Rodrigues Solitaire - Taxonomy and Evolution

Taxonomy and Evolution

François Leguat was the first to refer to the bird as the "Solitaire" (referring to its solitary habits), but it has been suggested that he borrowed the name from a tract mentioning the Réunion Solitaire. The bird was first scientifically named as a species of Dodo (Didus solitarius, based on Leguat's description) by Johann Friedrich Gmelin in the thirteenth edition of Systema Naturae.

Hugh Edwin Strickland and Alexander Gordon Melville suggested the common descent of the Solitaire and the Dodo in 1848. They dissected the only known Dodo specimen with soft tissue, comparing it with the few Solitaire remains then available. Strickland stated that although not identical, these birds shared many distinguishing features in the leg bones otherwise only known in pigeons. The fact that the Solitaire laid only one egg, fed on fruits, was monogamous and cared for its nestlings also supported this relationship. Strickland recognized its generic distinction and named the new genus Pezophaps ("pedestrian pigeon" in ancient Greek). The differences between the sexes of the bird were so large that Strickland thought they belonged to two species, naming the smaller bird Pezophaps minor. Later study of skeletal features by Alfred and Edward Newton indicated that the Solitaire was morphologically intermediate between the Dodo and ordinary pigeons, but differed from them in its unique wrist-knob.

For a long time the Dodo and the Rodrigues Solitaire (collectively termed "didines") were placed in a family of their own, the Raphinae; this was because their relationships to other pigeons was unresolved. They were also placed in a monotypic family each, Raphidae and Pezophapidae, respectively, due to the suggestion that hey had evolved their similar features independently. Recently, osteological and molecular data led to the dissolution of the family Raphidae, and the Dodo and Solitaire were placed in their own subfamily, Raphinae, nested within the Columbidae.

Comparison of mitochondrial cytochrome b and 12S rRNA sequences isolated from the femur of a Rodrigues Solitaire and the tarsal of a Dodo confirmed their close relationship, and their placement within the Columbidae. Genetic evidence was interpreted to show that the Southeast Asian Nicobar Pigeon is their closest living relative, followed by Crowned Pigeons of New Guinea, and the superficially Dodo-like Tooth-billed Pigeon from Samoa. The following cladogram, from Shapiro et al. (2002), shows the Dodo's closest relationships within Columbidae, a clade consisting of generally ground-dwelling island endemics.

A similar cladogram was published in 2007, differing in the inverted placement of Goura and Dicunculus, as well as the inclusion of the Pheasant Pigeon and the Thick-billed Ground Pigeon at the base of the clade. The 2002 study indicated that the ancestors of the Solitaire and the Dodo diverged around the Paleogene-Neogene boundary. The Mascarene Islands (Mauritius, Réunion, and Rodrigues) are of volcanic origin and less than 10 million years old, so both birds' ancestors likely remained capable of flight long after the separation of their lineages. The lack of mammalian herbivores competing for resources on these islands allowed the Solitaire and the Dodo to attain large size. The DNA obtained from the Oxford specimen is degraded, and no usable DNA has been extracted from fossil remains, so these findings still need to be independently verified.

A few reports mention other "Solitaires" from the Mascarenes. The term was also used for other species with "solitary" habits, such as the Réunion Ibis. Some scientists believed that Réunion was home to not only a white Dodo but also a white Solitaire (both of which are now believed to be misinterpretations of old reports). An atypical 17th-century description of a Dodo and bones found on Rodrigues (now known to have belonged to the Rodrigues Solitaire) led Abraham Dee Bartlett to name a new species, Didus nazarenus; it is now a junior synonym of this species.