Date of Award


Document Type


Degree Name

Master of Science


College of Arts and Sciences


Biology, MS

First Advisor

H. Thomas Goodwin

Second Advisor

Benjamin Navia

Third Advisor

Daniel Gonzalez-Socoloske


Thirteen-lined ground squirrels (Ictidomys tridecemlineatus) are omnivorous ground squirrels that range from Texas to Alberta and from Michigan to Utah. The amount and type of both animal and plant matter in their diet may vary both geographically and seasonally. The present study was motivated by prior work, based on δ13C analysis of incisor enamel of thirteen-lined ground squirrels from a colony in southwestern Michigan, suggesting that these animals underwent a pronounced, late-season shift in diet from predominantly C3 to C4 plants. However, this inferred dietary shift was not directly demonstrated. Here, I tracked diet of thirteen-lined ground squirrels in southwestern Michigan from June to October 2018 using fecal samples deposited by animals when live-trapped; squirrels were then released unharmed. In addition, I collected and identified voucher plant specimens at the field site.

Microhistological and isotopic analyses of fecal samples documented two primary seasonal shifts in diet. First, the microhistological data indicated a sharp decrease in arthropod consumption as summer turned to fall; this decrease correlated with the overall decrease in δ15N and 1/C:N (carbon:nitrogen; a measure of trophic position and degree of carnivory). These observations were consistent with a shift from a protein-rich early summer diet rich in arthropods to a fall diet with less protein. Secondly, the microhistological data demonstrated a dramatic late-season increase in consumption of grass seeds and glumes, which mirrored a sharp increase in δ13C. Both of these data sets indicated a shift from primarily C3-plants to a mixed or primarily C4-plant diet in August, with a C4-rich diet continuing into October. In summary, my study a) documented a previously unknown shift from high to low use of arthropods, a pattern that is contrary to findings of prior studies, and b) confirmed the late-season shift from C3- to C4-plants previously inferred from enamel isotope data.

Subject Area

Thirteen-lined ground squirrel--Food; Ground squirrels--Food

Creative Commons License

Creative Commons Attribution-No Derivative Works 4.0 International License
This work is licensed under a Creative Commons Attribution-No Derivative Works 4.0 International License.


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