Ecology and evolution of extravagant feather ornaments

Despite only a fraction of all organisms having evolved extravagant secondary sexual characters, these have played an important conceptual, theoretical and empirical role in evolutionary biology (Darwin, 1871; Cronin, 1991; Andersson, 1994). Secondary sexual characters such as the antlers of deer and the train of the blue peacock Pavo cristatus are presumed to have arisen and be maintained as a result of directional sexual selection. Individuals with more extreme character values are expected to bene®t in terms of male±male competition and/or female choice, respectively. An evolutionary understanding of sexual selection not only requires knowledge of the maintenance, but also of the origin of the characters in question. Almost the entire current surge in interest of sexual selection has been focused on the maintenance of secondary sexual characters based on observational and experimental studies of male±male competition and female choice (Andersson, 1994; Mùller, 1994). The origin of extravagant ornamentation is, however, equally interesting for a complete understanding of sexual selection. Andersson (1994) in his recent review of sexual selection listed only ®ve comparative studies of sexual ornamentation (Clutton-Brock et al., 1977, 1980; HoÈglund, 1989; Mùller & Pomiankowski, 1993; HoÈglund & SilleÂn-Tullberg, 1994), and none of these addressed the question of the ecological conditions being associated with the evolution of extravagant ornamentation. Models of sexual selection can be classi®ed in different ways (Andersson, 1994; Mùller, 1994), but here we adopt a classi®cation based on the kinds of bene®ts acquired by individuals of the choosy sex, usually females. Fitness bene®ts obtained by choosy individuals are either direct or indirect. Direct ®tness bene®ts can be obtained in terms of parental care, a high fertilizing ability, absence of contagious parasites, territory quality, nuptial gifts or other kinds of resources (reviews in Andersson, 1994; Mùller, 1994). Indirect ®tness bene®ts are either so-called good genes that enhance the viability of offspring (Zahavi, 1975, 1977; Hamilton & Zuk, 1982; Andersson,