Facultative scavenging by predatory carnivores is a prevalent but frequently underestimated feeding strategy. DNA-based methods for diet analysis, however, do not allow to distinguish between scavenging and predation, thus, the significance of scavenging on population dynamics and resource partitioning is widely unknown. Here, we present a methodological innovation to differentiate between scavenging and fresh prey consumption using prey RNA as a target molecule. We hypothesized that the rapid post-mortem breakdown of RNA in prey tissue should lead to a significantly lower detection probability of prey RNA than DNA when carrion rather than fresh prey is consumed. To test this hypothesis, ground beetles (Pseudoophonus rufipes (De Geer)) were offered either fresh or one-day-old dead Drosophila melanogaster fruit flies (carrion). The detectability of prey RNA and DNA in the beetles’ regurgitates was assessed with diagnostic Drosophila-specific RT-PCR and PCR assays at 0, 6, 12, 24 and 48 hours post-feeding. After fresh fly consumption, prey RNA and DNA were detectable equally well at all times. When carrion prey was consumed, the detection strength of prey RNA immediately after feeding was significantly lower than that of prey DNA and reached zero in most samples within 6 hours of digestion. Our findings provide evidence that prey RNA allows distinguishing between the consumption of fresh and scavenged prey, thereby overcoming a long-known weakness of molecular diet analysis. The assessment of prey RNA offers a generally applicable approach for examining the importance of scavenging in food webs to unravel its functional consequences for populations, communities, and ecosystems.

In feeding experiments, ground beetles (Pseudoophonus rufipes (De Geer)) were offered either fresh or one-day-old dead Drosophila melanogaster fruit flies (carrion). Batches of 10-14 regurgitates were collected per prey type (fresh or carrion prey) at 0, 6, 12, 24, and 48 h after feeding. The detectability of prey RNA and DNA in the beetles’ regurgitates was assessed with diagnostic Drosophila-specific RT-PCR and PCR assays. PCR products were separated and visualized with capillary electrophoresis and relative fluorescent units (RFU) were recorded for each sample as a relative measure of prey DNA or RNA. For more details, refer to our 2022 manuscript.

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