Individual hosts vary substantially in their parasite loads. However, whether individual hosts have consistently different loads remains uncertain. If so, hosts that have consistently high parasite loads may serve as key reservoirs or super-spreaders. Thus, identifying whether individuals persistently differ in their parasitism and the factors that explain these patterns constitute important issues for disease ecology and management. To investigate these topics, we examined nine years of tick counts in a wild population of sleepy lizards, Tiliqua rugosa. Lizards were individually marked, and throughout their activity season, often across several years, we repeatedly assessed lizards’ ticks (to stage – larva, nymph, adult male, and adult female – and species, either Bothriocroton hydrosauri or Amblyomma limbatum). Using these repeated individual measures, we determined whether tick counts were repeatable. Then, we tested predictors of average tick counts, particularly lizard mass, sex, behavioural type (aggression and boldness), and the distance between lizards’ home range centre and a road transecting the study site (an area of greater food and lizard activity). We found that lizards exhibited consistent individual differences in tick loads both within and across years. Within-lizard yearly average counts of larvae and nymphs were positively correlated. Lizards closer to the road tended to have more larvae and nymphs of both species and more adult B. hydrosauri. Sex did not affect tick counts. Mass differentially affected adult female A. limbatum and adult male B. hydrosauri tick counts. Intriguingly, lizards with above average aggression but below average boldness, or vice versa, tended to have higher average adult female B. hydrosauri tick counts. Ultimately, our results demonstrate that lizards differed consistently in their tick counts, indicating that lizard parasitism may constitute a phenotypic trait of the individual, with implications for both host-parasite dynamics and broader host ecology.

The following description is from Payne et al. (2020) but modified for brevity.

We studied sleepy lizards (Tiliqua rugosa) within an ~ 1.2 km² field site near Bundey Bore Station (33.888240° S, 139.310718° E). At our field site, sleepy lizards are primarily active during the austral spring, September to December (Bull 1987). The field site has a semi-arid Mediterranean climate and is dominated by chenopod shrubs. An east-west dirt road bisects the study area.

Sleepy lizards in the site harbour two major, lizard-specific tick species, A. limbatum and B. hydrosauri (Bull 1991). Sleepy lizards are the principal hosts for these tick species (Bull et al. 1981), which both have a three-host life cycle (Smyth 1973, Andrews and Bull 1980, Chilton and Bull 1994).

We collected data on lizards’ tick infestation as part of a long-term monitoring project of the sleepy lizards in our field site during their activity season, representing the years 2008 through 2017 (except 2012, when monitoring did not occur). During each year, lizards were relocated approximately every two weeks via radio telemetry, at which time they were weighed, and their ticks were counted, which included identification of species and stage (larva, nymph, adult female, adult male). In 2010 and 2014 through 2017, approximately three times per lizard per season, we scored a lizard’s response to a threat from an observer (observer threat, OT) and an offered banana, a preferred food item, in a perceived situation of risk (banana boldness, BB), such that higher ranks indicated greater aggression and boldness (described previously in Godfrey et al. 2012, Spiegel et al. 2015). Scores for aggression and boldness were repeatable (approximately 0.4 and 0.3, respectively, Payne et al. in prep). For each year for each lizard, we therefore calculated lizards’ within-season average score for each of OT and BB as a measure of BT. These averaged BT scores were used in analyses and are included in the provided datasets.

Tick variables

            In Payne et al. (2020), we consider five tick variables. Following previous authors (Leu et al. 2010, Wohlfiel et al. 2013), we combined larvae and nymphs as a measure of overall sub-adult tick infestation. We considered adults separately by species and sex as these categories can be reliably distinguished visually (whereas, the species and sex of larvae and nymphs cannot be reliably determined in the field).

For each of these five variables, we assessed repeatability (sensu Nakagawa and Schielzeth 2010). Estimates of repeatability for tick counts are less interesting if they simply reflect the ongoing presence of the same individual ticks. Because larvae, nymphs and adult female ticks often stay on a lizard for two to four weeks (Sharrad 1979, Chilton and Bull 1991), we used only tick counts for these groups that were taken 28 or more days apart (i.e., we used a lizard’s first observation, then the next observation 28 days or more after the first, etc.). We indicate these restricted datasets in text with the word “subset.” In some years female ticks were removed after each tick count. We combined the data from these removal years with the 28-day subset data from the non-removal years, denoting these combined datasets “subset-removal.” Since, contrary to other stages, adult male ticks can stay on their lizard hosts throughout the entire field season (Andrews and Bull 1980), we averaged adult male tick counts within each year for each lizard and then standardized those new values within a year (i.e., scaled to a mean of zero and standard deviation of one). We standardized within year because in some years male ticks were removed, which inflated average counts in non-removal compared to removal years, due to double-counting in the non-removal years.

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