ISOTOPIC PALEOECOLOGY OF MAMMUTHUS FROM SANTA ROSA ISLAND, NORTHERN CHANNEL ISLANDS, CA

Mammoths, Mammuthus columbi and M. exilis, are the most recognizable members of the insular Pleistocene fauna of the Northern Channel Islands of California. It has been suggested that body size played an important role in the behavioral isolation of M. columbi and M. exilis on the Northern Channel Islands; however, this hypothesis has not previously been tested. There is also a wide range of variation within Channel Islands mammoth tooth morphology that poses questions about the timing, cause, and number of speciation events within this insular fauna.

All teeth sampled for this study were originally identified as from M. exilis; however, several of the teeth fall within the size ranges observed for M. columbi. I used bulk (n=6) and serial (n=2) stable carbon and oxygen isotope data to interpret the ecological niche of this subset of Channel Islands mammoths. The calculated dietary δ13C values of all the mammoth teeth sampled, excluding one, suggests they ate a C3-dominated mixed feeder diet. The δ13C values from one specimen were outliers within this assemblage of teeth, suggesting a mixed feeder diet with a higher concentration of C4 plants than the other sampled teeth in this study. The stable carbon isotope data from this study are consistent with reconstructed plant communities from the Channel Islands during the Pleistocene, and Channel Islands mammoths likely ate a much more C3-dominated diet than mammoths in mainland North America. The dietary niche of Channel Islands mammoths was shaped by available vegetation and appears fairly consistent within this assemblage of teeth. From this dataset, there does not seem to be any significant relationship between tooth size and inferred dietary niche.

The range of calculated drinking water δ18O values from this assemblage of teeth are very high (2.6‰ to 4.5‰, VSMOW), compared to the modern δ18O value for precipitation in Santa Barbara County, CA (-6‰, VSMOW), which I interpret to reflect a dependence on surface waters sourced from condensed fog. By 11,800 cal BP, reduced conifer canopy cover combined with a reduction in fog events could have potentially limited surface water recharge during deglaciation due to decreased fog drip. In this way, Channel Islands mammoths may have needed to decrease their dependency on fog-sourced surface waters to avoid resource stress.