Ph.D. Candidate
Ecology, Evolution & Behavior Program
Department of Biology
Indiana University

E-mail: caink AT indiana DOT edu

Links

• Blog
  http://ww-junco.blogspot.com/
• Photos of research
  http://picasaweb.google.com/kristalcain

Dissertation description

When the optimal phenotype for males and females differs, the evolution of sexual dimorphism is favored. In many species, however, the degree of dimorphism is mild and numerous individuals express traits, e.g. body size or behavior, that fall between male and female norms along the distribution for that trait. Females in this group, or androgynous females, are of interest because they offer a natural experiment for understanding the sources and consequences of dimorphism. By investigating potential proximate causes for the development of sexual dimorphism a better understanding of how sex differences arise can be developed. Steroid hormones can modulate gene expression in a sex-specific way, and variation in hormone levels (or the timing of hormone exposure) constitutes a primary mechanism for the development of sex differences in vertebrates. Consequently, variation in hormone exposure may mediate the expression of androgynous traits, giving rise to covariation among these traits, owing to a common underlying mechanism. My research investigates how sexually dimorphic phenotypes develop by focusing on such androgynous individuals. Additionally, by relating these measures to fitness, an understanding of how selection and the mechanisms that give rise to sex differences interacts to maintain or constrain the evolution of further dimorphism can be developed.

My dissertation work looks to answer four related questions:

1. Does androgyny in one trait covary with androgyny in other traits, indicating a common mechanism mediating trait expression?
2. Does exposure to androgens during development predict the degree of androgyny?
   
3. Does adult ability to produce androgens covary with androgynous phenotypes?
   
4. Does the degree of androgyny relate to fitness, and if so, and how does this inform our understanding of the development of sex differences?

The dark-eyed junco (Junco hyemalis), a mildly dimorphic songbird, will be used to explore these questions. The junco has been the focus of extensive investigation including a long-term study of the effects of experimentally elevated testosterone in both sexes. In the research described herein, females will be measured for a variety of traits known to be sexually dimorphic in this species, namely reproductive behavior and morphology, to determine if they covary. Developmental exposure will be estimated using a morphological proxy, digit ratio or 2D: 4D. Adult ability to produce androgens will be determined using a repeatable dynamic measure that evaluates individual ability to produce and secrete androgens (GnRH challenge). Finally, fecundity and survival fitness measures of these individuals will be estimated in order to determine how, and in which direction, selection is acting.