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\begin{document}

\title{Chigger mite \emph{(Eutrombicula alfreddugesi)} ectoparasitism does not
contribute to sex differences in growth rate in eastern fence lizards
\emph{(Sceloporus undulatus)}}



\author[1]{Nicholas Pollock}%
\author[2]{Hailey Conrad}%
\author[3]{Henry John-Alder}%
\affil[1]{University of Texas Arlington Department of Biology}%
\affil[2]{Virginia Tech}%
\affil[3]{Rutgers University}%


\vspace{-1em}



  
  \date{May 24, 2024}


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\selectlanguage{english}
\begin{abstract}
\emph{(Sceloporus undulatus)} is a female-larger, sexually-size
dimorphic lizard species that is heavily parasitized by chigger mites
\emph{(Eutrombicula alfreddugesi)}. In particular, the intensity of mite
parasitism is higher in male than in female juveniles during the period
of time when sex differences in growth rate lead to the development of
sexual size dimorphism (SSD). Sex-biased differences in fitness costs of
parasitism have been documented in other species. We tested whether a
sex-biased growth cost of parasitism contributes to the development of
SSD in \emph{S. undulatus}. We measured growth and mite loads in two
cohorts of unmanipulated, field-active yearlings by conducting
descriptive mark-recapture studies during the activity seasons of 2016
and 2019. Yearling males had consistently higher mid-summer mite loads
and consistently lower growth rates than females. However, we found that
growth rate and body condition were independent of mite load in both
sexes. Furthermore, growth rates and mite loads were higher in 2019 than
in 2016. Our findings suggest that juveniles of \emph{S. undulatus} are
highly tolerant of chigger mites and that any costs imposed by mites
must be at the expense of functions other than growth. We conclude that
sex-biased mite ectoparasitism does not contribute to sex differences in
growth rate and the development of SSD.%
\end{abstract}%



\sloppy


Chigger mite (\emph{Eutrombicula alfreddugesi} ) ectoparasitism does not
contribute to sex differences in growth rate in eastern fence lizards
(\emph{Sceloporus undulatus} )

Hailey Conrad\textsuperscript{1,2\textbf{*}} , Nicholas B.
Pollock\textsuperscript{3,4,5\textbf{*}} , Henry
John-Alder\textsuperscript{1,6}

\textsuperscript{1} Department of Ecology, Evolution, and Natural
Resources, Rutgers University, New Brunswick, New Jersey 08901, USA

\textsuperscript{2} Present Address: Department of Biological Sciences,
Virginia Polytechnic Institute and State University, Blacksburg,
Virginia 24061, USA

\textsuperscript{3} Graduate Program in Ecology and Evolution, Rutgers
University, New Brunswick, New Jersey 08901, USA

\textsuperscript{4} Present Address: Department of Biology, University
of Texas Arlington, Arlington, Texas 76019, USA

\textsuperscript{5} Corresponding Author E-mail:
nicholas.pollock@uta.edu

\textsuperscript{6} Rutgers Pinelands Field Station, Rutgers University,
New Lisbon, New Jersey 08064, USA

\textsuperscript{\textbf{*}} \textbf{Co-First Authors, arranged
alphabetically}

\emph{NBP, HC, and HJ-A conceived the study and designed the
methodology; NBP and HJ-A collected the 2016 data, and HC and HJ-A
collected the 2019 data; HJ-A led the data analysis; HC and NBP led the
writing of the manuscript. All authors contributed critically to the
drafts and gave final approval for publication.}

\emph{Abstract}

\emph{Sceloporus undulatus} is a female-larger, sexually-size dimorphic
lizard species that is heavily parasitized by chigger mites
(\emph{Eutrombicula alfreddugesi} ). In particular, the intensity of
mite parasitism is higher in male than in female juveniles during the
period of time when sex differences in growth rate lead to the
development of sexual size dimorphism (SSD). Sex-biased differences in
fitness costs of parasitism have been documented in other species. We
tested whether a sex-biased growth cost of parasitism contributes to the
development of SSD in \emph{S. undulatus} . We measured growth and mite
loads in two cohorts of unmanipulated, field-active yearlings by
conducting descriptive mark-recapture studies during the activity
seasons of 2016 and 2019. Yearling males had consistently higher
mid-summer mite loads and consistently lower growth rates than females.
However, we found that growth rate and body condition were independent
of mite load in both sexes. Furthermore, growth rates \emph{and} mite
loads were higher in 2019 than in 2016. Our findings suggest that
juveniles of \emph{S. undulatus} are highly tolerant of chigger mites
and that any costs imposed by mites must be at the expense of functions
other than growth. We conclude that sex-biased mite ectoparasitism does
not contribute to sex differences in growth rate and the development of
SSD.

\textbf{Keywords:} body size, mite load, parasites, tolerance, sexual
size dimorphism, SSD

\emph{Introduction}

Parasitism is ubiquitous in animals and is frequently associated with
fitness costs in the host organism (Dobson et al., 2008; Hicks et al.,
2018a; O'Connor \& Bernhardt, 2018). Sub-lethal costs of parasitism can
reduce host fitness by reducing energy expenditure on growth, foraging,
or reproduction (Booth et al., 1993; Lehmann, 1993; Martin et al., 2003;
Reed et al., 2008; Gooderham \& Shulte-Hostedde, 2011; Lin et al., 2014;
Hicks et al., 2018a, 2018b). Furthermore, in agricultural applications,
broad-spectrum anti-parasitic agents such as Ivermectin are used to
improve growth, weight gain, onset of reproductive maturity, and
pregnancy rate in cattle and other animals (Larson et al., 1995; Mejia
et al., 1999; Loyacano et al., 2002; Powell et al., 2009). However,
while parasites frequently inflict fitness costs, there are cases where
no apparent cost of parasitism can be detected (Brown et al., 2006;
Mayer et al., 2015). Indeed, an apparent lack of costs of parasitism
tends to be underreported (S\selectlanguage{ngerman}ánchez et al., 2018; Hasik \& Siepielski,
2022) and may thus be more common than is appreciated.

In many species, males are more heavily parasitized than females and
thus may bear greater costs of parasitism (Zuk \& McKean, 1996; Aubret
et al., 2005; Curtis \& Baird, 2008; Krasnov et al., 2012; Dudek et al.,
2016). When parasites impose a growth cost, sex differences in the
intensity of parasitism may contribute to the development of sexual size
dimorphism (SSD) (Potti \& Merino, 1996; Pontier et al., 1998). In the
present study, we investigated whether ectoparasitic Trombiculid mites
inflict a sex-biased growth cost and contribute to the development of
SSD in eastern fence lizards (\emph{Sceloporus undulatus} ).

Eastern fence lizards (\emph{Sceloporus undulatus} ) in the New Jersey
pinelands are heavily parasitized by larval \emph{Eutrombicula
alfreddugesi} (Pollock \& John-Alder, 2020), a widespread species
commonly known as chigger mites, or simply chiggers. As larvae, chiggers
are generalist ectoparasites, which feed on their host's digested,
liquefied skin and lymph until they fully engorge, drop off, and molt
into the free-living nymphal stage (Sasa, 1961). In the New Jersey
pinelands, is not uncommon for an individual lizard to host hundreds of
chiggers at one time (Pollock \& John-Alder, 2020). Given that lizards
mount a local inflammatory response to mites (Goldberg \& Bursey, 1991;
Goldberg \& Holshuh, 1992), chiggers may plausibly impose costs on their
lizard hosts, especially when present in great numbers.

During the peak summer months of the \emph{S. undulatus} activity
season, chigger mites are abundant in the environment, all lizards are
parasitized by mites (100\% prevalence of ectoparasitism), and mite
loads on lizards reach their highest levels of the year (Pollock \&
John-Alder, 2020). Furthermore, mite loads vary by an order of magnitude
among individual lizards, and the rank ordering of the number of mites
on individual lizards is significantly concordant from week to week,
indicating consistent inter-individual differences in mite
ectoparasitism (Pollock \& John-Alder, 2020). By early July, when
juvenile females grow faster than males and sexual size dimorphism (SSD)
is developing to its full extent, mite loads are higher on yearling
males than on any other age-sex class. This is a system in which fitness
costs of ectoparasitism -- and potentially sex-biased costs of
ectoparasitism -- should be readily apparent. In the present study, we
investigated the relationship between mite ectoparasitism and growth
rate in yearling juveniles of \emph{S.undulatus} . Our broader goal was
to understand whether a sex-biased growth cost of ectoparasitism
contributes to the development of SSD in a female-larger lizard species.

\emph{Materials and Methods}

We investigated relationships between chigger mite loads and lizard
growth rates in yearling juveniles of \emph{Sceloporus undulatus} in the
New Jersey Pinelands National Reserve during June and July of 2016 and
2019. We conducted our studies in Colliers Mills Wildlife Management
Area (Burlington County, New Jersey, USA; 40.1° N, 74.4° W), which we
chose because of its relative abundance of lizards. Our study site was a
0.045 km\textsuperscript{2} patch of oak-pine forest bounded by open,
mowed fields and man-made sand paths, with a variably open canopy,
sparse understory due to prescribed burns, and abundant tree debris on
the forest floor.

Eggs in this population are laid from late May into June and begin to
hatch in late July. Depending on weather, the neo-natal activity season
for hatchlings can extend into mid-November. Lizards emerge from winter
inactivity as yearlings in March of the next year and grow rapidly to
reach the size of reproductive maturity by the end of the first full
activity season. Following a second winter of inactivity, lizards
re-emerge as reproductively mature adults at approximately 20 months of
age, in the spring of their second full activity season (Haenel \&
John-Alder, 2002). Males and females are initially indistinguishable in
body size, but sexual divergence in body size occurs most rapidly in
yearlings during June/July of the first full activity season, coinciding
with rising levels of plasma testosterone in males and faster growth
rates in females (Skelly \& John-Alder 2002; Cox et al. 2005). With the
attainment of the size of reproductive maturity at the end of the first
full (yearling) activity season, females are approximately 10\% larger
than males (Haenel \& John-Alder, 2002). In both years of our study, we
applied mark-recapture techniques to sample the same lizards repeatedly
for measurements of body size and mite loads. In 2016, we found 55
yearling females and 70 yearling males. In 2019, we found 42 yearling
females and 41 yearling males. Of these, we made measurements and
counted mites on 39 females and 55 males in 2016, and 28 females and 20
males in 2019. All measurements each year were collected by the same
individual (NBP in 2016, HC in 2019) to prevent possible measurement
biases in SVL, body mass, or mite counts. We captured lizards with a
slip-noose or by hand. Upon capture, we measured snout-vent length (SVL,
mm) using a ruler and body mass to 0.1 g using a
Pesola\selectlanguage{english}\textsuperscript{?} spring scale. We determined sex by the
presence (male) or absence (female) of enlarged post-cloacal scales, and
we unambiguously distinguished yearlings from older age-classes by their
smaller size during the month of June (Haenel \& John-Alder, 2002). Each
lizard was given a unique toe clip for permanent identification and a
unique dorsal paint mark just anterior to the base of the tail (a region
not preferred by mites) for visual identification.

To determine mite loads, we used a 10X hand lens in the field to count
the number of mites on each lizard at the time of capture. In 2016, we
counted mites on recaptured lizards at weekly intervals from June 9
through July 14, although not all lizards were recaptured every week. In
2019, we counted mites on recaptured lizards on a less structured
schedule, with averages of 1.6 +- 0.1 counts per lizard in June (median:
2; range: 1 to 3) and 1.9 +- 0.1 counts per lizard in July (median: 2;
range 1 to 4). Average intervals between counts were 10 +- 0.7 days in
June 2016 (median: 12 days) and 13 +- 1.0 days in July 2016 (median: 13
days).

In 2019, we assessed the accuracy of mite counts by comparing the number
of mites counted by eye in the field to the number of mites counted by
using ImageJ (National Institutes of Health, USA) to analyze photographs
of the same lizards taken at the time of mite counts in the field with a
Nikon D7200 digital camera and a Sigma 105mm F2.8 EX DG OS HSM Macro
lens. Mite loads determined by these two methods ranged from 7 to 424
mites/lizard and were highly correlated (Supporting Figure 1; n = 19;
slope: 1.06 +- 0.05, 95\% CI: 0.94--1.17, R\textsuperscript{2}: 0.997).

For a direct comparison of 2016 versus 2019, we selected comparable
three-week periods: 22--24 June through 13-14 July 2016 versus 24--27
June through 15--17 July 2019. These three-week periods included
measurements of SVL and mite counts taken in weeks 3, 4, 5, and 6 of the
2016 study (Pollock \& John-Alder, 2020) and in two intensive recapture
efforts of 2019. Intervals between measurements of SVL, thus time
intervals for calculating growth rate, invariably differed due to uneven
recapture success. In 2016, we demonstrated that growth rate calculated
as ((SVL\textsubscript{2} -SVL\textsubscript{1}) / (time in days)) was
equal to growth rate computed as the slope of SVL plotted as a function
of date of recapture for each lizard (Supporting Figure 2; slope: 1.04 +-
0.07, 95\% CI: 0.91--1.17, F\textsubscript{1,54} = 253.28,
\emph{p}\textless{} 0.001). In 2019, we demonstrated that growth rate
calculated as ((SVL\textsubscript{2} -SVL\textsubscript{1}) / (time in
days)) was independent of the number of days between measurements of SVL
(Supporting Figure 3; slope: -0.001 +- 0.003, F\textsubscript{1,41} =
0.12, \emph{p} = 0.736). To analyze relationships between growth rate
and mite load separately for the months of June and July, we averaged
each lizard's mite counts taken within the focal periods for each month.
For example, June's mite count in 2016 was the average of mites counted
in weeks 3 and 4 of that study, or the single mite count for lizards
captured in only one of those weeks.

\emph{Data Analysis}

All statistical analyses were done using SAS version 9.4 (SAS Institute
Inc., Cary, NC, USA). Data were log-transformed to linearize
relationships between dependent variables and SVL. We compared overall
growth rates between years and between sexes by using proc GLM to
regress growth rate versus the earliest SVL measurement of the year for
each lizard, with year and sex as covariates. We calculated body
condition separately in June and in July as the residual of log mass
regressed on log SVL using measurements taken in the first and last
weeks of the interval defined above. To analyze effects of mite load on
growth rate, we removed the effects of body size, sex, and year on
growth rate and mite load by computing residuals from separate
regression analyses of growth rate versus SVL and mite load versus SVL,
with sex and year as main effects. We then regressed residual variation
in growth rate or body condition versus residual variation in mite load.
We analyzed effects of residual mite loads on residual overall growth
rates separately for June and July because of the substantial difference
in environmental mite exposure between the two months and because mite
loads become higher on yearling males than on females in the first two
weeks of July (Pollock and John-Alder, 2020).

\emph{Results}

Mite loads, mite load ranges, and growth rates of male and female
yearling lizards are summarized in Table 1. Overall, growth rate was
negatively correlated with SVL, higher in females than in males, and
higher in 2019 than in 2016 (Figure 1; overall: F\textsubscript{3,82} =
25.97, \emph{p} \textless{} 0.001; effect of sex: (F\textsubscript{3,1}=
30.91, \emph{p} \textless{} 0.001; effect of year: (F\textsubscript{3,1}
= 16.52, \emph{p} = 0.001). Sex differences in growth rate were similar
in 2016 and 2019, resulting in the development of significant sexual
size dimorphism (SSD) early in July of both years. In 2019, for example,
the difference between female and male SVL, and thus the development of
SSD, had become statistically significant by 14 July (Figure 2;
F\textsubscript{1,25} = 5.24, \emph{p} = 0.031).

Mite loads tended to increase with body size (SVL) in June and in July,
although in both months these size effects failed to achieve
conventional statistical significance (Figure 3; June:
F\textsubscript{3,1} = 3.17, p = 0.077; July: F\textsubscript{3,1} =
3.78, \emph{p} = 0.055). Controlling for SVL, mite loads were higher on
males than on females in July (F\textsubscript{3,1} = 6.63, \emph{p} =
0.011), but not in June (F\textsubscript{3,1} = 0.09, \emph{p} = 0.764),
and were higher in 2019 than in 2016 in June (F\textsubscript{3,1} =
8.32, \emph{p} = 0.004) but not in July (F\textsubscript{3,1} =
0.83,\emph{p} = 0.364).

Growth rate was not correlated with mite load in June or in July (Figure
4). Residual values used for these analyses remove variation due to
differences in SVL, sex, and year, thus allowing the analysis to focus
solely on a potential correlation between growth rate and mite load. In
both June and July, correlations between residual variation in growth
rate and residual variation in mite load were convincingly
non-significant (June: F\textsubscript{1,84} = 0.08, \emph{p} = 0.777,
R\textsuperscript{2} = 0.001; July: F\textsubscript{1,70} =
0.12,\emph{p} = 0.727; R\textsuperscript{2} = - 0.012).

Similarly, mite load had little effect on body condition. In June, the
negative effect of mite load on body condition attained marginal
statistical significance (Figure 5; F\textsubscript{1,140} =
4.16,\emph{p} = 0.043, \emph{R\textsuperscript{2}} = 0.022), while in
July this relationship was not statistically significant
(F\textsubscript{1,98} = 1.28, \emph{p} =
0.261,\emph{R\textsuperscript{2}} = 0.003).

\emph{Discussion}

In the host-parasite system we studied, chigger mites (\emph{E.
alfreddugesi} ) are highly abundant in the environment, and all age- and
sex-classes (adults and juveniles; males and females) of \emph{S.
undulatus} are heavily parasitized (Pollock \& John-Alder, 2020).
However, we found no evidence that mites impose a growth cost, despite
very high mite loads and consistent week-to-week rank ordering of mites
on individual lizards. Indeed, within cohorts of yearlings, growth rate
is unambiguously not correlated with mite load in either sex.
Furthermore, across cohorts, both growth rate and mite loads were higher
in 2019 than in 2016, which is opposite the prediction if mites imposed
a growth cost. At any given mite load, females grow more quickly than
males, which could suggest that mites impose a sex-biased growth cost.
However, even in captivity in the complete absence of mites, SSD
develops because females grow faster to become larger than males (Duncan
et al., 2020). Thus, we find no evidence that growth rate in yearling
juveniles of \emph{S. undulatus} is affected by chigger mite
ectoparasitism. It follows that the temporal correlation between the
development of SSD and the seasonal attainment of sex-biased mite loads
in July does not reflect a sex-biased growth cost of chiggers.

Our findings match those of several other studies that found no
significant effects of mite parasitism in lizards. In \emph{S. virgatus}
, Abell (2000) reported no relationship between mite load and body
condition, mating success, or survivorship to the following year, and
Smith (1996) found no significant relationship between mite load and
growth rate. In the anole species \emph{Norops polylepis} , Schlaepfer
(2006) found no sex differences in mite load and no growth costs
associated with mite ectoparasitism. Mite parasitism was not associated
with body condition in \emph{Gallotia atlantica} lizards (Garcia-Ramirez
et al., 2005). Patterson and Blouin-Demers (2020) found no relationships
between mite load and growth rates or survival in six different
populations of \emph{Urosaurus ornatus} . One caveat worth mentioning is
that mite loads in the studies cited here were substantially lower than
those on \emph{S. undulatus} in the present study (\emph{S. undulatus} :
0--435mites; \emph{U. ornatus} : 0--120; \emph{N.} \emph{polylepis} :
0--37 mites, \emph{G. atlantica} : 1--28 mites).

In contrast to our findings and the studies cited above, evidence of
costs of mite parasitism has been reported in several species, including
two close congeners of \emph{S. undulatus} . An experimental study
on\emph{S. virgatus} found twice as many mites on males as on females
and a strongly negative correlation between mite loads and growth rate
(Cox \& John-Alder, 2007). This result suggests that growth costs of
mite ectoparasitism are greater in males than females (Cox \&
John-Alder, 2007), in contrast to what had previously been reported
(Smith, 1996; Abell, 2000). In \emph{S. woodii} , Orton et al. (2020)
reported that both color quality and running endurance are negatively
associated with mite load. In collared lizards (\emph{Crotaphytus
collaris} ), Curtis and Baird (2008) found evidence of a growth cost
associated with heavy parasitism during the peak growing season. That
study reported high mite loads, an average of 178.3 +- 65.2 mites on
yearling male lizards, which are comparable to what we found on
\emph{S.} \emph{undulatus} in the present study.

Overall, the inconsistent narrative regarding potential growth and
fitness costs of mites suggests that effects of, or correlates of, mite
parasitism can depend on which life history trait is being investigated
and the timing of the studies, as ecological conditions (i.e.,
temperature, precipitation, food abundance, etc.) can fluctuate across
the years and have different effects on life history traits.
Furthermore, effects of mite parasitism can differ not only among
species but also between populations of the same species as observed
with the studies on \emph{S. virgatus} (Smith, 1996; Abell, 2000; Cox
and John-Alder, 2007). As such, it would be problematic to envision an
overall evolved strategy to mite parasitism at a species level.
Furthermore, future studies need to consider what life history trait is
being examined when investigating potential effects of ectoparasitism.

While not a major consideration in our study, we posit that future
studies on the costs of mite parasitism in lizards should consider
whether the host population in the system would be expected to have
evolved a strategy of resistance or tolerance towards the parasites in
their study design. Parasites and their hosts coevolve in an
evolutionary arms race in which parasites extract resources from their
hosts, while hosts try to prevent or minimize the fitness costs of
parasitism (May \& Anderson, 1983; Restif \& Koella, 2004; Carval \&
Ferriere, 2010). Within this context, hosts evolve strategies of
resistance or tolerance to parasites, or some combination of the two.
The ecological relationship between the populations of \emph{S.
undulatus} and \emph{E. alfreddugesi} in this study (i.e., high
environmental mite abundance and high host mite loads) is typical of a
host-parasite system in which the host would be expected to have evolved
tolerance to parasites (Raberg et al., 2009; Pollock \& John-Alder,
2020). Hosts can evolve tolerance so effectively that the reduction in
fitness caused by parasites may not be measurable (Raberg, 2014).
Studies that do not find a growth cost associated with parasitism may be
examining systems where the lizards have evolved a strategy of tolerance
towards the mites. For example, it has been hypothesized that mite
pockets evolved as a compensatory mechanism for reducing the harm caused
by mite ectoparasitism because they concentrate mites in areas better
equipped to heal and where they do not interfere with other functions,
such as movement and vision (Arnold, 1986; Salvador et al., 1999; de
Carvalho et al., 2006; Reed, 2014). Tolerance can be quantified using
the slope of the relationship between fitness (or its proxy) and
parasite burden (Burgan et al. 2019). Given that we found a slope of
zero in the relationship between growth rate and mite load, our
population of eastern fence lizards is highly tolerant of chiggers by
the criteria we evaluated (Ra\selectlanguage{english}berg et al., 2007; Ra\selectlanguage{english}berg, 2014). Ornate
tree lizards (\emph{Urosaurus ornatus} ) have also been found to be
highly tolerant of mite parasites (Paterson \& Blouin-Demers, 2020).
Host-parasite systems in which hosts appear to be highly tolerant of the
parasite are not well-suited for investigating sex-biased costs of
parasitism because any costs of parasitism are not easily measurable.

If chigger mites in our study population are in fact extracting a
substantial cost from yearling \emph{S. undulatus} , the cost may be at
the expense of traits and functions other than growth. Rapid growth is
expected to be under strong selection to ensure that yearlings grow to
the minimum size of reproduction by the end of their first full activity
season (Adolph \& Porter, 1996; Haenel \& John-Alder, 2002).
Furthermore, female body size is positively correlated with clutch size,
suggesting strong fecundity selection on body size and thus yearling
growth (Angilletta et al., 2001; Haenel \& John-Alder, 2002; Cox et al.,
2003; Brandt \& Navas, 2011; Jim\selectlanguage{ngerman}énez\selectlanguage{english}-Arcos et al., 2017). Thus,
potential explanations for the absence of growth costs could include
that costs of parasitism are 1) traded off against other functions, 2)
too small to be of any consequence, or 3) compensated by increased
dietary consumption. We now consider each of these possibilities.

If mites do impose costs on juveniles of \emph{S. undulatus} , it seems
likely that a potential growth cost might be traded off against other
traits (Adolph \& Porter, 1996). For example, reduced activity is one of
the apparent costs of parasitism in common lizards (\emph{Lacerta
vivipara} ) and Western fence lizards (\emph{Sceloporus occidentalis} )
(Clobert et al., 2000; Megia-Palma et al., 2020). In the present study,
we did not quantify a specific measure of activity in \emph{S.
undulatus} . However, in a 2019 companion study conducted at our study
site and on the same population of \emph{S. undulatus} we found that
home range area is greater in yearling males than in females (1101 +- 327
m\textsuperscript{2} vs 233 +- 62 m\textsuperscript{2}) and is not
correlated with growth rate (F\textsubscript{2,1} = 0.14, \emph{p} =
0.708), even while mite loads are greater in males than in females
(Conrad, 2019; Yawdosyn, 2019). Home range area can be used as a proxy
for energy expenditure on activity to assess the possibility of a
trade-off against activity instead of growth (Christian \& Waldschmidt,
1984; Hews, 1993; Main \& Bull, 2000; John-Alder et al., 2009). Thus,
our evidence suggests that mites do not impose an activity cost of mite
ectoparasitism. However, while home range area can be used as a first
approximation of activity, a study of the close congener \emph{S.
occidentalis} reported that lizards infected with malarial parasites
maintained the same home range area as unparasitized lizards but showed
reduced daily activity (Schall \& Houle, 1992). Thus, time-intensive
focal observations would be required for a definitive determination of
whether mites impose an energetic cost on \emph{S. undulatus} as seen
through reduced activity.

Lacking evidence of costs of mite parasitism, it is possible that mites,
even in high numbers, may not impose a measureable energetic cost,
either directly or indirectly. We estimated the energetic cost of a
single chigger mite to be approximately 0.004 J/day using estimates of
chigger mite body size (body length = 0.4 mm, body mass = 0.9 \selectlanguage{greek}μ\selectlanguage{english}g;
Johnson \& Strong, 2000) and the metabolic scaling exponent of 0.75
(West et al., 2002). Taking the mean mite loads of yearling male lizards
in June 2016 (58 mites), July 2016 (138 mites), June 2019 (93 mites),
and July 2019 (119 mites), the average energetic costs of mite
ectoparasitism for yearling male lizards would have been 0.232 J/day in
June 2016, 0.552 J/day in July 2016, 0.372 J/day in June 2019, and 0.476
J/day in July 2019. For comparison, the energetic cost of growth is
about 130 J/day for yearling males (Cox et al., 2005). Thus, in the
aggregate, mites imposed an energetic cost that was a mere
0.002--0.004\% of the energy cost of growth.

The energetic cost of direct energy extraction by mites is unlikely to
be detectable as a growth cost because of its low value. These
estimations, however, do not account for the potential indirect
energetic costs of mounting an inflammatory immune response or the
accompanying stress to the lizard in response to mite ectoparasitism,
which we did not measure. However, potential energetic costs of immune
or stress responses are far from certain. A meta-analysis by van der
Most et al. (2011) found that selection for growth compromises immune
function, but selection for immune function does not appear to affect
growth. This suggests that the costs of growth are large relative to the
costs of immune function. A study of parasites in side-blotched
lizards,\emph{Uta stansburiana} , found increased immunocompetence
associated with parasitism, but no other measurable costs (Spence et
al., 2017). Despite the lack of measurable costs in \emph{U.
stansburiana} , it can be costly for a host to up-regulate their immune
system in response to parasites, forcing life-history trade-offs over
time (Sheldon \& Verhulst, 1996; Lochmiller \& Deerenberg, 2000; French
et al., 2009). Overall, while there may be some energetic cost to
immunity, those costs are likely to small to be detectable as costs to
growth. However, more research is certainly needed in this area.

Finally, growth and body condition may not have been associated with
mite ectoparasitism in \emph{S. undulatus} because environmental
conditions favorable for ectoparasitism were at the same time favorable
for abundant prey, including other arthropods and possibly even mites
themselves. Thus, even while they were heavily parasitized by mites,
lizards would be enabled to increase their energy consumption and
compensate for costs of a high parasite load. The spring of 2019 had
more precipitation and higher temperatures than the spring of 2016
(http://climate.rutgers.edu/stateclim\_v1/nclimdiv/), and we saw an
increase in both environmental mite abundance and lizard growth in 2019
as compared to 2016. Increased precipitation would have increased soil
moisture and humidity, which are positively correlated with the
abundance of arthropods, including mites (Zippel et al., 1996; Kardol et
al., 2011; Prather et al., 2020). Temperature has also been shown to be
positively correlated with arthropod abundance (Lessard et al., 2011).
Mite activity is increased by a combination of moderately high
temperatures and high humidity (Clopton and Gold, 1993). At the same
time, higher temperatures can create opportunities for increased daily
activity in \emph{S. undulatus} , while also contributing to increased
growth rates and faster maturation (Adolph and Porter, 1996). Greater
arthropod abundance may have increased food availability for \emph{S.
undulatus} as well, facilitating increased growth. A greater
understanding of the ecological interactions between \emph{S. undulatus}
, mites, and other arthropods could show if lizards predate upon the
mites or increase their feeding rates on other arthropods due to
increased mite ectoparasitism.

In summary, there are many possible explanations for why we did not find
a growth cost associated with mite parasitism in either sex of \emph{S.
undulatus} in this study, even when males had higher mite loads than
females. Female-larger SSD still developed in our study population. So,
it is safe to conclude that sex-biased costs of mite parasitism are not
contributing to the development of SSD in our study population
of\emph{S. undulatus} . The lizards in our study population appear to be
highly tolerant of mite ectoparasitism, but definitive experimental
studies are required to rule out various trade-offs and effects of
environmental food abundance. As such, while our study population also
exhibits the common male-biased pattern of ectoparasitism, it also shows
that we should be careful in making conclusions about the implications
of sex differences in parasite intensity and prevalence.

\emph{Acknowledgments}

We would like to thank C. Anderson, R. Antigues, J. Azzolini, M. Bolan,
N. Bryan, J. Kerr, M. Xie, and A. Yawdoszyn for their assistance in
lizard capturing and data recording, without which this study would not
be possible. Funding for data collection was provided by the Graduate
School of Rutgers University, National Science Foundation (1754934), and
Hatch Multi-State Project (NJ17240). Animal capture was approved by the
New Jersey Department of Environmental Protection, Division of Fish and
Wildlife (SC2015090; SC2019007).

\emph{Data Availability Statement}

The data that support the findings of this study will be deposited in
the Dryad Digital Repository upon acceptance.

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\textbf{Table 1.} June and July mite loads, mite load ranges, and growth
rates of male and female yearling \emph{S. undulatus} in 2016 and 2019.
Mite loads are mean +- 1 SE (n). Significant within-sex differences
between years are indicated in bold.\selectlanguage{english}
\begin{longtable}[]{@{}lllllll@{}}
\toprule
\begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\textbf{Male}\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\textbf{Female}\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\textbf{Mite Load}\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\textbf{Mite Load}\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\textbf{Growth Rate}\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\textbf{Mite Load}\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\textbf{Mite Load}\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\textbf{Growth Rate}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\textbf{Year}\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\textbf{June}\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\textbf{July}\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\textbf{June}\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\textbf{July}\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
2016\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
58 \selectlanguage{ngerman}± 4 (55) (range: 1-153)\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
138 \selectlanguage{ngerman}± 9 (36) (range: 45-302)\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.24 \selectlanguage{ngerman}± 0.01 (32)\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
60 \selectlanguage{ngerman}± 5 (41) (range: 3-132)\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
112 \selectlanguage{ngerman}± 9 (36) (range: 10-239)\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.31 \selectlanguage{ngerman}± 0.01 (39)\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
2019\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
93 \selectlanguage{ngerman}± 11 (20) (range: 14-180)\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
119 \selectlanguage{ngerman}± 17 (8) (range: 41-204)\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.31 \selectlanguage{ngerman}± 0.02 (10)\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
92 \selectlanguage{ngerman}± 13 (28) (range: 2-286)\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
123 \selectlanguage{ngerman}± 18 (22) (range: 21-315)\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.41 \selectlanguage{ngerman}± 0.02 (20)\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
p\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\textbf{\textless{} 0.001}\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
0.692\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\textbf{\textless{} 0.001}\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\textbf{\textless{} 0.001}\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
0.204\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\textbf{\textless{} 0.001}\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

\textbf{Figure 1.} Log growth rate as a function of log June SVL for
male and female yearling \emph{S. undulatus} from descriptive
mark-recapture studies in 2016 and 2019. Growth rates were higher in
2019 than 2016, females grew faster than males in both years, and growth
rate decreased with increased SVL.

\textbf{Figure 2.} Snout-vent length of male and female yearling\emph{S.
undulatus} across the weeks during the 2019 descriptive mark-recapture
study. Growth rate differed between males and females, resulting in the
development of significant sexual size dimorphism by 15-July.

\textbf{Figure 3.} Log mite load as a function of log SVL for male and
female yearling \emph{S. undulatus} in June (a) and July (b) from
descriptive mark-recapture studies in 2016 and 2019. Mite loads were
higher in 2019, but only in June. Males had higher mite loads than
females, but only in July.

\textbf{Figure 4.} Residual growth rate (mm/day) as a function of
residual mite load from combined 2016 and 2019 descriptive
mark-recapture studies for June (a) and July (b). Growth rates of males
and females were not significantly affected by mite load in either month
of either year.

\textbf{Figure 5.} Body condition as a function of residual mite load
from combined 2016 and 2019 descriptive mark-recapture study for June
(a) and July (b). In June, lizards with higher mite loads tended to have
slightly lower body condition. In July, however, body condition was not
significantly affected by mite load.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-1/Fig-1}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-2/Fig-2}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-3/Fig-3}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-4/Fig-4}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-5/Fig-5}

\end{center}
\end{figure}

\textbf{Hosted file}

\verb`Supplemental Figures.docx`  available at \url{https://authorea.com/users/579387/articles/622973-chigger-mite-eutrombicula-alfreddugesi-ectoparasitism-does-not-contribute-to-sex-differences-in-growth-rate-in-eastern-fence-lizards-sceloporus-undulatus}

\selectlanguage{english}
\FloatBarrier
\end{document}

