Many conspicuous ornamental traits in animals are expressed in both males and females. Despite this, most research has focused on sexually dimorphic ornamentation. Mutual ornamentation has often been viewed as a result of either a nonadaptive genetic correlation between the sexes or similar selection pressures in both sexes. Here, we review the theoretical underpinning and empirical evidence for these ideas. Few studies have attempted to test empirically whether a genetic correlation between the sexes can constrain the evolution of sexual dimorphism, and the results have been mixed. By contrast, there is good evidence that mutual ornaments can have a signal function in both sexes, especially in terms of mate choice. Other possible signalling functions have received little attention. Social status signalling is especially likely to be important, because competition over nonsexual resources is more balanced between the sexes than sexual competition. There is a need for experimental studies that explicitly test these hypotheses simultaneously in both sexes.

The spectacular diversity in sexually selected traits in the animal kingdom has inspired the hypothesis that sexual selection can promote species divergence. In recent years, several studies have attempted to test this idea by correlating species richness with estimates of sexual selection across phylogenies. These studies have yielded mixed results and it remains unclear whether the comparative evidence can be taken as generally supportive. Here, we conduct a meta-analysis of the comparative evidence and find a small but significant positive overall correlation between sexual selection and speciation rate. However, we also find that effect size estimates are influenced by methodological choices. Analyses that included deeper phylogenetic nodes yielded weaker correlations, and different proxies for sexual selection showed different relationships with species richness. We discuss the biological and methodological implications of these findings. We argue that progress requires more representative sampling and justification of chosen proxies for sexual selection and speciation rate, as well as more mechanistic approaches.

The results presented in this paper describe the short-and long-term toxicity of arsenate in Silene vulgaris. Shortterm toxicity, measured as inhibition of root elongation, depended on phosphate nutrition, arsenate being much less toxic at high phosphate supply. At low phosphate levels more arsenic was taken up by the plants. Under chronic exposure, toxicity (measured as inhibition of biomass production) did not increase with time. In addition, the accumulation of phytochelatins (PCs) as a function of toxicity and duration of exposure was studied. Shortterm PC accumulation (over a 3 d period) was positively correlated with exposure. Isolation of peptide complexes from prolongedly exposed plants showed that PC # , PC $ and PC % were present, although the latter not until at least 3 d exposure. Arsenic co-eluted mainly with PC # and PC $ . Fractions containing PC % were devoid of As, probably due to dissociation of the complexes during extraction or elution. The breakdown of PCs after arresting As exposure was very slow. This could explain the continuous accumulation of PCs throughout longer periods of As exposure.Key words : Silene vulgaris, arsenic toxicity, arsenic detoxification, phytochelatins. Arsenic is taken up mainly by plant roots as arsenate (AsO % $−) (Macnair & Cumbes, 1987) through the phosphate-uptake system (Asher & Reay, 1979). Once the arsenate, As(V), is taken up it is reduced to arsenite, As(III), by glutathione (GSH) (Thompson, 1993). Only in phosphate-deficient conditions is arsenate subsequently methylated in plants (Nissen & Benson, 1982). Between the successive methylation steps, GSH serves to reduce the intermediate products (Scott et al., 1993 ;Thompson, 1993). Arsoniumphospholipids in freshwater plants (Nissen & Benson, 1982) and arsenic sugars in marine brown algae (Edmonds & Francesconi, 1981) have also been identified.Mostly there is little transport of As to the aboveground parts of the plants. Dicotyledonous plants appear to transport more As to the shoots than monocotyledonous plants (Otte, 1991). The form in *Author for correspondence (fax j31 20 444 7123 ; e-mail elsesnel!bio.vu.nl). which the As is transported is unknown. There is some indication that dimethylarsenic acid is transported to the shoots (Marin et al., 1993).Increased As levels may cause toxic symptoms in plants, such as a decrease in plant growth and fruit yield (Carbonell-Barrachina et al., 1995), root discoloration and root plasmolysis, wilting and necrosis of leaf tips and leaf margins (Machlis, 1941), and a decrease in photosynthetic capacity (Marin et al., 1993).Some authors have reported the accumulation of heavy metal-binding, thiol-rich phytochelatins (PCs) on exposure to As (Grill et al., 1986a(Grill et al., ,b, 1987Maitani et al., 1996). Phytochelatins have the structure (γ-glu-cys) n -gly, where n l 2-11 (Grill et al., 1985), and are produced in plants on exposure to a variety of heavy metals and metalloids (Gekeler et al., 1989). Phytochelatins are synthesized from GSH (Hayashi et ...

The Seychelles Warbler Acrocephalus sechellensis is a rare island endemic which, from 1920 to 1988, occurred only on Cousin Island (29 ha) in the Seychelles. Despite the saturated nature of this population and the possibility of obtaining higher reproductive success on new nearby islands, inter‐island dispersal by Seychelles Warblers is extremely rare (0.10%). We test the hypothesis that Seychelles Warblers show an adaptation typical for island birds: a low‐cost reduced‐size flight apparatus. We compared the anatomy of the flight apparatus (wing shape, wing loading, skeletal parts and musculature) of Seychelles Warblers with that of three closely related migratory Acrocephalus species (Eurasian Reed Warbler A. scirpaceus, Australian Reed Warbler A. australis and African Reed Warbler A. baeticatus). Seychelles Warblers do not differ from the migratory warblers in pectoral mass and skeletal attachment area relative to body mass, wing shape and wing loading. Seychelles Warblers show the morphological structures required for sustained flight, but may have the behavioural reluctance to cross what they may regard as extensive bodies of water.