Fitness Costs of Parasites Explain Multiple Life-History Trade-Offs in a Wild Mammal

A longstanding prediction of life history theory states that parasitism can provide a mechanistic link between current and future fitness, thereby mediating life history tradeoffs. Demonstrating this mechanism in the wild involves linking a reproductive cost for parasitism with a cost of parasitism for fitness, which requires high-resolution longitudinal monitoring of individuals. Here, we examined associations among reproductive investment, immunity, parasitism, and fitness-related traits in a wild population of individually identified red deer (Cervus elaphus). Using path analysis, we constructed and analysed possible links among these traits, particularly investigating whether costs of lactation for survival and fecundity were likely to act through increased parasitism. Lactation was associated with increased strongyle parasitism, which was then associated with decreased subsequent fitness for all four fitness-related traits we examined. In addition, lactation was directly associated with later parturition date the following year, and increased helminth-specific antibody levels were associated with increased probability of reproduction. This study offers observational evidence for parasite mediation of multiple life history tradeoffs in a wild mammal, and supports the value of considering life history investment, immunity, and parasitism simultaneously when examining fitness consequences in disease ecology.