Browsing by Author "Nowicki, Stephen"

A long-held assumption in animal behavior is that females and males differ fundamentally in their mating strategies. Females are thought to be more choosy because female reproduction typically is limited by parental investment. Males, on the other hand, are expected to compete among themselves for access to females or resources, since male reproduction is limited primarily by mating access. This dichotomy is challenged by the increasing realization that males can be choosy and females also compete aggressively. It remains unclear, however, if and how selection acts on aggressive behavior in the context of intrasexual competition among females (reviewed in Chapter 1). In this thesis, I use a population of free-living tree swallows (Tachycineta bicolor) to test predictions about the selective pressures shaping aggressive behavior in females. First, using an experimental manipulation of nest site availability, I demonstrate that more aggressive females have a competitive edge in acquiring nestboxes, a critical limiting resource required for breeding (Chapter 2). This result shows that more aggressive females are more likely to breed and, thus, that females experience direct selection to be aggressive in the context of competition for mating opportunities. Next, I demonstrate a fitness cost of female aggression (Chapter 3): high levels of aggression in females are not associated with the quantity of offspring, but instead, more aggressive females had offspring of lower quality (i.e. reduced mass). Using a cross-fostering approach, I explore the causal link between female aggression and offspring mass, and I find that a trade-off between female aggression and maternal care best accounts for this cost of aggression. Site differences may create variation in how selection shapes female aggression, but the overall finding that more aggressive females have lower quality offspring indicates that this cost may work counter to selection favoring aggressive behavior in the context of competition over nestboxes. Understanding the evolution of female aggressiveness in a biparental system is incomplete without examining how males may alter the selective environment shaping female behavior. In Chapter 4, I explore the potential role of a female's mate in offsetting the costs of aggression. Males appear to mitigate these costs for their female partners, but not by compensating for poor parenting by aggressive females. Instead, females invest more heavily in reproduction, laying more and larger eggs, when mated to a male that is more different from her own phenotype. If this differential investment outweighs the cost of aggressiveness in terms of offspring quality, then male phenotype may play a key role in understanding the selective pressures shaping the evolution of aggressive behavior in females. Altogether, this dissertation explores the costs and benefits of female aggressive behavior. The focus on aggressiveness as a sexually selected trait in females provides a much needed parallel to the wealth of information already known about the selective pressures shaping sexually selected traits in males.

Sexual selection has long been framed as a process that ends when copulation is achieved. However, in species with polyandry (multiple mating by females), competition persists after mating inside the female's reproductive tract, where sperm from multiple males must then compete to fertilize a female's eggs. This post-mating process, known as sperm competition, is thought to be just as strong as the competition to secure a mate. Because sperm competition has only recently been observed, its evolutionary role remains largely unknown. In this dissertation, I use field, laboratory and computational approaches to understand the evolution of sperm competition in two ways: (1) by testing a possible source of variation in sperm competition within species, and (2) by examining how variation in sperm competition results in DNA evolution across species. My study system is the Agelaius clade of New World blackbirds, a group of songbirds with predicted variation in the intensity of sperm competition. In the first half of the dissertation, I explore the factors that affect how intensely sperm competition is experienced in a population. In Chapter 1, I assess the relationship between genetic diversity and extra-pair paternity (EPP, a proxy for sperm competition) in seven continental and one island population of red-winged blackbird (A. phoeniceus). I find that while genetic diversity varies significantly across populations, the population with the lowest amount of genetic diversity exhibits similar rates of EPP as the more diverse populations, providing no support for a relationship between genetic diversity and EPP rate. This result suggests that genetic diversity by itself is not an determining factor in EPP variation. In Chapter 2, I characterize the mating system of the endangered yellow-shouldered blackbird (A. xanthomus) and provide the first evidence that it, too, engages in EPP despite having low genetic diversity. I additionally present a conservation genetics profile of the species, showing that the yellow-shouldered blackbird's low effective population size and genetic diversity, both likely due to a recent bottleneck, may be increasing its vulnerability to extinction. I suggest ways in which future management decisions might account for the genetics of a small population. In the second half of the dissertation, I examine whether sperm competition itself can drive the molecular evolution of a species. I focus on the evolutionary patterns of seminal fluid proteins (Sfps), which are transferred with sperm during copulation and are known targets of sperm competition. I describe in Chapter 3 the transcriptomic and proteomic techniques I use to identify protein-coding genes in a non-model organism, presenting the first list of seminal fluid proteins in a songbird. I contrast the protein profile of the blackbird with the protein profile of insect and mammalian Sfps. Finally, in Chapter 4, I use eight of the proteins identified from the list to look for patterns of positive selection on these proteins. Specifically, I test whether Sfps evolve faster in species with mating systems featuring high levels of sperm competition than in species with mating systems featuring low levels of sperm competition. I first compare EPP rates measured from the previous two species with a third species, the tricolored blackbird (A. tricolor), and find that all three experience similar levels of sperm competition. From the catalog of genes derived in Chapter 3, I select, sequence and search for evidence of rapid evolution in six candidate Sfps and two control genes. I find that not only is there no evidence for positive selection in any of these genes, there is strong evidence for purifying selection and furthermore very low levels of diversity within and divergence across species. Reasons for these unexpected preliminary findings could be both microevolutionary or macroevolutionary in nature and warrant larger-scale studies, especially across a broader sample of taxa and across species with greater variation in sperm competition. Taken together, this dissertation describes the relationship between mating systems, sperm competition and post-mating adaptations. By examining the effect of mating system on protein divergence, it links sexual selection with molecular evolution while generating behavioral, genetic, transcriptomic and proteomic resources for future comparative studies.