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 (Sánchez 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 (Sceloporus undulatus ).
Eastern fence lizards (Sceloporus undulatus ) in the New Jersey
pinelands are heavily parasitized by larval 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 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 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.