Sexual size dimorphism in the Common Crane, a monogamous, plumage-monomorphic bird

Male-biased sexual size dimorphism (SSD) is common and related to male mating success in polygynous birds, but also occurs in monogamous species, in which sexual selection can be strong. In these species, SSD inmorphological traits not directly related to reproductive function, such as locomotory or feeding structures, has remained difficult to explain. We present the first biometric data of an archetypalmonogamous bird, the Common Crane (Grus grus), based on molecular sexing techniques. Males were heavier and larger than females, with weight showing the largest SSD, followed by wing, central toe, and head length in adults and juveniles. These sexual differences were also present in nine crane pairs where both adults were measured, with values being positively correlated between pair members, suggesting assortative mating and female choice as the main mechanisms driving the evolution of SSD. Since central toes and bills are used as weapons during male-male combats, intra-male competition could also be involved as a sexual selection mechanism. Our results suggest that these small but significant sexual differences in certain morphological traits have probably evolved mainly through sexual selection in this monogamous, plumage-monomorphic species.