A comparative study on the evolution of reversed size dimorphism in monogamous waders

Sexual dimorphism in size is common in birds. Males are usually larger than females, although in some taxa reversed size dimorphism (RSD) predominates . Whilst direct dimorphism is attributed to sexual selection in males givin g greater reproductive access to females, the evolutionary causes of RSD ar e still unclear. Four different hypotheses could explain the evolution of R SD in monogamous birds: (1) The 'energy storing' hypothesis suggests that l arger females could accumulate more reserves at wintering or refuelling are as to enable an earlier start to egg laying. (2) According to the 'incubati on ability' hypothesis, RSD has evolved because large females can incubate more efficiently than small ones. (3) The 'parental role division' hypothes is suggests that RSD in monogamous waders has evolved in species with paren tal role division and uniparental male care of the chicks. It is based on t he assumption that small male size facilitates food acquisition in terrestr ial habitats where chick rearing takes place and that larger females can ac cumulate more reserves for egg laying in coastal sites. (3) The 'display ag ility' hypothesis suggests that small males perform better in acrobatic dis plays presumably involved in mate choice and so RSD may have evolved due to female preference for agile males. I tested these hypotheses in monogamous waders using several comparative methods. Given the current knowledge of t he phylogeny of this group, the evolutionary history of waders seems only c ompatible with the hypothesis that RSD has evolved as an adaptation for inc reasing display performance in males. In addition, the analysis of wing sha pe showed that males of species with acrobatic flight displays had wings wi th higher aspect ratio (wing span(2)/wing area) than non-acrobatic species, which probably increases flight manoeuvrability during acrobatic displays. In species with acrobatic displays males also had a higher aspect ratio th an females although no sexual difference was found in non-acrobatic species . These results suggest that acrobatic flight displays could have produced changes in the morphology of some species and suggest the existence of sele ction favouring higher manoeuvrability in species with acrobatic flight dis plays. This supports the validity of the mechanisms proposed by the 'displa y agility' hypothesis to explain the evolution of RSD in waders. (C) 1999 T he Linnean Society of London.