Sex roles and reproductive behaviour in gobies and guppies : a female perspective
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- Institutt for biologi 
This thesis focuses on reproductive dynamics, behaviour and selection from a female perspective. The aim was to explore variation in sex roles and potential for selection on females, as well as the selective forces working on females, within a sexual selection framework. One subject that has received particular interest in this thesis is environmental influence on reproductive behaviour and sex roles and how this affects selection on females. In a field study on common gobies (Pomatoschistus microps) availability of nest sites seemed to affect which sex was courting (Paper I). When nests were abundant males courted and competed over matings, whereas when nests were scarce males competed over nest sites, while females courted and competed over matings. In the two-spotted goby (Gobiusculus flavescens), female reproductive behaviour was influenced by male availability (Paper II). In our population, the change in male availability was caused by a decline in the operational sex ratio (OSR) as the season progressed. Thus, early in the season males were abundant and also the courting sex, whereas late in the season few males were around and females were courting the few males available. The shift in the OSR over the season potentially affects the opportunity for females to be choosy. This could occur because of a reduced number of males available, but also because males may become choosy when the sex ratio is female biased. Females might be expected to adjust their choosiness to fit the current potential for mate choice. As expected, when given a choice between two males varying in size in a laboratory study, female two-spotted gobies preferred large males early in the season but reduced their choosiness for male size later on in the season (Paper III). Selection on females should also be affected by male behaviour, such as male mate choice. Male two-spotted gobies were found to slightly prefer larger females in a two-choice study performed from mid to end of the season (Paper IV). Therefore, male choice should produce only a minor selection pressure on female size in this species. Depending on ecological circumstances females are expected to interact and compete with each other to a lesser or higher degree. To test the effect of female-female interactions on female resource allocation to reproduction and growth, female guppies (Poecilia reticulata) were held in different competitive situations (either alone, with large, or small competitors). There was no effect of treatment on growth, however females in the competitive treatments produced fewer mature eggs than females held alone in spite of ad lib. food levels (Paper V). It therefore seems as if female guppies spend resources on interactions with other females and as if this negatively affects female reproductive success. This thesis illustrates that there are several selection forces working on females within sexual selection. Sex roles and behaviours in relation to reproduction show adaptive plasticity in response to environmental factors. The OSR can be linked to environmental conditions and sex roles, however the usefulness of OSR for predicting sex roles is questioned. Life history aspects are important for reproductive behaviour and fitness and should be taken into account when studying sexual selection. Since sex roles are flexible within a system, the distinction between conventional and reversed sex roles may be misleading. Furthermore, studying selection processes and consequences on both sexes provides a fuller understanding of sexual selection, sex roles and mating patterns.