Experimental ectoparasite removal has a sex-specific effect on nestling telomere length

Parasites are a strong selective force that can influence fitness-related
traits. The length of chromosome-capping telomeres can be used to assess
the long-term costs of parasitism, as telomere loss accelerates in
response to environmental stressors and often precedes poorer survival
prospects. Here, we explored the sex-specific effects of ectoparasite
removal on morphology and telomere length in nestling tree swallows
(Tachycineta bicolor). To do so, we experimentally removed blowfly
(Protocalliphora spp.) larvae from nests using Permethrin, a
broad-spectrum insecticide. Compared to water-treated controls,
insecticide treatment of nests had a sex-biased effect on blood telomere
length: ectoparasite removal resulted in significantly longer telomeres in
males but not females. While this treatment did not influence nestling
body mass, it was associated with reduced feather development regardless
of sex. This may reflect a relaxed pressure to fledge quickly in the
absence of parasites, or alternatively, could be a negative side effect of
permethrin on morphology. Exploring robust sex-specific telomere dynamics
in response to early-life environmental pressures such as parasitism will
shed light on sexual dimorphism in adult life histories and ageing.