Early predictors of female lifetime reproductive success in a solitary hibernator: evidence for “silver spoon” effect.

The research study focuses on the factors in the early life of female yellow ground squirrels that can predict how successful they will be at reproducing during their lifetime. The scientists found that the weight of the squirrels when they stopped nursing and their rate of weight gain as well as the density of their population, among other factors, all had an impact on their eventual ability to reproduce.

Study Overview

• The research involved long-term observation of yellow ground squirrels (Spermophilus fulvus) to assess what early-life characteristics could determine female reproductive success.
• The early-life traits examined included weaning weight, weight gain rate, date of emergence from the natal burrow, and environmental factors such as litter size, juvenile population density, and post-weaning precipitation.

Key Findings

• A significant variation was noted across all five components of lifetime reproductive success or LRS (survival to adulthood, adult lifespan, and lifetime numbers of weaned litters, weanlings, and yearling offspring).
• The number of offspring was correlated with each other and increased with lifespan, highlighting the importance of longevity over fecundity (the ability to produce an abundance of offspring) as a predictor of LRS.

Impact of Early-Life Conditions on Survival

• The study revealed that survival to adulthood was most affected by early-life conditions. Squirrels with greater weaning weight, those born farther from human settlements, and those born in lower population densities had higher survival rates.
• Population density at birth especially stood out as a reliable predictor of all LRS components and was found to negatively impact adult lifespan and the number of offspring.

“Silver Spoon” Effect

• The “silver spoon” effect refers to the phenomenon where squirrels born under favorable conditions (having higher weights and experiencing fast growth) were observed to have better reproductive outcomes.
• The early growth rate of squirrels positively affected the chances of reproduction after their first hibernation cycle and the number of weaned offspring.
• This early-life advantage leading to better fitness outcomes in adulthood, however, has only been observed in a few mammalian species.