Evolutionary ecology of competitive phenotypes in females

Like males, females also have exaggerated weapons and displays that may be important in male mate choice or female-female competition. Much less is known, however, about the potential function of these traits in females. An often overlooked, potentially sexually-selected trait that is pervasive among female animals is aggressive behavior directed at other females. While some instances of female-female aggression are firmly grounded in the realm of natural selection (e.g. competition for food resources), other cases, such as competition for access to males or nesting sites, provide a nearly identical parallel to sexual selection via male-male competition for females and territories. My behavioral ecological work has investigated a range of fitness costs and benefits of intrasexual aggression in female tree swallows (Tachycineta bicolor). I am currently extending this work both empirically and theoretically, investigating the ecological and life history parameters that affect the evolutionary trajectory of competitive phenotypes in females.

• Do females benefit from high levels of aggression? (Click to reveal details)

To determine whether females experience direct selection to be aggressive, a key step is to ask whether variation in aggressiveness maps onto variation in reproductive skew among females. The tree swallow (Tachycineta bicolor) is an obligate secondary cavity-nester with limited nesting sites, making it an ideal system to test whether more aggressive females have a competitive advantage in securing a nesting site. After assaying the aggressiveness of a population of tree swallows (photo on left), I experimentally reduced the number of nesting cavities. More aggressive females were more likely to obtain nesting cavities after the experimental reduction, meaning more aggressive females were more likely to mate and breed, relative to less aggressive females. Aggressive behavior therefore appears beneficial in the context of female-female competition for a critical mating and breeding resource (Rosvall, 2008). Other female animals also benefit from aggression in the context of competition for mating success, suggesting that sexual selection favoring aggressiveness in females is widespread (Rosvall, 2011 + associated commentaries)

• Are there fitness costs associated with high levels of aggression? (Click to reveal details)

  Growing evidence that female ornaments and armaments may be important for female reproductive success suggests a reevaluation of the costs of these female sexual traits. I found that high levels of aggression in female tree swallows are not associated with the quantity of offspring, but instead more aggressive females have offspring of lower quality (i.e. lower pre-fledging mass). I have used a variety of experimental field approaches (cross-fostering, hormone implants, and wing-clipping) to show that several different mechanisms likely account for these costs. Most importantly, females trade-off investment in mating effort (aggression) and parental effort (incubation, feeding), and thier mates do not mitigate the degree to which these trade-offs affect offspring quality (Rosvall, 2010, 2011c). Altogether, these costs of female aggression (i.e. reduced provisioning and incubation, leading to lower quality nestlings) may work counter to selection favoring aggressive behavior in the context of competition over nestboxes.

Evolution and plasticity of neuroendocrine mechanisms of behavior

Understanding the physiological and genomic mechanisms underlying behavioral variation is a key step in linking behavior with evolution because it can point to potential targets of selection in phenotypic evolution. The degree to which social experience or environmental change tinker with these mechanisms is another critical issue in a broad understanding of behavioral mechanisms. I am currently exploring individual, sex, and population differences in neural and genomic mechanisms of behavior in the dark-eyed junco (Junco hyemalis), with the ultimate goal of linking variation in gene expression to functional variation in behavior.

• Do males and females share common behavioral mechanisms? (Click to reveal details)

The evolutionary significance of aggression in females cannot be completely understood without considering its proximate mechanisms. In tree swallows, I found that testosterone (T) plays a role in mediating aggressive behavior. Looking across songbird species, however, evidence is mixed as to whether T predicts aggressiveness, and in fact, strong aggressive responses do not depend upon a T surge (Rosvall, Reichard et al. 2012). Other mechanisms, such as variation in neural sensitivity to T, may also be important (see below). Identifying when and why some species are behaviorally responsive to elevated T and when and why others are not is an important evolutionary question, currently without a clear solution (Rosvall, in revision). With collaborators at IU, I am now asking whether males and females process T in the same or different ways, at a genomic level, and whether sexually dimorphic and tissue-specific responses to hormones may provide solutions to sexual conflict (Peterson, Rosvall et al. in review).

• How do hormone-mediated traits and mechanisms of behavior evolve? (Click to reveal details)

  Testosterone is thought to play a major role in phenotypic evolution, but individual variation in hormone levels do not always map onto individual differences in behavior. Since individual variation is the raw material of evolution, this paradox poses a problem for understanding how hormone-mediated traits, like aggressive behavior, may evolve. Using the junco (at left), I have shown that natural variation in measures of the brain's ability to process steroid hormones predicts functional variation in aggressive behavior. In Rosvall et al. (2012a), we found strong and significant relationships between aggressive behavior in free-living birds and the abundance of mRNA for androgen receptor, estrogen receptor, and aromatase in behaviorally relevant brain areas. We are in the process of extending this approach to recently diverged subspecies of junco. Results thus far reveal exciting insights into how neuroendocrine mechanisms change as populations diverge into species.

• How do social challenges prime an animal for future social instability? (Click to reveal details)

  Competition is pervasive in the lives of animals. Much research has investigated how socially-stimulated hormonal shifts may prepare animals for success in future social challenges (e.g. Challenge Hypothesis, Winner Effect), but the genomic mechanisms of this preparatory effect are not clear. I am currently using a species-specific microarray designed for the junco, to investigate how specific global changes in gene expression facilitate an organismal response to social instability, and how responses in brain and periphery are coordinated (NIH R21 funding). Check back soon for updates!