Fossil Wyoming Ground Squirrels

Thomas Goodwin (Biology)

Seasonal variation in diet of fossil Wyoming ground squirrels (Urocitellus elegans) from Porcupine Cave, CO

With funding from two previous faculty research grants, my collaborators and I have demonstrated that carbon-isotope profiles (delta13C) from incisor enamel of modern thirteen-lined ground squirrels (Ictidomys tridecemlineatus), collected through the active season, preserve a fine-scale record of seasonal changes in diet, with a spike in use of C4 vegetation in late summer. We obtained delta13C readings along the growth axis of each tooth, using laser ablation coupled with a gas chromatography isotope ratio mass spectrometer (GC-IRMS), which allows high-resolution sampling (on average, one sample/3 days of tooth growth) and analysis of very small samples.

Here, we will apply these methods to test the hypothesis that diet varied seasonally during the lives of Wyoming ground squirrels (Urocitellus elegans) preserved as fossils in Pleistocene (“Ice Age”) deposits at Porcupine Cave, CO. We will estimate “time of season” based on position of enamel relative to a hibernation mark on the tooth (methods in Goodwin et al. (2005)), and, for young individuals, stage of dental development (calibrated to modern ground squirrels). Then, we will micro-sample along the growth surface of enamel using the same methods employed in study of modern specimens. Because values of delta13C primarily reflect proportions of C3 and C4 vegetation in diet for terrestrial herbivores, we expect to make inferences about seasonal distributions of these two vegetation types–and, by extension, gain insight into paleoenvironmental conditions in the vicinity of Porcupine Cave. We will also test the hypothesis individual ground squirrels varied substantially in their time of entry into hibernation.

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