Stable isotopes are widely used in ecosystem studies to decipher trophic relationships between species. Using stable isotopes as trophic markers requires knowledge of the stable isotope ratios of consumer and putative prey, and of trophic discrimination factors (TDFs), the difference between the stable isotope ratios of predator and prey. TDFs differ among species, tissue sampled and the consumer’s physiological state. We tested whether consumer ontogeny also influences its stable isotope ratios by measuring the δ¹³C and δ¹⁵N values, and their associated TDFs, of African penguin (Spheniscus demersus) blood components (whole blood, red blood cells, plasma) among four age classes (chicks, fledglings, juveniles and adults). Blood was sampled from captive penguins concurrently with their diet. Ontogenetic effects on TDFs were complex, depending on the blood component and isotope considered. Blood components differed in their sensitivity to consumer physiology, but TDFs for both carbon and nitrogen were generally lower for the younger age classes. Age class influenced the stable isotope compositions of whole blood and red blood cells with chicks and fledglings exhibiting significantly lower δ¹³C and δ¹⁵N values than juveniles and adults. Plasma showed no effect of ontogeny. The results indicate age class affects stable isotope ratios and TDFs through ontogenetic changes in physiology. Therefore, when uncertainty concerning TDFs exists in studies of wild animals, stable isotope analysis should preferably be combined with other dietary techniques.

稳定同位素广泛用于生态系统研究，以破译物种之间的营养关系。使用稳定同位素作为营养标记需要了解消费者和假定猎物的稳定同位素比率，以及营养鉴别因子 (TDF)，即捕食者和猎物的稳定同位素比率之间的差异。TDF 因物种、取样的组织和消费者的生理状态而异。我们通过测量非洲企鹅 ( Spheniscus demersus)的 δ ¹³ C 和 δ ¹⁵ N 值及其相关的 TDF 来测试消费者个体发育是否也影响其稳定同位素比率) 血液成分（全血、红细胞、血浆），分为四个年龄段（雏鸡、雏鸟、青少年和成年）。从圈养企鹅身上采集血液样本，同时进行饮食。对 TDF 的个体发育影响是复杂的，这取决于所考虑的血液成分和同位素。血液成分对消费者生理机能的敏感性不同，但碳和氮的 TDF 在较年轻的年龄段通常较低。年龄等级影响全血和红细胞的稳定同位素组成，雏鸡和雏鸟的 δ ¹³ C 和 δ ¹⁵显着降低N值大于幼鱼和成鱼。血浆对个体发育没有影响。结果表明年龄等级通过生理学的个体发生变化影响稳定同位素比率和 TDF。因此，当野生动物研究中存在 TDF 的不确定性时，最好将稳定同位素分析与其他饮食技术结合起来。