P-28 Seasonal Variation in Diet of Thirteen-lined Ground Squirrels in Southwestern Michigan

Thirteen-lined ground squirrels (Ictidomys tridecemlineatus) are omnivorous rodents known to eat a variety of plant and animal matter. Prior work on a population of these ground squirrels in southwestern Michigan hypothesized a sharp shift in diet in late summer, from a diet rich in C₃ plants to one rich in C₄ plants. This hypothesis was based on an abrupt shift in the carbon isotope composition of teeth from squirrels collected in the region but has not been directly confirmed. Here, we present more direct data on diet, based on isotopic evidence and microhistological analysis of fecal samples collected from squirrels. These data confirm that squirrels in a local colony shifted diets during the study season. From June to mid-July, squirrels ate abundant arthropods and C₃ vegetation. In August and later, squirrels ate few arthropods and abundant C₄ vegetation. The latter observation supports the hypothesis originally developed to account for a carbon isotope shift in teeth.