Effects of extrinsic and intrinsic factors on individual tick load in an introduced Siberian chipmunk (Tamias sibiricus) population

Background/Question/Methods

Temporal and intra-population variations of the parasite load may reflect variation in physiological and behavioral processes and influence demography and population dynamics. In introduced species, which can acquire new parasites from their new environment (spillback process), thorough knowledge of the processes underlying variation in infestation can play a key role in our understanding of the invasion success. Here we question the relative roles of extrinsic and individual variation in overall variation of the parasite load. Based on a longitudinal analysis of Capture-Mark-Recapture data, we investigated the effects of environmental (habitat, tick availability in the habitat) and intrinsic (sex, age, birth season) factors on the individual native tick burden (mainly Ixodes ricinus) in a population of the Siberian chipmunk (Tamias sibiricus) introduced in the Forest of Sénart (France). The analysis was based on 3142 data on tick load in 867 individuals, monthly recorded between March and October during nine consecutive years. Our large dataset enabled us to formulate precise predictions on the role of the various potential intrinsic factors of variation in the tick load and to contrast these predictions with a null model in which the individual tick load depends only on the availability of ticks in the environment.

Results/Conclusions

As expected from the parasites’ life cycle, the average tick load in chipmunks shows strong variation among seasons and, to a lesser extent, years. However, intra-seasonal differences in tick availability in the habitat explains only modest proportion of variance (<10%) in these changes, which are critically linked to intrinsic variations occurring at both the inter- and intra-individual levels. Based on linear mixed modeling, our analysis uncovers in particular, an interaction between sex and age, in which no sex difference occurs in juveniles, while male adult chipmunks harbor significantly more ticks than females. In contrast, although spring-born juveniles are more heavily infested than summer-born ones, this difference disappears at adult stage. Overall our results illustrate how the relationship between parasite availability and parasite burden is mediated by individual host characteristics, leading to substantial variation in the shape of intra-annual dynamics of infection among sexes and ages.