Sexual selection in the sex-role reversed Eurasian dotterel Charadrius morinellus.

Current theory concerning the evolution of parental care posits that one or both of the sexes may reduce parental care when the fitness gained from alternative reproductive opportunities exceeds that from continuing with parental care. If this situation leads to a difference between the sexes in potential rate of reproduction, intra-sexual competition for mates will occur in the sex with the higher potential reproductive rate, and the other sex will display mate choice (called inter-sexual selection). In the majority of avian species biparental care is the most common form of parental care. Polygyny is the most common consequence of polygamy; males compete for access to females and females exercise mate choice. Sex-role reversal is rare (ca. 3% of species in the world) but presents an opportunity to test the basis of theories of the evolution of parental care and sexual selection. This thesis presents the results of a three-year study of sexual selection and sex-role reversal in a breeding population of the Eurasian dotterel, Charadrius morinellus, in the Cairngorm region of Scotland. In this species, the female is the competitive sex and the male usually provides all of the parental care; females are sequentially polyandrous. Dotterel are dependent on a widely dispersed breeding habitat. Rather than defending territories, females compete for access to males on mating arenas (Chapter 2). After producing a clutch, females return to the arenas in order to compete for further males, this leads to a female-biased skew in the operational sex ratio (Chapter 3). The proximate factors which produces the skew in the sex ratio predict the temporal variation in the potential for sexual selection (Chapter 3). There is considerable intra-sexual plumage variation which is correlated with behavioural variability (Chapter 4). Bright females are more aggressive than dull females and thus initiate and win more fights (Chapter 4). Bright females also perform more courtship than do dull females and therefore get mates earlier in the season, as, via assortative mating, do bright males (Chapter 4). Females are, however, also the more choosy sex; females prefer to court bright males which are, in turn, likely to be in better physical condition than duller males (Chapter 4). Bright males are less likely to desert the clutch in bad weather conditions (Chapter 5). Females therefore appear to choose males on the basis of their ability to incubate their clutch. Male dotterel, because of their high level of parental care and relatively low potential rate of reproduction, are expected to exhibit behaviour which has been selected to protect their paternity. This prediction is upheld; male dotterel assure paternity of the brood for which they care through a combined strategy of prolonged, close mate-guarding and strategically timing copulations to the few days immediately before egg-laying (Chapter 6). Active female choice for male quality related to phenotypic traits which are also influenced by the environment is contrary to the predictions of the conventional theory of mate choice. In sex-role reversed species, because the potential reproductive rate of the male is lower, male choice is expected. A new model of mate choice is introduced and used to study the effects of differences between the sexes in both the relative potential reproductive rate and relative variation in mate quality (Chapter 7). This model is successful in predicting the form of mate choice observed for all permutations of variation in potential reproductive rate and mate quality. Finally, sex-role reversed mating arenas are compared with polygynous lekking (Chapters 3,4,7). Constraints on the potential reproductive rate of females determine that the direction of mate choice will only rarely be reversed in non-territorial sex-role reversed species (Chapter 7). Therefore, although the mating arenas of dotterel resemble leks, an important component of true lekking, active mate choice by the non-competitive sex, is absent.