Fitness costs of parasites explain multiple life history tradeoffs in a wild mammal

Reproduction in wild animals can divert limited resources away from immune
defence, resulting in increased parasite burdens. A longstanding
prediction of life history theory states that these parasites can harm the
individual, reducing the organism’s subsequent fitness and producing
reproduction-fitness tradeoffs. Here, we examined associations among
reproductive allocation, immunity, parasitism, and subsequent fitness in a
wild population of individually identified red deer ( Cervus elaphus ).
Using path analysis, we investigated whether costs of lactation for
downstream survival and fecundity were mediated by changes in strongyle
nematode count and mucosal antibody levels. Lactating females exhibited
increased parasite counts, which were in turn associated with
substantially decreased fitness in the following year in terms of
overwinter survival, fecundity, subsequent calf weight, and parturition
date. This study offers observational evidence for parasite regulation of
multiple life history tradeoffs, supporting the role of parasites as an
important mediating factor in wild mammal populations.