Isotopes in coyotes: Calibrating scat and different tissues to diet

Background/Question/Methods

As studies using stable isotopes to infer the diets and foraging behaviors of wild animals increase, the importance of species-specific diet-to-tissue discrimination factors is becoming more apparent. Many authors have shown that different tissues have different isotopic values, due either to isotopic routing or to variable rates of isotopic turnover. To make comparisons across studies that use a variety of tissue types, we determined tissue-to-tissue apparent C and N isotope enrichment factors (ε¹³* and ε¹⁵*, respectively) between (1) bone collagen and hair keratin, (2) collagen and scat, and (3) keratin and scat from road kill coyotes (Canis latrans). To validate the use of C and N isotope values measured in scats themselves, we compared diet predictions based on traditional scat-analysis techniques (frequency of occurrence and % by volume) with mixing model predictions derived from stable isotope analyses of scats and scat contents. Together, these two lines of inquiry will enable comparisons between past coyote diets determined from bone collagen C and N isotope values with modern coyote diets determined through non-invasive sampling of scats.

Results/Conclusions

For the 3 coyote specimens analyzed to date, the mean ε* value between collagen and keratin is consistently small (< 1‰ for both ε¹³* and ε¹⁵*), while mean ε¹³* and ε¹⁵* values between collagen and scat (4.4‰ ± 1.7, 2.0‰ ± 0.9) and between keratin and scat (4.2‰ ± 0.8, 1.4‰ ± 0.8) are considerably higher. ε¹³* values between both collagen and scat and keratin and scat from an individual gray fox (Urocyon cinereoargenteus) are only half as large, 2.4‰ and 2.0‰ respectively, and more similar to values calculated for mammalian herbivores. This suggests that ε¹³* values between scat and other tissues may vary along a carnivory gradient, with greater ε¹³* values found in more carnivorous animals. Based on results from 8 mtDNA-verified coyote scats collected at Año Nuevo State Park, CA, we found that scat C and N isotope values and traditional scat-analysis methods predict complimentary, but not identical, coyote diet compositions. All three methods indicate that terrestrial mammals comprise the greatest proportion of coyote diets, followed by fruit/seeds/vegetation, and then by marine mammals. Stable isotopes are advantageous; they are comparatively quick to measure and provide a means to better estimate the importance of larger animals to coyote diets.