Abstract
Evaluating relationships between host body condition indices (BCIs) and parasite infections is a common approach for assessing the negative effects of parasites on host health. Researchers generally calculate BCIs from host body length-mass relationships and expect negative correlations between BCIs and infection levels; however, empirical results are often highly variable among host–parasite systems. Here, we hypothesize that two overlooked factors—host body size and external temperature– underlie this variability by modulating the metabolic costs associated with infection. Both of these are particularly well-known factors that strongly influence host and parasite ecology and physiology. We specifically predict that the negative effects of parasites are stronger in smaller hosts due to their higher mass-specific metabolic rates, requiring costly energetic allocation toward resistance or tolerance. We also predict that higher temperatures exacerbate these effects by increasing metabolic stress and reducing the host's ability to mitigate infection costs. We tested these predictions using 165 fish BCI-metazoan parasite infection relationships extracted from 51 published studies providing the required data and compiled systematically. Contrary to our initial predictions, our meta-regression models revealed no significant effects of host size (body mass) or temperature on the BCI-infection relationship after accounting for other drivers, such as parasite taxa and types of BCI measures used. We discuss potential mechanisms underlying this pattern, including counter-adaptive strategies employed by fish hosts to maintain body condition even under suboptimal conditions.