Should growing and adult animals fed on the same diet show different delta 15N values?

Animals usually show a delta 15N value that is slightly higher than that of their food. The value of this enrichment appears to be fairly constant among species (approximately 3/1000). This phenomenon is more and more widely used in ecological research to study individual diets or the trophic structure of whole communities. However, very little is known about the mechanisms that actually explain how this trophic isotopic enrichment occurs. Most ideas about these mechanisms are only expressed verbally, so that it is difficult to get a clear picture of what is actually known, and how these pieces of knowledge interact. We propose a very simple model that describes mathematically what is currently known on the trophic isotopic enrichment phenomenon in animals. This model cannot replace actual measurements and investigations about the mechanisms explaining the phenomenon. However, it allows the clarification of some ideas such as what conditions have to be fulfilled in order for the trophic isotopic enrichment to occur. Our formalization accounts for all the known features of the trophic isotopic enrichment phenomenon that we consider in this paper in both a qualitative and quantitative manner (at least for orders of magnitude). A prediction of the theory, that can be tested, is that growing animals should show the same delta 15N values as those of adults fed the same diet if the total amount of nitrogen they assimilate during their growth is large compared to the total nitrogen content of their adult body. It seems likely that this condition is fulfilled in most cases.