Seasonal Changes in Body Mass, Insulin, and Glucocorticoids of Free-Living Golden-Mantled Ground Squirrels

Abstract

The aim of the present study was to determine the relationship between seasonal variation in levels of insulin, cortisol, and corticosterone and body mass in a natural population of golden-mantled ground squirrels ( Spermophilus saturatus) over the active season, from April to August. The body mass of females is at an active season minimum on emergence from hibernation, increases during pregnancy and lactation, and shows a further rise during fattening, reaching a peak in August. Males are heavier than females on emergence from hibernation, lose mass during the mating period, then gradually increase to a maximum in August. Plasma insulin titers generally reflected these patterns of change in body mass. Levels in males were almost double those of females on emergence, then decreased as body mass was lost during mating. Male values increased in July and peaked in August, coincident with the body mass maximum. Females, in contrast, showed very low insulin levels in April on emergence and increased levels during lactation, continuing high into the fattening period. Corticosterone levels were low in both sexes at the beginning of the season and rose throughout most of the active season; female levels exceeded those of males during the lactation phase. Cortisol titers gradually decreased over the first half of the active season in both sexes, but later increased to a seasonal maximum at the end of the season, coincident with the peak in body mass. Insulin may act as an anabolic agent to promote fat deposition, and also as a signal of total body fat content to influence the transition from mass gain at the end of the active season to mass loss during hibernation. The influence of the glucocorticoids is less clear, but cortisol appears to exert a predominantly anabolic influence on seasonal fattening.