Structure and variation in the hindlimb musculature of the woodcreepers (Aves: Passeriformes: Dendrocolaptinae)

The Dendrocolaptinae (woodcreepers), a clade of neotropical passerine birds, form an adaptive radiation with a spectrum of body sizes and bill shapes. Woodcreepers are scansorial, climbing vertical tree trunks supported by their forward toes and stiffened tail. The hindlimb musculature was dissected and described for 42 of the 50 species representing all genera, and for 14 outgroup species. Structural, functional, developmental and evolutionary aspects of muscular variations are analysed. Woodcreepers have extensive ossification of leg tendons. There is intraspecific variation in the degree of ossification, and interspecific variation in the occurrence of ossification between muscles. Intraspecific variation in muscle structure was apportioned according to a published classification. Nine muscles showed variation of the minor, singular, mimicking and incongruous types, but explosive variation was lacking. Some muscles are more prone to variation than others. Ten muscles showed interspecific variations of four types, for which new terms are proposed: occurrence variations; attachment variations in origins and insertions; structural variations in size, shape, or fibre arrangement; and relational variations with other muscles. Variations in the presence of a muscle component did not occur. Discrimination of intraspecific variations from interspecific variations is discussed.