Is the Intrasexual Competition in Male Red Deer Reflected in the Ratio of Stable Isotopes of Carbon and Nitrogen in Faeces?

SIMPLE SUMMARY: Stable isotope analysis is a technique increasingly used for studying many aspects of wildlife behaviour, such as diet, migrations, and trophic relationships. However, scarce information is available on how isotopes are assimilated into the body under conditions such as nutritional stress situations or intrasexual competition for mates. In this study, we evaluated how intrasexual competition between male red deer from free-ranging populations in Andalusia (Spain) is reflected in the ratio of stable isotopes of carbon and nitrogen (δ(13)C and δ(15)N) in faeces. Our results showed that in populations with high intrasexual competition δ(15)N values in faeces were significantly lower than in low intrasexual competition populations. No differences were found for δ(13)C. This study provides a basis for interpreting δ(15)N values related to different protein usage under various nutritional stress situations in male red deer and novel applications of the use of nitrogen isotopes. ABSTRACT: Isotopic analysis of carbon and nitrogen in faeces is a reliable methodology for studying ecology in wildlife. Here, we tested this technique to detect variations in carbon and nitrogen isotopic ratios (δ(13)C and δ(15)N) in two different intrasexual competition scenarios of male Iberian red deer (Cervus elaphus hispanicus) using faeces of individuals collected during hunting actions in South-eastern Spain. The carbon isotopic ratio (δ(13)C) was not found to be significant, likely due to similar diet composition in all individuals. However, the nitrogen isotopic ratio (δ(15)N) was found to be lower in populations where sexual competition between males during the rut was higher compared to low-competition populations. Therefore, this study suggests a different use of proteins by an individual male red deer depending on the sexually competitive context in which he lives. Although further research is needed, these results show the potential of isotopic analysis as a tool for studying individual and populational variations in the level of intrasexual competition, with implications in evolutionary ecology and population management.