Phylogenetic and Ecological Trends in Specialization: Disentangling the Drivers of Ectoparasite Host Specificity

1. Ecological specialization reflects both evolutionary and ecological processes. For parasitic taxa, ecological specialization can be assessed as the degree to which a parasite species will associate with certain host species, a property known as host specificity. 2. Ectoparasitic bat flies have been previously reported as highly host specific, presumably due to a history of coevolution with their bat hosts. However, there is conflicting evidence of coevolution between bats and bat flies. Resource-driven competition between parasite individuals and between species may also be important in explaining patterns of bat fly specificity. 3. To test the importance of evolutionary and ecological factors on bat fly specificity, we collected and identified 21 bat fly species from 16 host bat species from the State of Rio de Janeiro in Brazil. We generated a bat fly species phylogeny from molecular data, estimated d’ specialization values (a metric of specificity), and used linear and cophylogenetic models to compare the importance of various drivers of parasite ecological specialization. 4. We found that bat fly co-occurrence frequency (a proxy for interspecific competition) and mean infection intensity (a proxy for intraspecific competition) best predicted patterns of bat fly specialization. Co-occurrence frequency had a significantly negative association with specialization, while mean infection intensity has a significantly positive association with specialization. Coevolutionary congruence had a small effect size and did not significantly predict parasite specialization. 5. We found multiple shifts toward more generalized host niches across the bat fly phylogeny. Our results suggest that ecological processes such as resource-driven competition may be more important than evolutionary processes in shaping bat fly host specialization networks. 6. Bat flies showed variable degrees of host specialization, parasitized phylogenetically distant host species, and showed low phylogenetic congruence to their hosts. This suggests that as a group, bat flies may show flexibility in their host preference phenotypes and may change their host associations in the face of environmental disturbance. ### Competing Interest Statement The authors have declared no competing interest.