Prey-to-fox isotopic enrichment of ³⁴ S in bone collagen: Implications for paleoecological studies

The trophic enrichment factor (TEF) is a parameter reflecting the difference in isotopic ratio between a consumer's tissues and diet, used in isotopic ecology and paleoecology to track dietary habits. The TEF of sulfur is believed to be low, but was, until now, only documented in a limited number of taxa. In this study we use a subfossil accumulation of bones from a red fox (Vulpes vulpes) den to verify the TEF for sulfur in fox bone collagen.

Collagen was extracted from 30 samples of subfossil bones, including foxes and their prey. The δ³⁴ S values of the bone collagen samples were measured with an elemental analyzer connected to an isotope ratio mass spectrometer. The TEF was calculated as [Δ³⁴ S = (mean δ³⁴ S in predator) - (mean δ³⁴ S in prey)], using taphonomic indices to estimate the mean diet, and calculated separately for different age classes of the predator.

We modeled 12 variants of TEF for different estimations of the diet composition and for three fox age classes (adult, subadult, and juvenile). The estimated TEF values range from -0.54 to +0.03‰ and are similar to TEFs known for other mammals. Absolute TEF values are nearly equal to or lower than the analytical error, which is ±0.4‰.

For the first time, we present direct δ³⁴ S data for the bone collagen of a free-living predator and its naturally selected prey. Our results indicate very low or even slightly negative TEF values for sulfur. Furthermore, according to our results, the δ³⁴ S value should not be considered a reliable indicator of trophic position in terrestrial food webs but rather, it should be used to disentangle different food webs based on different primary producers.