The Great Bustard (Otis tarda) is one of the heaviest flying birds and the most sexually dimorphic living bird. Adult males weighed 2.48x more than females, and their linear measurements were 18–30% larger. Weight increased between the prebreeding and breeding seasons by 16% in females and 20% in males. Sexual size dimorphism emerges very early in development and explains why growth in males is so costly. Weight and central toe length were hyperallometric when related to wing length in males but isometric in females and varied more in males, as compared with females and with other male traits. Although hyperallometry and high variability have frequently been used to invoke sexual selection as a driving force, our results support different functional hypotheses for the evolution of each trait. Male—male competition is intense in this lekking species, and high rank among males and access to females are weight-dependent. Thus, sexual selection has likely pushed male weight close to the limit imposed by powered flight. Because Great Bustards are mostly cursorial, the hyperallometry of the central toes of males in relation to wing length most likely evolved for support and balance.
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