Effects of mite load on growth and body condition in Sceloporus undulatus

Parasitism is nearly ubiquitous in animals and is frequently associated
with fitness costs in host organisms, including reduced growth, foraging,
and reproduction. In many species, males tend to be more heavily
parasitized than females and thus may bear greater costs of parasitism.
Sceloporus undulatus is a female-larger, sexually-size dimorphic lizard
species that is heavily parasitized by chigger mites (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 investigated whether there are
growth costs of mite ectoparasitism, at a time coinciding with sex
differences in growth rate and the onset of SSD. If there are sex-biased
growth costs of parasitism, then this could suggest a contribution to the
development of SSD in 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 S. undulatus are highly
tolerant of chigger mites and that any costs imposed by mites may 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, therefore, does not contribute to the development of SSD.