High-resolution stable isotope profiles of modern elephant (Loxodonta africana) tusk dentin and tail hair from Kenya: Implications for identifying seasonal variability in climate, ecology, and diet in ancient proboscideans

Stable isotope ratios in tissues of large mammalian herbivores record diet and climate information integrated over large spatial areas and can be used to study modern and fossil ecosystems. Sound interpretation of data requires that tissue growth rates be determined accurately and that ecological and behavioral variables that influence stable isotope ratios of tissues be measured and related to experienced environmental conditions assessed through field observations, remote sensing data, and meteorological records. If well-understood in modern herbivores, stable isotopes from closely-related extinct taxa have tremendous potential for resolving paleodiet, paleoenvironment, and paleoclimate of terrestrial ecosystems. We present multiyear, high-resolution (i.e., weekly) stable isotope records from bioapatite in tusk dentin (δ¹³Cdₑₙₜᵢₙ and δ¹⁸Odₑₙₜᵢₙ) and tail hair (δ¹³Cₕₐᵢᵣ and δ¹⁵Nₕₐᵢᵣ) of an African elephant (Loxodonta africana) from Kenya that was fitted with a GPS collar intermittently over a five year period and observed for nearly a decade. GPS and observational data provide behavioral, life history, and location information. Normalized Difference Vegetation Index (NDVI), precipitation, and isotopic data from plants and water provide further constraints for interpreting isotope profiles. We determine tusk and hair growth rates using a combination of histological and geochemical approaches, including bomb-curve radiocarbon, that confirm approximately weekly resolution in the stable isotope profiles. Tusk dentin isotope profiles spanning the periods 1982 to 1987 and 2000 to 2006 record weekly variability in δ¹³Cdₑₙₜᵢₙ, where increases of up to 4.5‰ from baseline values due to diet switches from predominantly C₃ browsing to mixed C₃ browsing and C₄ grazing occur during the twice-yearly (biannual) rainy seasons. The δ¹³Cₕₐᵢᵣ values show a similar trend. The δ¹³C profiles served as a proxy for seasonal changes in rainfall, vegetation, and diet. The δ¹⁸O of tusk bioapatite varied approximately biannually up to 5‰, likely reflecting increases in the proportion of plant water ingested during the wet season. Using a least squares inverse filter, we show that NDVI can be used to predict δ¹³C of dentin and vice versa, offering the possibility to reconstruct seasonal changes in vegetation and rainfall in the geologic past. Our results demonstrate that high-resolution tusk isotope profiles serve as a proxy for seasonality of diet and precipitation, and thus can be used to reconstruct aspects of elephant life history, vegetation, and climate at unprecedented resolution from modern and fossil proboscidean samples.