring weaning weight (condition: p = 0.34; litter size: p = -0.85) and growth (condition: p = 0.28; litter size: p = -0.88). Pup growth rate was the only variable with an effect on pup survival to yearling age. Influences of maternal investment in young did not appear to persist to yearling age, as suggested by the lack of association between maternal traits and size of offspring at emergence from hibernation the following spring (R2 = 0.34).