Renée Duckworth
Research page

1. Evolution of Aggression: Correlations across contexts

Correlations among traits can have important consequences for their evolution. A major focus of my dissertation research has been to understand both how behavioral correlations form and what are their evolutionary consequences. Much of this work has been conducted in western bluebirds (Sialia mexicana).

Western bluebirds are ideal for this study for several reasons. First, as secondary cavity nesters, they depend on nest cavities to reproduce but cannot excavate their own. So they aggressively compete with both conspecifics as well as heterospecifics for this crucial limited resource. Second, western bluebirds are somewhat unique among North American passerines in that most males are highly philopatric and will return to almost the exact location of birth to breed. This has allowed me to collect detailed multi-generational pedigrees for males for which I know both the fathers' and the sons' behaviors and can examine behavioral correlations across multiple contrasting contexts.

I have found that male aggression is highly correlated across the contexts of nest defense against tree swallows (Tachycinetus bicolor), an important heterospecific competitor, and the context of male-male competition for females. Male-male aggression has important fitness consequences because more aggressive males invest less in parental care and fledge fewer offspring.

I'm currently exploring whether the phenotypic correlation of inter and intraspecific aggression is underlain by a genetical correlation. If so, then sexual selection for males that are more aggressive during male-male competition might have important consequences for the evolution of interspecific aggression. My current studies explore the evolutionary dynamics of the competing forces of sexual and natural selection in shaping aggressive behavior.

2. Ecological consequences: Rapid evolution of aggression and reciprocal range changes

Western bluebirds were common in western Montana until the 1930's at which time their numbers declined drastically until they were extirpated from the state. During this same time, mountain bluebird numbers also declined, however, unlike western bluebirds, they maintained small local populations in some parts of the state. In the last few decades, implementation of large scale nest box programs has led to the rapid re-colonization of their ranges. As these two species come into contact in areas of recent overlap, a clear pattern has emerged with western bluebirds repeatedly and rapidly displacing mountain bluebirds.

To investigate the role of aggression in this range expansion, I experimentally measured aggressive behavior across eight populations that differ in duration of coexistence of the two species. I found that western bluebirds are generally more aggressive than mountain bluebirds. Moreover, aggression of western bluebirds is higher in new populations where mountain bluebirds are common, but once western bluebirds become the dominant species, aggression decreases in a few generations. These rapid behavioral shifts are most likely due to opposing selection pressures in edge versus interior populations combined with the propensity of more aggressive males to disperse and establish new populations.

Higher aggression of western bluebirds allows them to compete more effectively for nest cavities and territories and ultimately exclude mountain bluebirds from an area.

I'm currently exploring the idea that western bluebirds' lower investment in parental care might play a central role in their poor performance in extreme climatic conditions and limit them to lower elevations. While mountain bluebirds may trade-off high competitive ability for the ability to survive in a wide range of habitats. Future studies will compare reproductive success and behavioral differences among the species in high and low elevation populations to test this idea. Ultimately, this work will provide crucial insights into the coexistence of the two species.

3. Reproductive Strategies: The role of hormones and the importance of context

In collaboration with Alex Badyaev, I have been investigating the proximate causes of variation in male reproductive strategies in house finches. Males pursue different parental strategies based on the elaboration of their plumage coloration. Bright (red) males invest very little in offspring care compared to dull (yellow) males. Variation in parental behavior is closely related to variation in the hormone prolactin (a hormone produced by the pituitary that stimulates the expression of parental behavior in many vertebrates). In turn, bright and dull males differ in their expression of prolactin – bright males have very low prolactin levels during parental care stages of breeding while dull males have high prolactin levels.

We are currently teasing apart the causal links between prolactin elevation, parental behavior and male plumage color by experimentally switching male behavior using hormonal implants. How do females respond when the phenotypes are reversed and bright males suddenly become better parents than dull males?

Ultimately, we’d like to understand, not only the proximate physiological mechanisms that influence male parental behavior but also, what cues in the environment stimulate males to pursue one parental tactic over another. So far, we’ve found that males’ plumage coloration and hence their reproductive tactic are partly age-dependent and partly influenced by environmental context. Male plumage color during their first molt depends largely on their physiological condition, however, in older males, plumage coloration is mostly a function of their pairing status in the previous year.

4. Sexual selection: Implications of condition-dependent testosterone elevation

Why do females evolve apparently arbitrary preferences for males with elaborate sexual traits? One answer to this question is that male ornaments are linked to the health and physical condition of the male. Because testosterone influences a wide variety of sexual traits, it may be a proximate link between ornamental traits and male health, providing a simple explanation of female preference for many different sexual traits such as song production, courtship behaviors, and elaborated physical traits.

In collaboration with Mary Mendonca and Geoff Hill, I found that experimentally elevating testosterone levels of captive males resulted in a more rapid onset of coccidial infection. However, in a natural setting males with higher testosterone levels had lower levels of infection. What can explain these seemingly contradictory results? In the wild, testosterone elevation is condition-dependent. Only males in good condition can elevate testosterone and these males are also the healthiest males. This suggests that male condition plays an integral role in mediating the relationship between testosterone and disease resistance and may explain why a female would benefit from choosing a male displaying elaborated sexual traits.