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.

Avian haemosporidians are a diverse group of parasites that infect birds worldwide and have been a major focus of research for decades. Yet, few studies have identified the drivers of infection at the intraspecific host level. We aimed to study the drivers of prevalence and diversity of haemosporidian parasites infecting a common North American songbird species, the American Robin (Turdus migratorius), which breeds across most of the continent. We found little seasonal variation in haemosporidian prevalence in robins, although prevalence of Leucocytozoon increases with breeding latitude in this host species. We also detected a significantly positive relationship between the robin breeding latitude and coinfection with different haemosporidian parasite lineages. Additionally, robins infected with Plasmodium had a significantly better body condition than those of uninfected robins, which could be due to migratory culling. When matched against known haemosporidian lineages, our results suggest that robins harbor a higher diversity of haemosporidian parasites than previously known. The results of this study suggest that comparisons of common, widespread bird species such as robins across their range could help unveil novel aspects of the haemosporidian–host relationship and how such a relationship may change under current and future rapid environmental change.

Biodiversity varies across the world and is influenced by multiple factors, such as environmental stability and past historical events (e.g., Panama Isthmus). At same time, organisms with unique life-histories (e.g., parasites) are subject to unique selection pressures that structure their diversity patterns. Parasites represent one the most successful life-strategies, impacting directly and indirectly the ecosystem by cascading effects on host fitness and survival. Here, I focused on a highly diverse, prevalent, and cosmopolitan group of parasites (avian haemosporidians) to investigate the main drivers of regional parasite diversity on a global scale. To do so, I compiled data from four global datasets on (i) avian haemosporidian (malaria and malaria-like) parasites, (ii) bird species richness, (iii) avian functional traits, and (iv) climate data. Then, using generalized mixed models, I evaluated potential drivers of haemosporidian diversity. I found that haemosporidian diversity is driven by both host regional diversity and functional traits, and by environmental conditions. In other words, parasite diversity increased with increasing host richness and higher numbers of resident and territorial birds. Further, greater temperature seasonality was also positively correlated with parasite diversity. Hence, regions harboring the greatest resident/territorial avian diversity (e.g., neotropics) and/or higher temperature seasonality (e.g., North America) generally harbor the highest diversity of haemosporidian parasites. Overall, I demonstrated that haemosporidian parasite diversity is intrinsically associated with their hosts’ diversity and functional traits.