A matter of taste : the role of compatibility in mate preferences in great tits

By choosing the right partner individuals can gain reproductive benefits and can increase their reproductive success. These benefits can be direct, when the offspring’s quality or quantity is increased by the behaviour or investment of the mate, or indirect, when the quality of the offspring is increased by the genetic contribution of the mate. While previously it was often thought that reproductive benefits were primarily caused by the higher quality of these preferred partners, more and more studies suggest that also partner compatibility may increase reproductive success. Mate compatibility may be especially important when individuals differ in their preferences for a mate. Although uniform preferences are often assumed, an increasing number of studies indicate that individuals have a preference for different trait values. Moreover, these differences in preference can depend on the chooser’s own trait values, indicating that individuals may prefer a mate that is the best fit for them, rather than the universally ‘best individual’. A number of studies on captive populations have found that the fitness benefits from mating with a certain individual differ between individuals, suggesting individual differences in preference and both direct and indirect benefits of mating with these compatible mates. However, it is still unclear how compatibility and perceived mate attractiveness affect reproductive success in a wild population. Additionally, to understand fitness consequences and the evolution of mate preferences, preference and choice should be studied as two distinct processes. To our knowledge, no study thus far has combined all three and tested what the fitness benefits of mate preferences are under mate choice constraints in a wild population. Therefore, the overall aim of this thesis was to study the role of individual differences in mate preferences and its effects on reproductive success, by comparing measured mate preference with the resulting choice and relating the pair compatibility with different aspects of reproductive success. When measuring mate preferences results may be affected by how individuals evaluate and compare the available options they are presented with. Preferences are often assumed to be absolute, assigning a fixed, absolute value to a cue or potential partner they encounter. In reality however, individuals may compare the available options, in which case the social context plays an essential role in the preference for each potential partner. For this study I used a 6-choice mate choice setup to test great tits for their preferences in a mate. Using such a test design allowed individuals to comparatively evaluate potential partners. Additionally, presenting individuals with 6 choice options allowed for the measurement of non-linear preferences. I reveal that great tits use a combination of both absolute and comparative evaluation and that the social context affected the measured responses to stimulus groups. This suggests that great tits can flexibly use both evaluation methods, depending on the situation they encounter potential mates in. With such knowledge of how a study species encounters and evaluates potential mates, it is possible to choose the most appropriate experimental design and analysis to obtain reliable measurements of mate preferences. The ability to more accurately quantify preference is expected to increase our understanding of mate preferences, mate choice, and ultimately sexual selection. By measuring preferences for traits in a mate I found that there were no population-wide mate preferences for the different bird traits that we measured (heterozygosity, intensity yellow breast plumage, black breast stripe size and body condition) and that individual preference slopes for (at least three of) these traits differed between individuals. Thus, rather than preferring the same universally attractive individuals, individuals may prefer a mate that is compatible to themselves. Moreover, individuals may differ in which traits they find important in a mate. Compatibility between pair mates can be dependent on the combination of the pairs’ trait values. Here I show that preferences for certain traits were dependent on the choosers’ own traits, suggesting preference for compatibility for these traits. By testing individual for their preferences for genetic characteristics I found that both sexes preferred mates that had heterozygosity levels similar to themselves, and not those with which they would optimise offspring heterozygosity. Moreover, I found that these preferences for similarity in heterozygosity were in reflected in direct benefits for reproductive success. In the subsequent field experiment where I cross fostered offspring, foster parents with more similar heterozygosity levels had higher reproductive success, despite the absence of assortative mating patterns. This suggests that direct fitness benefits underlie mate preferences for genetic characteristics and that the potential for selection for preferences persists despite constraints on mate choice. Another trait that is thought to play a large role in pair compatibility is behavioural compatibility. Especially for species with biparental care behavioural traits and behavioural compatibility may be important for reproductive success. Here I studied the role of personality and behavioural compatibility in the provisioning behaviour in particular, by recording parental provisioning behaviour of great tit pairs with known exploratory behaviour. By experimentally cross-fostering offspring I was able to distinguish between the pre-hatching and rearing effects of parental personality on offspring weight and fledging probability. Here I found that the combination of personalities in a pair did not affect their provisioning behaviour. Instead, I found that although all birds provision on a trade-off between visit rate and prey volume, fast birds brought in smaller prey for a given visit rate resulting in a lower overall amount of prey delivered to their offspring. This led to faster explorers bringing a lower total prey volume to their offspring. Thus, personality may certainly play a role in direct benefits that individuals gain from finding a mate, however, this benefit was not dependent on the choosers’ own traits. Despite the differences in provisioning behaviour, there was no effect of pair compatibility on provisioning performance. However, it is possible that these effects of personality are context dependent, or only become apparent when pairs encounter challenges in their breeding attempt. To gain a better insight into how selection pressures may favour certain combinations of personalities these personality effects should be measured over more different contexts. Additionally, a context dependence of behavioural compatibility suggests a fluctuating selection on personality traits and pair compatibility, and with it a potential for fluctuating selection on preferences for mate personality. Individuals have been shown to have increased reproductive success when they are able to mate with individuals that they perceive as attractive. Since here individuals have been found to differ in their preferences, and some traits can be more or less important for different individuals depending on their own traits, each individual can differ in how attractive they perceive the same potential mate. Here I investigated if the attractiveness of an individual, as perceived by its mate, increased the reproductive success of the pair. On the basis of preference tests, I calculated preference functions for each individual and in the subsequent breeding season estimated a measure of how attractive each individual perceived the mate they were paired with. Here I found that while individuals differed in their preference for a mate, they were not more likely to obtain a mate that they perceived as attractive than under random mating. Additionally, I found no effects of perceived mate attractiveness on female reproductive investment, offspring weight or fledging probability. Although we found no effect of our measure individual mate attractiveness on reproductive success, this method of estimating preferences and individual mate attractiveness gives the potential for further studies studying pair compatibility and reproductive success. A better understanding of preferences for compatibility rather than quality and the benefits related to this may shed light on the adaptive value of individual differences in preference for mates. By measuring both preference and choice I found that, quite often, preferences are not reflected in the mating patterns that we find in the population. However, I did find that these preferences were reflected in reproductive benefits. Thus, while not always being able to, those individuals that did find a mate according to their preferences benefitted from compatibility with their mate. Mate choice is therefore the result of choosing a partner according to ones’ preferences whilst under physical and environmental constraints. This emphasizes the importance of measuring both preference and choice to fully understand how sexual selection acts on preferences. To conclude, in this thesis I investigated the role of compatibility on mate preferences, mate choice and reproductive success. I revealed how individuals evaluate potential partners and how perceived mate attractiveness may affect reproductive success. Here I have found that mate preferences for a compatible, rather than universally attractive, partner may have evolved to gain direct or indirect benefits from pairing. Additionally, while there may be constraints on the choice of a mate, finding a compatible mate can increase reproductive success. These results also illustrate the importance of considering mate preferences and mate choice two distinct processes, both of which should be studied to better understand how sexual selection acts on preferences.