Does bigger mean sicker? A systematic review and meta‐analysis of the intraspecific relationship between host body size and parasite infection across vertebrate taxa

Elucidating the determinants of parasite infection levels of individuals within a population is a basic but important step for understanding the roles of parasites in natural systems. Among intraspecific studies on vertebrate hosts and metazoan parasites, host body size (length/mass) is probably the most frequently reported factor determining parasite infection levels (intensity/abundance), where positive relationships are usually observed. However, whether the positive correlations between host size and infections represent a general rule remains unclear, as do the factors determining the strength of these correlations. Here, by conducting meta‐analyses using 793 infection‐versus‐body size correlations, we tested whether the positive relationship between host body size and parasite infections represents a general pattern across all vertebrate host taxa and major metazoan parasite groups. Results showed that heterogeneity among studies was very high ( I ²  = 96.05%), and as a result, overall correlations were relatively weak, though the mean effect size was significantly positive ( Zr = 0.206). These results indicate that the presumed positive relationships between host body size and infections are not strongly supported as a general rule, and that the strength of correlations varied greatly among host–parasite systems and among ecological contexts. We also identified several drivers of the relationships such as host groups, the range of the body size, and infection measures used, suggesting that not only biological processes but also statistical noise associated with these methods influence the relationships. To our knowledge, this study provides the first quantitative evidence from meta‐regression showing that positive relationships between host body size and infection are not a general pattern and are highly heterogeneous among host–parasite systems. Based on these findings, we stress the importance of testing rather than assuming host size–infection relationships in any given host–parasite system, and considering both biological and other underlying processes, to better understand the determinants of infection in nature.