P-23 High-resolution Record of Temporal Variation in δ13C from Intra-Tooth Sampling of Incisor Enamel of Free-ranging Thirteen-lined Ground Squirrels

Buller Hallway

We tested the hypothesis that δ13C profiles of incisor enamel from free-ranging thirteen-lined ground squirrels (Ictidomys tridecemlineatus) record seasonality in diet. We used laser ablation to densely sample lower incisor enamel (on average, one sample per 3 days of tooth growth) from 20 squirrels collected in SW Michigan, and analyzed samples using GC-IRMS. The basal ~20-35 increments of record always displayed a negative trend in δ13C, probably reflecting contamination by organic carbon in developing enamel; we excluded these values. At one colony, squirrels collected within 25 m of a cornfield displayed significantly higher and more variable δ13C (variable C4-rich diet; mean=-5.03, SD=2.38, n=6) than did squirrels from mowed lawn distant from the cornfield (C3-rich diet; mean=-13.92, SD=0.61, n=6); this pattern persisted from early May-late August and was confirmed to reflect dietary differences (δ13C of fecal pellets). Both groups displayed a positive excursion in δ13C in late August. Late-season use of C4-rich vegetation was also documented at a second colony (n=8) without access to a cornfield, studied in late summer through hibernation. Isotope values peaked rapidly from mid-August to mid-September, and subsequently dropped precipitously to hibernation. We suggest rodent incisors as possible recorders of fine-scale seasonal variation of interest to paleoecologists.