Telomere length is a biomarker of biological ageing and lifespan in various vertebrate taxa. Evidence is accumulating that telomeres shorten more rapidly when an individual is exposed to environmental stressors. Parasites are potent selective agents that can cause physiological stress directly or indirectly through the activation of the host’s immune system. Yet to date, empirical evidence for a role of parasites in telomere dynamics in natural populations is limited.

Here we show experimentally that exposure to ectoparasitic hen fleas (Ceratophyllus gallinae) during growth results in shorter telomeres in female, but not male, great tit (Parus major) nestlings. Females had significantly longer telomeres than males when growing up in experimentally deparasitized nests but because of the sex-specific effects of parasitism on telomere length, this sexual dimorphism was absent in birds growing up in experimentally infested nests. Our results provide the first experimental evidence for a role of ectoparasitism in telomere dynamics in a natural vertebrate population, and suggest that the costs of infection manifest in sex-specific ways.