Active, explorative, and infected: a meta‐analysis of individual variation in behaviour and parasitism

Behavioural variation among individuals strongly influences how individuals interact with their environment and, consequently, their exposure to parasites. Parasite infections, in turn, can alter host behaviour by imposing energetic costs or manipulating hosts to enhance transmission. Our meta‐analysis of 224 effect sizes from 74 studies examined the links between behaviours measured in standardized assays and infection across study designs and host and parasite types. Activity and exploration were the most frequently studied behaviours and accounted for most of the significant relationships detected in our analysis. Experimental studies showed that infected animals displayed lower activity and exploration, indicating energetic constraints on energetically demanding behaviours, while boldness and aggressiveness remained largely unchanged. These relationships varied with parasite type and host role. Microparasite infections were associated with reduced boldness, whereas endoparasites were linked to decreased activity–exploration, particularly in intermediate hosts. This effect appears less consistent under natural conditions, where responses may be modulated by other ecological and biological factors. Together, these findings indicate that host behavioural responses to parasitism are highly context dependent, as reflected by the substantial heterogeneity observed, underscoring the need for broader integration across host–parasite systems, behavioural traits, and study designs to better understand links between animal behaviour and parasitism.