Differential Parasitism Between Two Suitable Cowbird Hosts

Host choice by the brood parasitic Brown-headed Cowbird (Molothrus ater) is an evolved response to host suitability, resulting in patterns of differential parasitism rates among species within a community. In the ponderosa pine (Pinus ponderosa) forests of the Colorado Front Range, we recorded that Western Wood-pewee (Contopus sordidulus) is infrequently parasitized (1%, n = 259 nests) by the Brown-headed Cowbird, whereas the Plumbeous Vireo (Vireo plumbeus) is heavily parasitized (51%, n = 292). To account for differences in parasitism rates on these species we experimentally parasitized pewee nests with cowbird eggs, and we compared host aggression towards cowbird models, host nest attentiveness, nest placement, and egg-laying dates in these species. Pewees accepted cowbirds eggs and reacted more aggressively towards the cowbird model than the control model, were more attentive at their nest sites than vireos, and placed their nests higher and closer to the trunk than vireos. Egg-laying dates for vireos and cowbirds overlapped more than the egg-laying dates for pewees and cowbirds. We suggest that temporal asynchrony in host availability, coupled with differences in nest placement and behavior at the nest, help to account for the observed differences in parasitism rates between these two species.

Dietary differences between phenotypes with symmetrical and asymmetrical pelvis in the stickleback Gasterosteus aculeatus

Differential parasitism among phenotypes within populations can result from intrinsic factors such as immunocompetence or extrinsic factors such as ecological overlap with pathogens. In a recent study of a population of sticklebacks (Gasterosteus aculeatus) from Boulton Lake, British Columbia, relative cestode infections were related to pelvic-girdle symmetry. Adult sticklebacks with an asymmetrical pelvis had a higher incidence of infections than fish with a symmetrical pelvis, yet the pattern was reversed among yearlings. In the current study we test whether this unexpected result for yearlings, which is inconsistent with general theory coupling asymmetry with reduced immuno competence, might be due to ecological factors rather than to differences in immunocompetence. We analyze the diet of 9089 uninfected sticklebacks collected during a 15-year study and show that male and female yearling sticklebacks with an asymmetrical pelvis are more benthic than symmetrical phenotypes. This could reduce the probability of exposure to pelagic copepods, the primary hosts of the cestode Schistocephalus solidus, and thereby lead to lower infection rates relative to pelagic fish. This finding would account for the unusual reversal in relative parasitism in this population and raises the possibility of finding further diet-induced causes of differential parasitism in studies of asymmetry.

Differences in attachment of water mites on water boatmen: further evidence of differential parasitism and possible exclusion of a host from part of its potential range

This paper is the latest study on the distribution of two sympatric species of water boatmen in saline lakes in British Columbia. It is a further examination of a study by Smith, who found that parasitic water mites have higher prevalence and abundance on Cenocorixa expleta than on C. bifida. He hypothesized that this is one of the main factors responsible for the small populations of C. expleta in lakes below 13 000 µS · cm-1. We performed laboratory experiments with the mite Eylais euryhalina on two different wing morphs of the two host species. Over 8 days, we found higher prevalence and abundance of mites on the predominant, flight-incapable morph of C. expleta than on the predominant, flight-capable morph of C. bifida. Eylais euryhalina attached over most of the abdominal terga of C. expleta, but generally attached only to the permanently non-sclerotized thoracico-abdominal membrane of C. bifida. Infecting both wing morphs of both species showed that locations of mite attachment differed according to the wing morph and the associated differences in sclerotization, whereas the effect of species was equivocal. We hypothesize that a factor in C. expleta's exclusion is the greater potential mite-attachment area of its predominant, unsclerotized morph.