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theses

A growing body of evidence indicates that testosterone (T) plays an important role in regulating patterns of growth in lizards. Testosterone has also been found to facilitate the development of male-typical coloration and a suite of male behaviors that increase reproductive success. However, while T promotes male fitness through these characteristics, it appears to hinder fitness through direct molecular inhibition of growth and through indirect potential costs associated with increased parasitism. The relationship between T and ectoparasitism is complicated by seasonal variation in host circulating T levels and ectoparasite life cycles. It is unclear whether sex differences in ectoparasite loads are present year-round, are present only when circulating T is high in males, or are present only when ectoparasite abundances are high. Furthermore, it is often assumed that because ectoparasites feed by taking nutrients and energy from their hosts, then ectoparasites likely impact host growth. Effects of ectoparasitism on host growth may be particularly high in males if they have greater ectoparasite loads than females. This could indirectly lead to slower male growth and smaller overall male body size. To address the lack of information regarding direct and indirect effects of T on growth, seasonal variation in sex- biased ectoparasite loads, and of the relationship between ectoparasitism and growth, I investigated (1) whether growth inhibition in eastern fence lizards (Sceloporus undulatus) is regulated through androgen or estrogen receptors, (2) seasonal correlations of mite loads with environmental mite abundances, and (3) whether sex differences in growth are correlated with sex differences in mite loads. I found that DHT inhibits male growth in S. undulatus, suggesting that T inhibits growth through direct androgenic molecular regulation. Furthermore, as indicated by the negative correlation between male growth and mite load, I found that T may also inhibit growth in males indirectly through costs associated with increased mite parasitism. Mite loads on S. undulatus varied seasonally, with peak mite loads occurring during months of high environmental mite abundance, coincident with seasonally high circulating T in yearling males and negatively correlated with male growth. This suggests that mites may impose a cost to growth in S. undulatus and contribute to male growth-inhibition, sex-specific growth rates, and the development of sexual size dimorphism (SSD).

ETD_7593

Ph.D.

Includes bibliographical references

by Nicholas Pollock

doi:10.7282/T32V2JFJ

ETD doctoral

The author owns the copyright to this work.