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Bolund, Elisabeth (2009): Condition dependence and fitness consequences of sexual traits in zebra finches. Dissertation, LMU München: Faculty of Biology



The focus of this thesis is on sexual selection within a behavioural and evolutionary framework. Sexual selection is concerned with the factors that determine reproductive success. I have used a laboratory population of zebra finches to address fundamental questions that remain unanswered despite decades of research on this model organism in studies of sexual selection in monogamous species. The aim of this thesis was to perform a rigorous investigation of how male zebra finches achieve reproductive success. To do this, I tried to evaluate the signalling value of candidate traits and investigate the mechanisms that lead to reproductive success through both intra- and intersexual selection. The honesty of signals has had a special place in my work. Current theory suggests that secondary sexual signals should be costly to be able to function as honest indicators of male quality in a mate choice context. If ornaments are costly to develop or maintain, this would lead to condition dependent expression of ornaments. I tested the condition dependence of a broad range of traits so that I could compare the condition dependence of sexual and non-sexual traits and between traits in males and females (chapter 1). I found that zebra finches are remarkably resilient to stressful conditions during early development and can develop normally in most traits with very little consequences for fitness or longevity. These results indicate that not all traits that appear to be sexually selected characters will exhibit condition dependence. To study fitness consequences, I used an aviary breeding set-up. This allowed me to study the process of sexual selection in a socially complex environment where natural selection pressures had been relaxed. Since only selection acting on the genetic component of a trait can lead to evolutionary change, I used a quantitative genetic approach to look at selection pressures. Contrary to the prevailing view, I found that beak colour and courtship song rate and song complexity were not subject to female choice and not relevant for reproductive success (chapter 3). Beak colour was also not important in male-male competition (chapter 2). These findings necessitate a re-evaluation of the function of these traits. I suggest that courtship song rate reflects male reproductive strategies, while song structure is important for individual recognition in this highly social species and the beak colour might function as a signal of breeding status. The strongest candidate for a condition-dependent trait with consequences for fitness was body size (measured as tarsus length). Tarsus length was dependent on early condition (chapter 1), played a role in intrasexual competition (chapter 2) and was related to reproductive success in the aviaries (chapter 3). However, the quantitative genetic approach revealed that the selection was acting mainly on the environmental, not the genetic component of tarsus length, explaining why sexual size dimorphism has not evolved. Contrary to the expectations of honest signalling theory, males with a higher undirected song rate (i.e. non-courtship song) were less successful in the aviaries (chapter 4). However, females invested more into reproduction when paired to a high undirected song rate male, indicating that this type of song might function in reproductive stimulation of the partner and maintenance of the pair bond. Furthermore, female reproductive investment can be influenced by male quality. In contrast to previous studies, I found that females invested more when paired to a less attractive male (chapter 5). This investment pattern fits with the breeding system of this species, since life-time monogamy and a short lifespan means that the chances of breeding with a higher quality male in the future is very low. Thus, females should do the ‘best of a bad job’ and compensate by investing more when paired to a less attractive partner. Collectively, this work has provided several unexpected insights that call for a reinterpretation of several of the classical views of this model organism. This illustrates how novel experimental approaches can provide new insights into the process of sexual selection, also in well-studied model systems.