Stable isotope analysis of ectoparasites as a tool for understanding trophic interactions with mammalian hosts.

Climate change is expected to expand the geographic ranges of ectoparasites, increasing the transmission of vector-borne diseases and necessitating a better understanding of ectoparasite-host trophic dynamics. Haematophagous ectoparasites can serve as valuable subsamples of their hosts, retaining isotopic values that reflect dietary information in both their blood meals and tissues. However, differences in the life histories and feeding strategies of lice, fleas and ticks may influence how host isotopic composition is preserved. Here, stable isotope values of carbon (δ¹³C) and nitrogen (δ¹⁵N) were used to investigate trophic interactions between ectoparasites and their mammalian hosts in three pairings: lice (Anoplura: Polyplacidae; n = 101) from Eurasian red squirrels Sciurus vulgaris L. (Rodentia: Sciuridae), fleas (Siphonaptera: Ceratophyllidae; n = 92) from fat dormice Glis glis L. (Rodentia: Gliridae) and ticks (Ixodida: Ixodidae; n = 16) from European hedgehogs Erinaceus europaeus L. (Eulipotyphla: Erinaceidae). Our findings indicate that ectoparasites reflect the dietary patterns of their hosts, with lice exhibiting the closest isotopic values, followed by fleas and ticks. All parasites had significantly higher δ¹⁵N values than their hosts, indicative of trophic enrichment, but their δ¹³C values varied. Notably, we found that the presence of a blood meal did not significantly affect the isotopic values found in lice and fleas, while ticks showed a significant difference between exoskeleton and blood meal in δ¹³C values. This study highlights the importance of understanding how the life histories of parasite species influence the preservation of isotopic host signals in order to be able to utilise stable isotope analyses of ectoparasites to infer host dietary niches and preferences, with broader implications for understanding host-parasite dynamics and disease transmission pathways.