Feather mite ecology and morphology: exploring how obligate ectosymbionts of birds are impacted by life on hosts
Creators: Cook, Andrew
Publisher: name: University of Alberta Library
Year: 2022

<p>Feather mites (Acariformes: Sarcoptiformes: Analgoidea and Pterolichoidea) are a group of small arachnids that inhabit the feathers of nearly every species of bird. Life as ectosymbionts has shaped the morphology and ecology of feather mites, whose entire life cycles (from egg to adult) are reliant on a host. Within this host-symbiont system, I explored several small- and large-scale patterns across many host species and their ectosymbiotic mites.<br>
Vane-dwelling feather mites are adapted to life on the flight feathers of their hosts. Most ecological work on feather mites treats all individuals as identical, which obscures potential variation in host use based on life stage or sex. Using birds from the order Passeriformes caught between dawn and noon, I demonstrate that adult and juvenile vane-mites in the genera Amerodectes and Proctophyllodes (both Proctophyllodidae) use the feather’s surface differently. Juvenile mites are almost entirely restricted to areas immediately adjacent to the feather shaft and seldom occur on the flat surface of the vane, while adults are found on both regions equally. Female mites had fuller guts than males, suggesting they are feeding later into the morning than males, which may be focusing their energies on searching for mates. To fully understand feather mite use of their habitat, it is clear that future research should differentiate sexes and age classes.<br>
Also working within the vane-mites, I show that these mites are following predictions of Harrison’s Rule. Harrison’s Rule is the observation that larger-bodied hosts tend to have larger-bodied symbionts. Although demonstrated across a wide variety of host-symbiont pairs, Harrison’s Rule has yet to be demonstrated in a mite-bird system. In lice living on flight feathers, this relationship is mediated by inter-barb distance (the space between the barbs of the wing). Using vane-dwelling mites from the genus Proctophyllodes and hosts from the order Passeriformes, I demonstrate that these mites do indeed follow Harrison’s Rule and that the host-symbiont size relationship may be mediated by inter-barb distance. Mite width was consistently the best correlate with host metrics, and is likely the aspect of mite morphology that selection is acting on. I discuss potential alternatives that may co-vary with both mite size and inter-barb distance (e.g. barb height) and recommend additional work to test whether inter-barb distance is the proximate driver.<br>
Macroevolutionary patterns of diversity are largely unknown for feather mites. Working in the host order Charadriiformes, I explore the relationship between host body size and mite diversity. Charadriiform birds have a wide range of sizes, from under 12 grams to nearly 2 kilograms. Predictions from island biogeography (the species-area relationship) are that larger hosts will have more diverse feather mite assemblages. Using data from published host-mite records and two methods of calculating diversity (simple species richness and a metric that takes phyletic distance into account), I found an unexpected pattern of decreasing diversity with increasing host size. This pattern was also true for the number of times particular species of birds had been examined for feather mites: large-bodied species had typically been examined for mites less often than small-bodied ones. Indeed, the number of studies of a particular host species and the number of known mites from that species was tightly correlated. The confounding of body size and sampling effort means that with currently available data, the biogeographical predictions cannot be accurately tested.<br>
I finally explore how island colonization and Harrison’s Rule may be affecting diversity and morphology of vane-dwelling feather mites. The Eastern Bluebird (Sialia sialis (Linnaeus 1758)) colonized Bermuda in the 1600’s, but it is unknown which of their feather mites they brought with them. Using samples collected from both mainland North America and Bermuda, I compared species compositions between the two host populations. Most species of feather mites known from mainland bluebirds were present in the samples from Bermuda. Amerodectes and Trouessartia populations on Bermuda did not show molecular evidence of differentiation from mainland populations. Since colonization, Bermudian Eastern Bluebirds appear to have become smaller than populations on the mainland. Using several metrics of mite size, I found that island mites were following Harrison’s Rule and were also smaller than their mainland counterparts.</p>