Meeting Abstract
Eastern fence lizards (Sceloporus undulatus) are heavily parasitized by chigger mites (Eutrombicula alfreddugesi), with mite counts >500 on some individuals. Among growing yearlings, mite counts vary by an order of magnitude, and the week-to-week rank ordering of mite counts is highly concordant. In yearlings, males are more heavily parasitized than females, even as mite populations change across the summer activity season. Thus, any potential costs of ectoparasitism may vary consistently among individuals and between males and females. Furthermore, exposure of lizards to mites may be increasing with climate change, potentially leading to higher ecological costs. We investigated growth costs of chigger mites in field-active yearling S. undulatus in three independent studies conducted in 2014-15, 2016, and 2019, and we attempted to manipulate ectoparasitism experimentally by administering Ivermectin in 2019. We found no evidence of a negative correlation between mite count and growth rate in either sex. Furthermore, Ivermectin had no effect on mite counts or growth rates. Yearling males had consistently lower growth rates than females, consistent with findings that testosterone inhibits growth in S. undulatus, but the inhibitory effect of testosterone on growth in males does not appear to be mediated through increased mite parasitism. Average mite intensities increased progressively across years, suggesting that exposure to mites may be increasing as the climate warms. Given these conditions, selection may favor the evolution of tolerance as opposed to resistance to mites. If so, further increases in mite populations due to climate change may have little effect on S. undulatus. Supported by NSF 1754934 and Hatch Multistate project no. NJ17240.
