Mate choice plays an important role in sexual selection and speciation. The evolution of mate choice is intriguing in cases where choosy individuals gain little except for genetic material from the mate and where the trait used as a criterion for the choice is costly to its bearer. The sensory exploitation hypothesis is an interesting idea that applies to such cases because it suggests that sexual preferences may arise as side-effects of preferences that are under selection in other contexts. The role of mate choice in speciation is strong but is debated because the reasons for population divergence in mate preferences and sexual traits are sometimes hard to explain. Also in this context sensory exploitation offers a potential explanation in that a link between natural and sexual selection may result in divergence in sexual selection whenever populations differ in natural selection. In this thesis, I test several aspects of this hypothesis in a species of fish, the swordtail characin (Corynopoma riisei). In this species males display a flag-like ornament that grows from the operculum. Because females respond to this ornament by biting at it, it has been proposed to act as a food-mimic. By manipulating female food type and quantity, and testing the resulting female preference for the male ornament, I find support for the theory that the preference has evolved through sensory exploitation and that females indeed appear to relate the ornament to a food item. Furthermore, I show that sensory exploitation can lead to morphological divergence among natural populations in this species. Apart from the flag-ornament, other courtship signals are also investigated. The results show that the relative importance of different signals may vary depending on receiver motivation. This suggests that various aspects of both male courtship signals and the conditions during which they are being signalled should be considered to gain a full understanding of mate choice and its role in sexual selection and speciation.

Chapter 1: Relative Concerns and the Choice of Fertility Empirical research has shown that people exhibit relative concerns, they value social status. If they value their children's status as well, what effect will that have on their decisions as parents? This paper argues that parents and potential parents are in competition for status and rank in the generation of their children; as a consequence richer agents may cut back on the number of children they have and invest more in each child to prevent children of lower income agents from mimicking their own children. This effect need not be uniform so that equilibrium fertility may e.g. be a U-shaped function of income, even when agents would privately like to increase fertility when they receive greater income. These findings have wide ramifications: they may contribute to our understanding of the working of the demographic transition; they also suggest that the low fertility traps seen in some developed countries are rather strongly entrenched phenomena; and they o er a new explanation for voluntary childlessness. Chapter 2: Relative Concerns and Primogeniture While pervasive in the past, differential treatment of children, i.e. different levels of attention and parental investments into children of the same parent, has become rare in modern societies. This paper offers an explanation based on technological change which has rendered the success of a child more uncertain for a parent who is deciding on how much to invest into each of his children. Within a framework of concerns for social status (or relative concerns), agents decide on how many children to have and how much to invest in each child. When their altruism towards each child is decreasing in the total number of children, it is shown that they may solve the trade-off between low investment, high marginal return children (that come in large numbers and hence hurt parental altruism) and high investment, low marginal return children (that come in low numbers) by demanding both types and hence practice differential treatment. Uncertainty over status or rank outcomes of children reduces the range of equilibrium investment levels intro children so that the difference in the numbers they come in is reduced. Eventually the concern for return dominates and differential treatment disappears. Chapter 3: Co-Evolution of Institutions and Preferences: the case of the (human) mating market This paper explores the institutions that may emerge in response to mating preferences being constrained in their complexity in that they can only be conditioned on gender not other characteristics of the carrier of the preferences. When the cognitive capacity of the species allows a sophisticated institutional setup of one gender proposing and the other accepting or rejecting to be adopted, this setup is shown to be able to structure the mating allocation process such that preferences evolve to forms that, conditional on the setup, are optimal despite the constraint on complexity. Nature can be thought of as delegating information processing to the institutional setup. In an application to humans it is shown that the mechanism of the model can help explain why men and women may exhibit opposed preferences in traits such as looks and cleverness. The anecdotal fact that women do not marry down while men do can be interpreted as a maladaptation of female preferences to modern marriage markets.

The sense of humour is a uniquely human skill and understanding humour is an important and rewarding part of social interaction. This thesis begins by discussing the definition of humour, followed by a review of the evidence we have that humour is an evolved and adaptive behaviour. Humour may play an important role in helping individuals to bond and signal cooperation, which may be further communicated by the humour style which is used to communicate. Research has also demonstrated that humour is an attractive quality in a mate, though the precise reasons for this are currently debated (Chapter 1). Empirical work in the first section of the thesis is consistent with evidence demonstrating that humour is attractive and sexually selected for. Chapter 2 tests the influence of modality and relationship context in an effort to further our understanding of why humour is attractive and provides evidence that more attractive people are rated as being funnier than less attractive people. Humour was also found to be more attractive for short-term relationships than long-term relationships, possibly due to the similarity between funniness and flirtatiousness. In Chapter 3, attractiveness ratings of vignettes in the style of personal advertisements, which contained either aggressive or affiliative humour, demonstrated the importance of humour style. An affiliative humour style was more attractive for long-term relationships whereas an aggressive humour style was more attractive for short-term relationships. Further testing provided evidence that humour styles were associated with personality traits which are highly relevant in a mating context, helping to explain the functions of different humour styles. The second section of the thesis examines the relationship between humour, cooperation, and dominance as an alternative explanation for the evolution of humour. Chapter 4 contains an extended introduction to the physical, verbal, and nonverbal cues to dominance and the sex differences that exist in expressive behaviours. Chapter 5 continues this theme and elaborates further on the function of humour in group situations, before providing empirical evidence of how humour is used in the context of a competitive ‘desert-island’ style conversation between same-sex dyads. Chapter 6 further expands on this line of research as empirical evidence presented in this chapter demonstrates that males may be using humour as a way of communicating the desire to cooperate with other males who are of a similar level of dominance. The communication of dominance is further examined in Chapter 7, where ethological evidence showed that males who were more physically dominant tended to knock doors with greater frequency than males who were less physically dominant. In the final chapter of the thesis (Chapter 8), the evolution of humour is discussed in light of the evidence presented in Chapters 2-7. The thesis presents evidence to suggest that humour production is an important skill for males for two reasons. Firstly, a good sense of humour is a highly attractive quality to females and may be a cue to genetic quality or good partner qualities, depending on the humour style used. Secondly, it may be important for males to use humour to signal cooperation to other males in order to form alliances. In females, the evidence presented in the thesis suggests that humour production may be a way for females to demonstrate romantic interest or flirtatiousness but the function of humour use between females remains largely inconclusive.

The evolutionary forces that led to the unprecedented expansion of the human brain and the extreme cognitive prowess possessed by humans have always attracted a great deal of attention from the scientific community. Presented here is a novel theoretical perspective, where the driving force on human brain evolution was the need for enhanced ability to infer social values of conspecifics in the face of degradation and loss of chemosensory signalling mechanisms necessary for social communication present in most mammals. The lack of chemosensory communication of biologically relevant information between humans in the face of the need to make adaptive and accurate social evaluations, led to an exaption of mammalian chemosensory brain regions for the more complex task of inferring social values from behavioural cues that are variable, ambiguous, or otherwise difficult to detect and interpret. This change in social processing from perceptual evaluation to inferential computation placed a premium on cognitive capacity, thus selecting for larger more powerful brains. These selective processes would have left an indelible mark on the human brain, where the human homologues of regions involved in mammalian conspecific chemical communication, in particular the target regions of this study the amygdala and ventromedial prefrontal cortex (VMPC), should be involved in the processing of biologically relevant information and social inference. Several experiments were conducted to examine the role of these brain regions in social inferential processing using the lesion deficit method. First, given that conspecific chemical communication is particularly relevant for biologically imperative evaluation for the purposes of reproduction, VMPC and amygdala damage may result in abnormal mate-related decisions. Second, normal social attributions exhibit the correspondence bias, however damage to the target regions may result in an abnormal lack correspondence bias. Third, the current hypothesis is contrasted with another leading hypothesis, the Social Brain Hypothesis whose proponents predict a relationship between group-size and social cognition. Finally, if the target brain regions are truly integral in inferring social information, then damage to these regions will interfere with the ability to utilize transitive inference in social situations, and potentially in using transitive inference in general. Damage to the target areas produces limited effects on mate-related decisions and preferences. However, the current hypothesis may suggest that the target brain regions are only involved when the problem is inferential in nature rather than simpler perception of social information. In support of this notion, damage to the target regions results in a lack of the correspondence bias when making economic decisions. This alteration in social attributions actually leads to more `rational' decision-making in this context. In contrast to the predictions of the Social Brain Hypothesis, damage to the target regions produces no observed reduction in social group size, nor is there any observed relationship between perspective-taking ability and group size. Finally, damage to the VMPC produces deficits in using transitive inference in a non-social context perhaps hinting at the underlying computations of this region in inferring social information. In conclusion, it appears that the notion that the human brain regions that have been exapted from their duties in chemosensation and communication in mammalian brains has at least some validity. Moreover, these brain regions have been shifted by evolution to a more computationally complex process of social inference possibly providing the push toward larger and more powerful human brains.

The ability to understand the complex mechanisms by which species communicate has been the focus of study for many years. The Adélie penguin (Pygoscelis adeliae) breeds in vocally challenging Antarctic colonies and has evolved a complex system of intra- and intersexual visual and vocal behaviours. Unique to the Adélie penguin, is the presence of two distinct display calls, the Loud Mutual Display (LMD) call and male Ecstatic Display Call (EDC). The EDC is used for both mate choice and territory defence; but apart from general structure and function this call remains basically unstudied. Therefore, the aim of this study is to examine the EDC with regard to its function (during mate choice) and variability over time, with location and with changing conditions. It is predicted that the EDC will be an individual call that is distinct from the LMD call. Additionally, the effects of an inclement breeding season on factors that affect breeding success will be examined using a logistic regression approach. The study took place over two breeding seasons(2000/1 & 2002/3) on Ross Island, Antarctica, with the majority of research being conducted at Cape Bird. Results of the study indicate that the EDC is an individual call that cannot reliably be described as a male display as females also give an EDC that is acoustically comparable. The similarities between the EDC and the LMD call seem to relate only to syllabic organisation (structure), as almost every call parameter was different between call types. Furthermore, several call parameters differed between Ross Island colonies. These parameters varied with both geographical location and colony size. In conjunction with variation in call parameters, the health and breeding success of the colonies was different. The stability of call parameters (from Cape Bird) indicated that parameters that separate colonies not only alter between breeding years, but also vary during a single breeding season. The function of the EDC as an honest signal during mate choice was supported by spectral analyses. These analyses found heavier males had significantly lower Frequency Modulation (FM) in the short repeated syllables of their calls than lighter males. Furthermore, a male’s breeding success could be predicted during the early breeding season by the FM. Logistic regression models showed that early male arrival, weight and good nest quality were most influential in predicting fledging success. The logistic regression method also showed that the parameters measured varied in their predictive ability as the season progressed. This study has not only enhanced the wealth of knowledge regarding Adélie breeding behaviour and developed our understanding of the vocal repertoire of this species, but also has implications for honest signalling theory, and the evolution and stability of vocal variation at different spatial and temporal scales.

<p>This thesis integrates the fields of sexual selection, parental investment and sex role theory by investigating mutual mate choice and mate competition in the sex role reversed deep snouted pipefish <i>Syngnathus typhle</i> (Pisces: Syngnathidae) through a series of laboratory experiments. In<i> S. typhle</i>, the female transfers her eggs to the male's brood pouch where they are nourished and oxygenated for about a month, when the male gives birth to the independent fry.</p><p>Mate choice was found to be adaptive. Both sexes benefited from mating with preferred partners in terms of increased offspring viability and got larger, or faster growing, offspring when mating with large fish. Females were also shown to prefer males with thicker brood pouches. Thus, females, the more competitive sex, had multiple preferences. Both male and female choice behaviour was found to be flexible and influenced by available information on partner quality. In addition, males, but not females, copied the mate choice of consexuals. </p><p>Both sexes were found to take their own quality in relation to surrounding competitors into account when deciding whether to display to potential partners. Male-male competition was found to influence both the mate choice of males and, potentially, overrule the mate choice of females. Males did not compete as intensely as females, nor did they use their sexual ornament in this context as females do. Rather, the ornament was used in interactions with females, and males that displayed more received more eggs.</p><p>The findings in this thesis emphasise the importance of not viewing mate choice and competition as opposite behaviours, but rather to apply a dynamic approach in mate choice studies, integrating choice and competition in both sex</p>

<p>This thesis focuses on reproduction in a simultaneous hermaphrodite with internal fertilization; the nudibranch <i>Aeolidiella glauca</i>. Unlike most other nudibranchs, where copulation is the rule, <i>A</i>. <i>glauca</i> was found to transfer sperm via external spermatophores that were attached to the partner's back. Despite elaborate courtship the actual spermatophore transfer, which always involved two animals only, was of short duration. In most matings (88%) spermatophores were reciprocally exchanged. </p><p><i>A. glauca</i> was further found to be very promiscuous. During mating and sperm transfer the receiver exerts considerable control over sperm, and manipulative behaviours designed to increase the donor's reproductive success are thus likely to have evolved. An example of such manipulative behaviour may be <i>A. glauca's</i><b> </b>unique spermatophore avoidance behaviour. I found that slugs carrying a sign of previous mating activity, i.e. a spermatophore, were discriminated against in a situation where mate choice was possible. The presence of spermatophores was further found to reduce slugs' ability to interrupt matings, and displace other slugs. Body size, however, had no direct effect on displacement in <i>A. glauca</i> as small slugs interrupted matings as successfully as large ones. Furthermore, pair formation and mating were found to be random with respect to size. This was true also for pairs formed in the field. </p><p>In addition to mate choice hermaphrodites may increase their reproductive success by differential sex allocation. I tested whether differing mate encounter rates had any effects on allocation to male and female function in <i>A. glauca</i>. Slugs with more mating opportunities mated more, and had higher proportional spermatophore production that others. As predicted they also laid significantly fewer eggs than slugs presented with partners less often.</p>

<p>A central goal of evolutionary biology is to understand the processes responsible for morphological, genetic and behavioural differentiation between sexes and among geographically distinct populations. Perhaps the most significant processes are genetic drift, natural selection, phenotypic plasticity and sexual selection. The main aim of this thesis was to investigate differentiation among individuals and populations of the sex-role reversed pipefish (<i>Syngnathus typhle</i>) and, consequently, determine which processes may be responsible for emerging patterns. This unique species is characterised by males predominately choosing amongst displaying females.</p><p>In this thesis I revealed, on a microgeographic scale, morphological differentiation without genetic divergence among populations. Interestingly, females differed in size whereas the males did not. For females in this sex-role reversed species, the costs of expressing a plastic phenotype may be outweighed by the potential gains from greater survivorship, higher fecundity or increased mating success. Thus, females gain the ability to make themselves as conspicuous and attractive to males as possible in the specific environment they are living. Moreover, behavioural experiments, which focussed on describing “personalities”, reproductive investment strategies, and mate-sampling tactics, also indicated that males as well as females had the behavioural plasticity required to adjust to the environment in which they live. To this end, using video playbacks as experimental stimuli may be especially rewarding in this species.</p><p>Overall, the studies in this thesis acknowledge the ability of species to fine-tune their phenotype to maximise fitness and, therefore, highlight the importance of considering patterns of differentiation in an environment-specific context. </p>

The ability to understand the complex mechanisms by which species communicate has been the focus of study for many years. The Ad��lie penguin (Pygoscelis adeliae) breeds in vocally challenging Antarctic colonies and has evolved a complex system of intra- and intersexual visual and vocal behaviours. Unique to the Ad��lie penguin, is the presence of two distinct display calls, the Loud Mutual Display (LMD) call and male Ecstatic Display Call (EDC). The EDC is used for both mate choice and territory defence; but apart from general structure and function this call remains basically unstudied. Therefore, the aim of this study is to examine the EDC with regard to its function (during mate choice) and variability over time, with location and with changing conditions. It is predicted that the EDC will be an individual call that is distinct from the LMD call. Additionally, the effects of an inclement breeding season on factors that affect breeding success will be examined using a logistic regression approach. The study took place over two breeding seasons(2000/1 & 2002/3) on Ross Island, Antarctica, with the majority of research being conducted at Cape Bird. Results of the study indicate that the EDC is an individual call that cannot reliably be described as a male display as females also give an EDC that is acoustically comparable. The similarities between the EDC and the LMD call seem to relate only to syllabic organisation (structure), as almost every call parameter was different between call types. Furthermore, several call parameters differed between Ross Island colonies. These parameters varied with both geographical location and colony size. In conjunction with variation in call parameters, the health and breeding success of the colonies was different. The stability of call parameters (from Cape Bird) indicated that parameters that separate colonies not only alter between breeding years, but also vary during a single breeding season. The function of the EDC as an honest signal during mate choice was supported by spectral analyses. These analyses found heavier males had significantly lower Frequency Modulation (FM) in the short repeated syllables of their calls than lighter males. Furthermore, a male���s breeding success could be predicted during the early breeding season by the FM. Logistic regression models showed that early male arrival, weight and good nest quality were most influential in predicting fledging success. The logistic regression method also showed that the parameters measured varied in their predictive ability as the season progressed. This study has not only enhanced the wealth of knowledge regarding Ad��lie breeding behaviour and developed our understanding of the vocal repertoire of this species, but also has implications for honest signalling theory, and the evolution and stability of vocal variation at different spatial and temporal scales.

This thesis focuses on reproduction in a simultaneous hermaphrodite with internal fertilization; the nudibranch Aeolidiella glauca. Unlike most other nudibranchs, where copulation is the rule, A. glauca was found to transfer sperm via external spermatophores that were attached to the partner's back. Despite elaborate courtship the actual spermatophore transfer, which always involved two animals only, was of short duration. In most matings (88%) spermatophores were reciprocally exchanged. A. glauca was further found to be very promiscuous. During mating and sperm transfer the receiver exerts considerable control over sperm, and manipulative behaviours designed to increase the donor's reproductive success are thus likely to have evolved. An example of such manipulative behaviour may be A. glauca's<b> </b>unique spermatophore avoidance behaviour. I found that slugs carrying a sign of previous mating activity, i.e. a spermatophore, were discriminated against in a situation where mate choice was possible. The presence of spermatophores was further found to reduce slugs' ability to interrupt matings, and displace other slugs. Body size, however, had no direct effect on displacement in A. glauca as small slugs interrupted matings as successfully as large ones. Furthermore, pair formation and mating were found to be random with respect to size. This was true also for pairs formed in the field. In addition to mate choice hermaphrodites may increase their reproductive success by differential sex allocation. I tested whether differing mate encounter rates had any effects on allocation to male and female function in A. glauca. Slugs with more mating opportunities mated more, and had higher proportional spermatophore production that others. As predicted they also laid significantly fewer eggs than slugs presented with partners less often.