The influence of food quantity on carbon and nitrogen stable isotope values in southern African spiny mice (Acomys spinosissimus)

Stable isotope analysis is frequently applied as a tool to examine dietary patterns in animals. However, some of the underlying assumptions associated with using this approach are increasingly being questioned. We carried out a controlled diet experiment on the southern African spiny mouse (Acomys spinosissimus Peters, 1852) to test a number of aspects relating to these assumptions and also examine the hypothesis that stable isotopes, especially δ[sup.15]N, can be used to provide evidence of nutritional stress. We compared the δ[sup.13]C and δ[sup.15]N values of livers and blood from animals that were fed ad libitum with animals undergoing a 10% reduction in food supply. Food-restricted animals showed no significant difference in δ[sup.15]N; however, δ[sup.13]C values of both liver and blood were depleted. Restricted animals also had a significantly lower C:N ratio. We examined the role of lipids and found following lipid extraction that both livers and lipids still showed the same separation in carbon values. Tissue-diet discrimination values were also calculated and found to be higher for both Δ[sup.13]C and Δ[sup.15]N compared with other mice species. Empirical values for discrimination rates were then compared with values calculated using an alternative method based on employing generic values and were found to be dissimilar, suggesting the use of generic values are not always appropriate. Our results highlight the need for greater understanding of the assumptions associated with using stable isotope analysis to examine diet and we suggest that studying a single species under captive conditions presents an ideal method to begin to test these hypotheses. Key words: Acomys spinosissimus, discrimination, food restriction, spiny mouse, stable isotope. Resume: L'analyse des isotopes stables est frequemment employee comme outil pour examiner les habitudes alimentaires d'animaux. Certaines des hypotheses qui sous-tendent cette approche sont toutefois de plus en plus remises en question. Nous avons mene une experience d'alimentation controlee sur la souris epineuse d'Afrique meridionale (Acomys spinosissimus Peters, 1852), afin de verifier differents aspects associes a ces hypotheses et d'examiner l'hypothese voulant que les isotopes stables, en particulier les valeurs de δ[sup.15]N, puissent etre utilises pour fournir des indices de stress nutritif. Nous avons compare des valeurs de δ[sup.13]C et δ[sup.15]N du foie et du sang d'individus nourris fi volonte a celles d'animaux assujettis aune reduction de 10 % de leur approvisionnement. Les animaux dont l'approvisionnement etait restreint ne presentaient aucune difference significative du δ[sup.15]N; cependant, les valeurs de δ[sup.13]N tant pour le foie que pour le sang etaient appauvries. Les animaux restreints presentaient egalement un rapport C:N significativement plus faible. Nous avons examine le role des lipides et constate que, apres extraction de ces derniers, les foies et les lipides presentaient toujours la meme separation de leurs valeurs de carbone. Des valeurs de discrimination tissusregime ont egalement ete calculees et etaient plus elevees, tant pour le Δ[sup.15]C que le Δ[sup.15]N, que celles d'autres especes de souris. Les valeurs empiriques des taux de discrimination ont ensuite ete comparees fi des valeurs calculees par une autre methode basee sur des valeurs generiques et se sont averees differentes, donnant a penser que l'utilisation de valeurs generiques n'est pas toujours bien avisee. Nos resultats soulignent la necessite d'une meilleure comprehension des hypotheses associees a l'utilisation de l'analyse des isotopes stables pour etudier le regime alimentaire, et nous suggerons que l'etude d'une seule espece dans des conditions de captivite constitue une methode ideale pour commencer a verifier ces hypotheses. [Traduit par la Redaction] Mots-cles: Acomys spinosissimus, discrimination, restriction alimentaire, souris epineuse, isotope stable.
Introduction Stable isotope analysis has been used to answer a number of questions relating to foraging and diet; for example, quantifying reliance on different food sources (Robb et al. 2012), [...]