I am interested in how intrasexual selection among females influences female behavior. My thesis focuses on the selective pressures shaping aggressive behavior in female tree swallows (Tachycineta bicolor ). Since 2002, I have been working with a population of tree swallows breeding in northwestern Pennsylvania near the Pymatuning Lab of Ecology (PLE) in Linesville. I measure aggressiveness using a behavioral bioassay, where I simulate a territorial intrusion, shown at the right. Altogether, my work looks at the potentially conflicting selective pressures of sexual and natural selection in shaping aggressive behavior in females. While common when studying sexually-selected male traits and behaviors, this perspective is rarely taken when trying to understand the evolution of similar traits in females.
Evidence for direct selection on aggressiveness in females
Typically, the presence of "male-like" traits (such as ornamental plumage) in females is explained via genetic correlation. I performed an experimental study to test the alternative hypothesis that female aggressiveness is the result of direct selection on females. After increasing competition for nestboxes by decreasing their availability, I found that more aggressive females were more likely to obtain nesting cavities, while male aggressiveness did not predict cavity acquisition. Because cavities are required for reproductive success, this result demonstrates that females gain direct benefits through their aggressiveness, suggesting that aggressive behavior in the the context of female-female competition over nesting cavities may be directly favored by selection (Rosvall, 2008 , Animal Behaviour. doi: 10.1016/j.anbehav.2007.09.038).
Aggressiveness is costly to females
While aggressive behavior may benefit females by allowing them access to a breeding site, aggressiveness is associated with a measurable fitness cost. Using two years of data from a cross-fostering study, I have found that offspring of more aggressive females have lower mass on day 12 post-hatch, a proxy for offspring quality. The mechanism by which this relationship occurs is not completely clear at this point, but it appears to be partially mediated by aggressive females provisioning offspring less frequently, and may relate to incubation or brooding behavior and offspring stress. I am currently using hormone analyses to test whether offspring of more aggressive females experience higher physiological stress.
Females may also experience direct costs associated with their aggression. Using a vertical test of flight in the field, I found that more aggressive females have a slower escape velocity to a simulated predator. This result suggests that there may be morphological trade-offs in flight ability that make females who excel at maneuverable flight (i.e. the type of flight used during aggressive interactions) less adept at generating the lift needed to escape a predator.
More results coming soon!
Correlates of aggressiveness in females
If aggressiveness is beneficial to females, why aren't all females aggressive? One answer to this question is that there are costs associated with being aggressive (above), and females may differ in their ability to handle these costs. Another possibility is that aggressiveness is constrained by other aspects of female phenotype. I have found that more aggressive females are heavier for their size (p < 0.000001), suggesting that aggressiveness is condition-dependent, much like many sexually-selected traits in males are linked to condition.
What about the males?
To date, my research has focused on the fitness effects of female aggressiveness. Understanding the costs and benefits of female aggressiveness is incomplete, however, without examining how males may alter the selective pressures shaping female aggressiveness. I have found no evidence of disassortative mating by aggressiveness, although pairs differing more in aggressiveness had more and larger eggs. When male provisioning was experimentally reduced, offspring were no worse in quality, but nestling mortality increased. Aggressive behavior was only associated with reduced offspring quality in control, not experimental, nests. Therefore, males may indirectly mitigate the costs of aggression for their female partners, although not by compensating for poor parenting by more aggressive females. Instead, females appear to invest more heavily in reproduction when mated to a male that is more different from her own phenotype. If this differential allocation outweighs the cost of aggressiveness, then male phenotype may play a key role in understanding the selective pressures shaping the evolution of aggressive behavior in females in general.
Ongoing Work
I am currently exploring the causal link between female aggression and reduced maternal care. In a recent field study, I found that testosterone-implanted females were significantly more aggressive than control-implanted females (p = 0.0004), and they incubated significantly less frequently.
