DOI: 10.25338/b8b310
title: Consistent individual differences in ecto-parasitism of a long-lived lizard host

description: 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.
description: The following description is from Payne et al. (2020) but
 modified for brevity. We studied sleepy lizards
 (<i>Tiliqua rugosa</i>) within an ~ 1.2
 km<sup>2</sup> 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,
 <i>A. limbatum</i> and <i>B. hydrosauri</i> (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. <i>Tick
 variables</i>             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
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