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.