Pezophaps Solitarius
Rodrigues solitaires grew to the size of swans, and demonstrated pronounced sexual dimorphism. Males were much larger than females and measured up to 75.7–90 centimetres (30–35 inches) in height and 28 kilograms (62 pounds) in weight, contrasting with 63.8–70 centimetres (25–28 in) and 17 kilograms (37 lb) for females. Its plumage was grey and brown; the female was paler than the male. It had a black band at the base of its slightly hooked beak, and its neck and legs were long. Both sexes were highly territorial, with large bony knobs on their wings that were used in combat. The Rodrigues solitaire laid a single egg that was incubated in turn by both sexes. Gizzard stones helped digest its food, which included fruit and seeds.
First mentioned during the 17th century, the Rodrigues solitaire was described in detail by François Leguat, the leader of a group of French Huguenot refugees who were marooned on Rodrigues in 1691–1693. It was hunted by humans and introduced animals, and was extinct by the late 18th century. Apart from Leguat's account and drawing, and a few other contemporary descriptions, nothing was known about the bird until a few subfossil bones were found in a cave in 1786. Thousands of bones have subsequently been excavated. It is the only extinct bird with a former constellation named after it, Turdus Solitarius.
Taxonomy
The French explorer 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 1689 tract by his sponsor Marquis Henri Duquesne, which used the name "solitaire" in reference to the Réunion ibis. The bird was first scientifically named in 1789 as a species of dodo (Didus solitarius, based on Leguat's description) by the German naturalist Johann Friedrich Gmelin in the thirteenth edition of Systema Naturae. In 1786, subfossil Rodrigues solitaire bones encrusted in stalagmite were discovered in a cave and sent to the French naturalist Georges Cuvier in about 1830. For unknown reasons, he stated they had recently been found on Mauritius, which caused confusion, until they were compared with other bones from Rodrigues that were found to belong to the same species.
The English naturalists Hugh Edwin Strickland and Alexander Gordon Melville suggested the common descent of the Rodrigues solitaire and the dodo in 1848. They dissected the only known dodo specimen with soft tissue, comparing it with the few Rodrigues 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 Rodrigues solitaire laid only one egg, fed on fruits, was monogamous and cared for its nestlings also supported this relationship. Strickland recognised its generic distinction and named the new genus Pezophaps, from ancient Greek pezos (πεζός 'pedestrian') and phaps (φάψ 'pigeon'). The differences between the sexes of the bird were so large that Strickland thought they belonged to two species, naming the smaller female bird Pezophaps minor.
Additional subfossils were recovered during the 1860s, but more complete remains were found during the 1874 transit of Venus, since an observation station was located on the island. Many of these excavations were requested by the English ornithologists (and brothers) Alfred and Edward Newton, who used them to describe the osteology of the bird in detail. Thousands of bones were excavated, and mounted skeletons were composed from the remains of several specimens. Study of skeletal features by the Newtons indicated that the solitaire was morphologically intermediate between the dodo and ordinary pigeons, but differed from them in its unique carpal knob.
Some scientists believed that Réunion was home not only to a white dodo, but also to a white bird similar to the Rodrigues solitaire, both of which are now believed to be misinterpretations of old reports of the Réunion ibis. An atypical 17th-century description of a dodo and bones found on Rodrigues, now known to have belonged to the Rodrigues solitaire, led the British taxidermist Abraham Dee Bartlett to name a new species, Didus nazarenus; it is now a junior synonym of this species.
At one point it was suggested that the skeleton of this species is the best described after that of humans. In spite of the evidence, some later scholars doubted Leguat's story, and the existence of the Rodrigues solitaire. In 1921, the American linguist Geoffroy Atkinson claimed Leguat's memoir was merely a novel, and that the man had never even existed, and in 1955, the British ecologist George Evelyn Hutchinson doubted aspects of the bird's biology mentioned by Leguat. Today, it is widely accepted that Leguat's memoirs are credible observations of the bird in life.
For many years the dodo and the Rodrigues solitaire were placed in a family of their own, the Raphidae (formerly Dididae), because their exact relationships with other pigeons were unresolved. Each was also placed in a monotypic family (Raphidae and Pezophapidae, respectively), as it was thought that they had evolved their similarities independently. Osteological and DNA analysis has since led to the dissolution of the family Raphidae, and the dodo and solitaire are now placed in their own subfamily, Raphinae, within the family Columbidae.
Evolution
In 2002, American geneticist Beth Shapiro and colleagues analysed the DNA of the dodo and the Rodrigues solitaire for the first time. 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. The genetic evidence was interpreted as showing the Southeast Asian Nicobar pigeon (Caloenas nicobarica) to be their closest living relative, followed by the crowned pigeons (Goura) of New Guinea, and the superficially dodo-like tooth-billed pigeon (Didunculus strigirostris) from Samoa. This clade consists of generally ground-dwelling island endemic pigeons. The following cladogram shows the closest relationships of the dodo and the Rodrigues solitaire within Columbidae, based on Shapiro et al., 2002:
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A similar cladogram was published in 2007, inverting the placement of Goura and Didunculus and including the pheasant pigeon (Otidiphaps nobilis) and the thick-billed ground pigeon (Trugon terrestris) at the base of the clade. Based on behavioural and morphological evidence, Jolyon C. Parish proposed that the dodo and Rodrigues solitaire should be placed in the Gourinae subfamily along with the Goura pigeons and others, in agreement with the genetic evidence In 2014, DNA of the only known specimen of the recently extinct spotted green pigeon (Caloenas maculata) was analysed, and it was found to be a close relative of the Nicobar pigeon, and thus also the dodo and Rodrigues solitaire.
The 2002 study indicated that the ancestors of the Rodrigues solitaire and the dodo diverged around the Paleogene–Neogene boundary. The Mascarene Islands (Mauritius, Réunion, and Rodrigues), are of volcanic origin and are less than 10 million years old. Therefore, the ancestors of both birds probably remained capable of flight for a considerable time after the separation of their lineage. The Nicobar and spotted green pigeon were placed at the base of a lineage leading to the Raphinae, which indicates the flightless raphines had ancestors that were able to fly, were semi-terrestrial, and inhabited islands. This in turn supports the hypothesis that the ancestors of those birds reached the Mascarene islands by island hopping from South Asia. The lack of mammalian herbivores competing for resources on these islands allowed the solitaire and the dodo to attain very large sizes. The dodo lost the ability to fly owing to the lack of mammalian predators on Mauritius. Another large, flightless pigeon, the Viti Levu giant pigeon (Natunaornis gigoura), was described in 2001 from subfossil material from Fiji. It was only slightly smaller than the Rodrigues solitaire and the dodo, and it too is thought to have been related to the crowned pigeons.
Description
The beak of the Rodrigues solitaire was slightly hooked, and its neck and legs were long. One observer described it as the size of a swan. The skull was 170 mm (6.7 in) long, flattened at the top with the fore and hind parts elevated into two bony ridges structured with cancellous bone. A black band (a contemporary description described it as a "frontlet") appeared on its head just behind the base of the beak. The plumage of the Rodrigues solitaire was described as grey and brown. Females were paler than males and had light-coloured elevations on the lower neck.
Sexual size dimorphism in this species is perhaps the greatest in any neognath bird. Males were considerably larger than females, measuring 75.7–90 centimetres (30–35 inches) in height and weighing up to 28 kilograms (62 pounds), whereas females were 63.8–70 centimetres (25–28 in) and weighed 17 kilograms (37 lb). This is only 60% of the weight of a mature male. Their weight may have varied substantially due to fat cycles, meaning that individuals were fat during cool seasons, but slim during hot seasons, and may have been as low as 21 kg in males and 13 kg in females. Though male pigeons are usually larger than females, there is no direct evidence for the largest specimens actually being the males of the species, and this has only been assumed based on early works. Though the male was probably largest, this can only be confirmed by molecular sexing techniques, and not skeletal morphology alone.
Members of both sexes possessed a large tuberous knob of bone exostosis situated at the base of the carpometacarpus of each wrist. Other wing bones also sometimes show similar structures. The knob was cauliflower-like in appearance, and consisted of up to two or three lobes. The knobs were about half the length of the metacarpus, were larger in males than females, and described as the size of a musket ball. One study measured the largest knob to be 32.9 millimetres (1.30 in) in diameter. The knobs vary in size across individuals, and were entirely absent from 58% of specimens examined for the study. These are thought to be immature birds, or birds without territory. The carpometacarpi of males without the knobs were smaller on average than those with it, but there was little difference between the females. In life, the knobs would have been covered by tough cartilaginous or keratinous integument, which would have made them appear even larger. Carpal spurs and knobs are also known from other extant as well as extinct birds. Within Columbidae, the crowned pigeons and the Viti Levu giant pigeon have outgrowths on the carpometacarpus which are similar to those of the female Rodrigues solitaire. Other well known examples are the steamer ducks, the torrent duck, sheathbills, screamers, the spur-winged goose, and the extinct Jamaican ibis, Xenicibis xympithecus.
The Rodrigues solitaire shared traits with the dodo, its closest relative, such as size and features in the skull, pelvis, and sternum. It differed in other aspects; it was taller and more slender than the dodo and had a smaller skull and beak, a flatter skull roof, and larger orbits. Its neck and legs were proportionally longer, and the dodo did not possess an equivalent to the carpal knob of the Rodrigues solitaire. Many skeletal features of the Rodrigues solitaire and dodo that are unique among pigeons have evolved to adapt to flightlessness. Their pelvic elements were thicker than those of flighted birds (to support their greater weight), and their pectoral region and wings were paedomorphic (underdeveloped, retaining juvenile features). However, the skull, trunk, and pelvic limbs were peramorphic, which means they changed considerably with maturity.
Contemporary descriptions
Apart from Leguat's rather simple depiction, the life appearance of the Rodrigues solitaire is only known from a handful of descriptions; no soft-tissue remains survive. Leguat devoted three pages of his memoirs to the Rodrigues solitaire, and was clearly impressed by the bird. He described its appearance as follows:
Of all the Birds in the Island the most remarkable is that which goes by the name of the solitary, because it is very seldom seen in company, tho' there are abundance of them. The Feathers of the Males are of a brown grey Colour: the Feet and Beak are like a Turkey's, but a little more crooked. They have scarce any Tail, but their Hind-part covered with Feathers is roundish, like the Crupper [rump] of a horse; they are taller than Turkeys. Their Neck is straight, and a little longer in proportion than a Turkey's when it lifts up his Head. Its Eye is Black and lively, and its Head without Comb or Cop. They never fly, their Wings are too little to support the Weight of their Bodies; they serve only to beat themselves, and flutter when they call one another. They will whirl about for twenty or thirty times together on the same side, during the space of four or five minutes. The motion of their Wings makes then a noise very like that of a Rattle; and one may hear it two hundred Paces off. The Bone of their Wing grows greater towards the Extremity, and forms a little round Mass under the Feathers, as big as a Musket Ball. That and its Beak are the chief Defence of this Bird. 'Tis very hard to catch it in the Woods, but easie in open Places, because we run faster than they, and sometimes we approach them without much Trouble. From March to September they are extremely fat, and taste admirably well, especially while they are young, some of the Males weigh forty-five Pounds.
Several of Leguat's observations were later confirmed through study of subfossil Rodrigues solitaire remains. The curved contour lines of the pelvis also support the roundness of its hind parts, which he compared to that of a horse. Also, a ridged surface appears at the base of the beak, indicating the position of the caruncular ridge, which Leguat described as a "widow's peak". Before fossils of the carpal knob were found, Strickland noted that the keel of the sternum of the Rodrigues solitaire was so well-developed as to almost indicate it had possessed the power of flight; however, since the humerus was very short he inferred that this was instead related to Leguat's claim that they used their wings for defence.
Leguat continued with an elaborate description of the female Rodrigues solitaire, which also appears to be the sex depicted in his illustration of the bird:
The Females are wonderfully beautiful, some fair, some brown; I call them fair, because they are the colour of fair Hair. They have a sort of Peak, like a Widow's upon their Breasts [beaks], which is of a dun colour. No one Feather is straggling from the other all over their Bodies, they being very careful to adjust themselves, and make them all even with their Beaks. The Feathers on their Thighs are round like Shells at the end, and being there very thick, have an agreeable effect. They have two Risings on their Craws [crop] and the Feathers are whiter than the rest, which livelily represents the fine neck of a Beautiful Woman. They walk with so much Stateliness and good Grace, that one cannot help admiring them and loving them; by which means their fine Mein often saves their Lives.
It has been proposed that Leguat's comparison between the crop of the female Rodrigues solitaire and the "beautiful bosom of a woman" (changed to "fine neck" in some editions of his memoirs) was out of longing for female companionship.
Leguat's statements were confirmed by another description by Julien Tafforet, who wrote in 1726:
The solitaire is a large bird, which weighs about forty or fifty pounds. They have a very big head, with a sort of frontlet, as if of black velvet. Their feathers are neither feathers nor fur; they are of light grey colour, with a little black on their backs. Strutting proudly about, either alone or in pairs, they preen their plumage or fur with their beak, and keep themselves very clean. They have their toes furnished with hard scales, and run with quickness, mostly among the rocks, where a man, however agile, can hardly catch them. They have a very short beak, of about an inch in length, which is sharp. They, nevertheless, do not attempt to hurt anyone, except when they find someone before them, and, when hardly pressed, try to bite him. They have a small stump of a wing, which has a sort of bullet at its extremity, and serves as a defence.