Good parasites make bad hosts: why is hyperparasitism uncommon among metazoan parasites

Hyperparasitism, in which a parasite exploits another parasite, represents a logical extension of parasite diversity and complexity within ecological networks. While common and ecologically significant in microbial systems, hyperparasitism among metazoan parasites appears comparatively rare. This discrepancy is intriguing given the high diversity and frequent co-occurrence of parasites within hosts. Here, we compile and review published cases of hyperparasitism involving metazoan parasites to assess their taxonomic distribution and ecological characteristics. We identify a wide range of hyperparasitic strategies, including opportunistic associations and highly specialized interactions. However, many published cases appear to be accidental or rare, suggesting that true hyperparasitism is even less frequent than reported. We argue that the relative rarity of hyperparasitism among metazoan parasites results from a combination of ecological and evolutionary constraints, including low encounter probabilities, dependence on multiple hosts, parasite aggregation, limited trophic accessibility, structural simplicity of parasite bodies, the presence of dual immune barriers imposed by both host and parasite, and, especially, energetic scaling constraints. Together, these factors restrict the conditions under which hyperparasitism can evolve and persist. Therefore, despite the vast diversity of parasites, their suitability as hosts is limited, and thus they rarely serve as hosts for other metazoan parasites. In this context, "good parasites" may indeed make "bad hosts."