Male sticklebacks display multiple ornaments, and these ornaments have been shown to be preferred by females in laboratory experiments. However, few field data exist, and it is not known whether these preferences are simultaneously or sequentially operative in a single population. We report correlates of reproductive success in two stickleback populations that differ in their ecology, over several periods within their breeding season. In both populations larger males had higher reproductive success, but not in all periods of the breeding season. Reproductive success increased with redness of the throat only in the Wohlensee population, and only in one period that was characterized by low average success. In the Wohlensee population, the parasitic worm Pomphorhynchus laevis is abundant, and reproductive success decreased with the presence of the parasite. In the Roche population, males with nests concealed in a plant had higher mating success. These nests were less likely to fail, suggesting that females preferred to spawn in concealed nests because of higher offspring survivorship. The different sexual traits appear to reveal different aspects of male quality (multiple message hypothesis): females probably find large males attractive because of their higher paternal quality, but it seems more likely that red males are preferred for better genetic qualities. Females also discriminate on territory quality, and male traits may be important in competition for these territories. The correlates of reproductive success were not consistent during the season, probably due to changes in the availability of ripe females. Such fluctuating selection pressures will contribute to the maintenance of genetic variation in sexual traits.

Vertebrates have various sex determining mechanisms. These have been broadly classified as either genotypic sex determination (GSD) or environmental sex determination (ESD). This terminology, however, may obscure the fact that mixtures between genotypic and environmental sex determination exist, or that genotypic and environmental sex determination may, in fact, be the extremes of a continuum. Sex ratio evolution plays an important role in the evolution of sex determining mechanisms. 7.2 Introduction This chapter starts with the proximate aspects of sex determining mechanisms (section 7.3). We introduce the traditional classification of sex determining mechanisms that exist in vertebrates (section 7.3.1) and the distribution of mechanisms among extant vertebrate taxa (section 7.3.2). At phylogenetically shallow levels, different mechanisms are present. We describe how the existence of either male or female heterogamety, or environmental sex determination is usually established for individual species or taxa (section 7.3.3). Cases of mixed sex determination, i.e. combinations of genotypic and environmental sex determination are also observed (section 7.3.4) and we caution this phenomenon has implications for sex identification by molecular markers (section 7.3.5). We stress that phenotypic sex generally has environmental and genetic components and discuss a model that attempts the unification of sex determination by stating that sex determination in all vertebrates is mediated by differential growth of the embryo (section 7.3.6). In the second part of the chapter we discuss the evolution of sex-determining mechanisms. Evolution from one system to another can be quite rapid (section 7.4). We stress that sex ratio selection plays an important role in the evolution of sex determining mechanisms (section 7.4.1). This usually leads to sex determining mechanisms that produce an unbiased sex ratio, but under some conditions mechanisms that bias the sex ratio are favoured. We conclude the chapter with an illustration of how one can investigate verbal models of the evolution of sex determination by means of mathematical models. We present a simulation model with which we

In July 1999, 49 three-spined sticklebacks were sampled from Enos Lake, Canada. Here a benthic/limnetic species pair is known to exist. In the sample, six individuals (12%) were of intermediate phenotype. These represent 17% of the 35 males in the sample. This large number of intermediates contrasts with the 1% in previous samples reported in 1984 and 1992 and may signal the collapse of the species pair. 2001 The Fisheries Society of the British Isles

Individual male three-spined sticklebacks in the field, often collected eggs for longer (up to 10 days) than had been reported for sticklebacks in captivity (3-6 days). The probability that a male stopped collecting eggs increased with the number of eggs already present, and possibly with the age of the eggs. Males with nests hidden under a plant were more likely to continue collecting than males with exposed nests. These results are discussed in the light of theoretical considerations that predict when males should stop collecting further eggs. 1999 The Fisheries Society of the British Isles

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