Host sex and age typically explain variation in parasitism of rock ptarmigan: implications for identifying determinants of exposure and susceptibility

Abstract

Measures of parasitism often differ between hosts. This variation is thought due in part to age or sex differences in exposure to parasites and/or susceptibility to parasitism. We assessed how often age or sex biases in parasitism were found using a large, multiyear (2006-2017) dataset of 12 parasite species of Icelandic rock ptarmigan (Lagopus muta). We found host traits (i.e. age and/or sex) accounted for significant variation in abundance of 11 of the 12 parasite species. We often found increased abundance among juvenile hosts, although significant adult biases were observed for three parasite species. Additionally, higher levels of parasitism by many species were observed for female hosts, contrary to frequent male biases in parasitism reported for other vertebrates. Abundance of six parasite species was best explained by interactions between host age and sex; some degree of decrease in abundance with host age was present for both male and female hosts for four of those parasite species. We consider various host and parasite traits that could account for observed singular and repeated patterns of age and/or sex biases in parasitism (e.g. age-and sex-related grouping behaviours, age-specific mortality in relation to parasitism, acquisition of greater immunity with age). This work provides a foundation for future studies investigating age-related differences in acquired immunity and age-specific parasite-mediated mortality for males and females, as well as studies on interactions between co-infecting parasite species.

Weighing the predictors: host traits and coinfecting species both explain variation in parasitism of Rock Ptarmigan

“…Summarizing the competing best models (Table 1) showed that host age explained variation in abundance of T. tenuis with adults having more worms than juveniles; those patterns agree with Nielsen et al. 2020. Thus, inclusion of two potential predictor parasites did not change host age– T. tenuis relations.…”

“…Hereafter, we are referring specifically to previous results of Nielsen et al. (2020) whenever current results are compared to previously established patterns with respect to host traits. These same effects of host traits were seen in three of four best models including three potential predictor parasites; age, but not the age:sex interaction, was also significant in the fourth best model (Table 2).…”

“…Previously, the louse fly, O. chloropus , showed a significant juvenile bias and age:sex interaction (juvenile bias specific to females) when controlling only for year of sampling as a random factor (Nielsen et al. 2020). Hereafter, we are referring specifically to previous results of Nielsen et al.…”

“…Briefly, the chosen parasite species were well represented parasitic fauna that could be identified to species (i.e., they were not exceedingly rare parasites, those for which ptarmigan could be considered accidental hosts, or parasites that were insufficiently described taxonomically—for those parasites, see Nielsen et al. 2020). One recent study has shown that host traits (age and/or sex and/or their interaction) accounted for significant variation in parasitism by 11 of the 12 species, while controlling for the random effect of year of study (Nielsen et al.…”

“…One recent study has shown that host traits (age and/or sex and/or their interaction) accounted for significant variation in parasitism by 11 of the 12 species, while controlling for the random effect of year of study (Nielsen et al. 2020).…”