Cryptic sexual size dimorphism in Malagasy plovers Charadrius spp.

Abstract

Many small plovers Charadrius spp. have sexually monomorphic plumage and cryptic sexual size dimorphism. The objective of our study was to assess the variation in body sizes between male and female plovers breeding in Madagascar. We collected blood samples and data on adult body sizes of four small plovers (Madagascar Plover Charadrius thoracicus, Kittlitz's Plover C. pecuarius, White-fronted Plover C. marginatus and Three-banded Plover C. tricollaris), and used molecular genetic markers to sex the adults. We found significant differences in body size among the four species, and between sexes. Furthermore, individuals from the southern ecoregion tended to be larger than in the western ecoregion. The Madagascar Plover's body size was significantly more dimorphic than the Kittlitz's and White-fronted Plovers. Breeding Malagasy plovers' show significant sexual size dimorphism (SSD): Madagascar Plover females were heavier and had longer wings than males, whereas the males had longer tarsi; in White-fronted Plover only wing length was different between the sexes. Taken together, our work reports SSD in small African plovers that exhibit monomorphic plumage, and we propose that SSD may be more common than currently acknowledged; we term this 'cryptic sexual size dimorphism'. Our results also suggest sexual selection and/or natural selection exert different pressures on body size in different Malagasy plover species.