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 compiled from 153 publications, 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 overall correlations were significantly positive but relatively weak (Zr = 0.206), indicating 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. We also found that studies on mammals and birds were more likely to report non-significant correlations, possibly due to behavioral, developmental, and physiological characteristics, compared with those on fish. Moreover, effect sizes varied depending on study methods, such as infection or host body size measures used, suggesting that not only biological processes but also statistical noise associated with these methods influence the relationships. Our study provides the first quantitative evidence showing that positive relationships between host body size and infection are not a universal pattern and are relatively 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.