When fly larvae feed on decaying animal tissue, their nitrogen isotope levels rise dramatically compared to the tissue they eat, showing that the larvae, not the decaying material, are responsible...

We analyzed the available evidence and found that fly larvae appear to be closely linked to the extreme rise in nitrogen isotope levels—δ¹⁵N—seen in decomposing tissue. The single assertion we reviewed, supported by 53.0 studies, suggests that when fly larvae feed on decaying animal tissue, their own nitrogen isotope values increase significantly more than what is present in the tissue itself. This pattern implies that the larvae, rather than the decaying material alone, play a central role in driving this isotopic shift. What we’ve found so far points to the larvae’s biological processes—likely related to how they metabolize and recycle nitrogen—as the main factor behind the unusually high δ¹⁵N levels. There is no evidence in our review that contradicts this observation. However, we also note that this conclusion is based on a single assertion, even though it is supported by a large number of studies. We do not yet have data on whether other organisms, environmental conditions, or stages of decomposition might also contribute to this effect. The evidence we’ve reviewed leans toward fly larvae being the primary driver of this isotopic enrichment, but we cannot say this is the only possible explanation. More research would be needed to rule out other factors or confirm how consistent this pattern is across different species, climates, or types of tissue. In everyday terms: if you’re seeing unusually high nitrogen isotope levels in a dead animal, the maggots feeding on it are likely the reason why—because of how their bodies process the nutrients, not just because the tissue is rotting.