Year of Award
Doctor of Philosophy (PhD)
Organismal Biology, Ecology, and Evolution
Department or School/College
Division of Biological Sciences
Doug Emlen, Tom Martin, Nathan Morehouse, Bret Tobalske
bird, blue, bluebird, coloration, corticosterone, sexual selection
University of Montana
Sexual selection acts on traits that increase mating success, either through mating preferences or intrasexual competition for access to mates. For traits to be honest, we expect sexually selected traits to reflect individual condition at the time of trait development. Furthermore, when sexual selection operates through mating preferences, we also expect traits to indicate benefits (direct or indirect) that females receive for exercising their preference. If sexual selection acts through differential success in intrasexual contests over mates, we expect traits to indicate resource holding potential, or fighting ability. These links between individual condition, trait quality, and performance maintain honesty, because high condition individuals have high quality traits, and conspecifics can therefore use information from sexually selected traits when entering contests or choosing between prospective mates.
Based on the above logic, we expect sexually selected traits to be consistently and positively related to performance. However, individuals may differ in their sensitivity to environmental variation such that sexually selected traits are not always honest indicators of individual condition, benefits to females or offspring, or competitive ability. Environmental variation could affect trait honesty if individuals vary in their ability to respond to environmental variation. For example, trait honesty may disappear in poor environments, if individuals with highly developed sexually selected traits only perform well in high quality environments. Alternatively, individuals with more elaborate traits may be more adept at responding to environmental challenge, and trait honesty could increase when environmental conditions are poor.
For my dissertation I examined variation in trait development and honesty under varying conditions in the mountain bluebird, Sialia currucoides. Mountain bluebirds display sexually dimorphic UV-blue coloration, and males with more intense coloration sire more offspring at their own nest and at other nests through extra-pair fertilizations. However, it is unclear what benefits and costs receivers experience when using this trait to asses mates or rivals, and what processes regulate the development of this sexually selected trait. Therefore, in chapter one I explored the function of this signal during agonistic contests for territories. I performed simulated territorial intrusions to understand whether male aggressive behavior was related to his coloration. I also measured food availability to determine if males with more intense coloration obtained higher quality territories. Overall, my results provide evidence for the function of this signal during agonistic contests. Furthermore, while a single aggressive behavior (number of attacks) was repeatable across the egg laying period, my integrated metric of aggression, which accounted for many aggressive behaviors and was related to male coloration, was not. Lastly, I found that males with more saturated coloration obtained territories with greater insect abundance.
In chapter two, I performed an experiment to understand the mechanisms of condition dependence of blue coloration. Individuals may vary in their sensitivity to environmental variation during trait development; such high condition individuals preserve trait quality during environmental challenge while poor condition individuals do not. Martin et al. (2011) suggest that endocrine systems are an important mediator of phenotypic variation because hormones both respond to environmental conditions and regulate internal response and resulting phenotype. The hormone corticosterone (CORT) is released by the adrenal glands in response to challenge to divert resources towards self-preservation. A rapid, transient increase in CORT can help individuals to survive challenging situations. However, prolonged or frequent CORT secretion can cause damage to other physiological systems and potentially decrease fitness. For example, CORT-implanted white crowned sparrows abandon their high-elevation breeding territories and retreat to low elevations during storms. This increases survival but decreases reproductive success. Also, CORT-implanted male song sparrows increase fat stores, but are less likely to respond aggressively to a simulated territorial intrusion. These links between environmental conditions and potential fitness consequences make CORT an ideal regulator of sexually selected traits.
To this end, I brought 14 hatch year mountain bluebirds into captivity to study how individual variation in CORT physiology during resource limitation predicted coloration. I wanted to understand whether CORT predicted blue coloration when resources were abundant, limiting, or both. I found that when birds were food limited, variation in CORT levels increased. Furthermore, CORT and coloration were negatively associated in my food limitation treatment, as predicted if poor condition males mount larger stress responses, but this association disappeared when birds were given ad lib food. I also measured feather structure variables to determine how coloration was related to feather quality and feather performance (measured as resistance to airflow) across my resource availability treatments. I found that the component of feather structure that was related to coloration (barbule density) was sensitive to CORT only when resources were limiting. Conversely, the feather structure variable that determined feather performance (interbarb distance) was not sensitive to CORT in either treatment and did not predict feather coloration. These results indicate that feather coloration in bluebirds is only sensitive to variation in physiology when resources were limiting, and that this was mirrored by concurrent changes in condition-sensitive feather structure. However, feather performance was not sensitive to individual variation in physiology or variation in environmental conditions.
While chapters one and two demonstrated that blue coloration is condition dependent and related to aggressive behavior, male bluebirds still provide a significant amount of parental care. This means that females may be using coloration to select mates and obtain benefits for themselves and their offspring. In chapter three, I examined the effect of environmental variation on the relationship between bluebird coloration and direct benefits to females, expressed as offspring quality (nestling mass). Three years of data on free-living bluebirds suggests that the relationship between male coloration and nestling mass varied across years and between broods. In some contexts, more elaborate males had heavier nestlings, while in other contexts they raised the lightest nestlings. I found that this variation was not driven by resource abundance, but instead appeared due to changes in optimal reproductive effort. When average nestling mass at my study site was higher, bluer males raised heavier nestlings than they did when average nestling mass was lower. This variation in nestling mass occurred independently of resource availability.
Overall, these results demonstrate that the honesty of blue coloration varies across environmental contexts. While coloration is positively related to aggressive behavior, males vary in their sensitivity to resource limitation, and blue coloration does not consistently predict direct benefits in the wild. My data indicate that the process of sexual selection is context-dependent, and sexually selected traits vary in the extent to which they predict individual performance and quality.
Berk, Sara A., "ENVIRONMENTAL VARIATION AND SEXUAL SELECTION IN THE MOUNTAIN BLUEBIRD (SIALIA CURRUCOIDES)" (2019). Graduate Student Theses, Dissertations, & Professional Papers. 11322.
© Copyright 2019 Sara A. Berk