A novel evolutionary pattern of reversed sexual dimorphism in fairy wrens: implications for sexual selection

Reversed sexual dimorphism (females being larger than males) occurs in several bird groups, including hawks and vultures (Accipitridae), falcons (Falconidae), sandpipers and snipe (Scolopacidae), phalaropes (Charadriidae), jacanas (Ja-canidae), skuas (Stercorariidae), boobies (Sulidae), frigate birds (Fregatidae), owls (Strigiformes), cuckoos (Cuculidae), hummingbirds (Trochilidae), manakins (Pipridae), and some ratites (Struthioniformes). In most cases, reversed sexual di-morphism (RSD) is present in many traits, and hence selection has been presumed to act non-independently on several characters (Lande and Arnold, 1983). Hence, RSD has commonly been discussed in terms of differences in body size (Mueller, 1990). In this study, we report a novel pattern of RSD in fairy wrens (Maluridae), which has important evolutionary implications for the ways in which sexual dimorphism can occur and the mechanisms of sexual selection. We examined patterns of morphological sexual dimor-phism, based on published data (Rowley and Russell, 1997; Schodde, 1982) and our own measurements (described later), within a molecular phylogeny for the Maluridae (Christidis and Schodde, 1997). This analysis revealed at least two independent occurrences of RSD in tail length (Figure 1). In one of these cases, the orange-crowned fairy wren Clytomyias in-signis, the reversed dimorphism is associated with a small relative increase in tarsus and wing length in the female, as is commonly observed in species with RSD. However, reversed tail dimorphism in red-backed, white-shouldered and white-winged fairy wrens (M. melanocephalus, M. alboscapulatus, and Malurus leucopterus, respectively) is not associated with an increase in size of other female traits (Table 1). In the first two species, the male is larger in terms of tarsus and wing length, but the female has a significantly longer tail. Two island subspecies of the third species (M. l. leucopterus and M. l. edouardi) show the same pattern, but the mainland subspe-cies (which we have used as the designate species for the phy-logeny) appears monomorphic in terms of tail length (M. l. leuconotus; Table 1). Although tail-length differences have previously been noted for fairy wren species (Schodde, 1982), the classification of RSD has not been formally described or quantitatively studied in any of these species. As far as we are aware, the RSD of a single trait (in an opposite direction to the body size dimorphism) is a novel pattern of evolution of tail elon-gation in birds. In a survey of published incidences of RSD, we could not find a single account that matched the morphological patterns observed in this cluster of three …