Rodrigues solitaire

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:[20][21]

A Nicobar pigeon, the closest living relative of the Rodrigues solitaire and the dodo according to DNA studies

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.[22] 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.[23] 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.[24]

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.[25] 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.[24] The lack of mammalian herbivores competing for resources on these islands allowed the solitaire and the dodo to attain very large sizes.[26] The dodo lost the ability to fly owing to the lack of mammalian predators on Mauritius.[27] 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.[28]

Skeletons of a female and a male, Hunterian Museum

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.[32] Leguat devoted three pages of his memoirs to the Rodrigues solitaire, and was clearly impressed by the bird.[34] 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.[29]

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".[4] 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.[8]

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.[29]

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.[34][15]

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.[29]

Pelvis of a female (1) and male (2), furcula (3) and gizzard stone (4–6)

Male pelvis from below and coccyx

Leguat stated that the Rodrigues solitaire fed on dates, whereas Tafforet mentioned seeds and leaves. No other accounts mention diet.[30] It has been suggested it ate latan palm fruits, for which it competed with the now extinct Cylindraspis tortoises. It is not known how the young were fed, but related pigeons provide crop milk.[34] The risings on the crop of the female may have covered glands that produced the crop milk. If the theory is correct, the birds may have practiced a division of labour, where the female stayed and fed the young crop milk, while the male collected food in the crop and delivered it to the female. It has been suggested that the maximum size attained by the solitaire and the dodo was limited by the amount of crop milk they were able to produce for their young during early growth.[39]

Several contemporary accounts state that the Rodrigues solitaire used gizzard stones. Dodos also did this, which may imply a similar diet.[34] Leguat described the stones in the following passage, mentioning that Rodrigues solitaires refused to feed in captivity:

Tho' these Birds will sometimes very familiarly come up near enough to one, when we do not run after them, yet they will never grow Tame. As soon as they are caught they shed Tears without Crying, and refuse all sustenance till they die. We find in the Gizzards of both Male and Female, a brown Stone, of the bigness of a Hen's Egg, 'tis somewhat rough, flat on one side and round on the other, heavy and hard. We believe this Stone was there when they were hatched, for let them be never so young, you meet with it always. They never have but one of 'em, and besides, the Passage from the Craw to the Gizard is so narrow, that a like Mass of half Bigness cou'd not pass. It serv'd to whet our Knives better than any other Stone Whatsoever.[29]

In 1877 three stones were found in a cavern on Rodrigues, each near a Rodrigues solitaire skeleton, and were inferred to be the gizzard stones mentioned by Leguat. One of the stones was examined and found to be dolerite: somewhat rough, hard and heavy, c. 50 g (1+3⁄4 oz), but hardly flat on one side as described by Leguat. This could be due to its association with a young individual.[9] Although Leguat asserted that the bird hatched with the gizzard stone already inside, in reality adults most likely fed the stones to their hatchlings.[34]

Photo collage restoration of a nesting female and a male in their environment[40]

The most detailed account of the reproductive habits of the Rodrigues solitaire is Leguat's. He described mating and nesting as follows:

When these Birds build their Nests, they choose a clean Place, gather together some Palm-Leaves for that purpose, and heap them up a foot and a half high from the Ground, on which they sit. They never lay but one Egg, which is much bigger than that of a Goose. The Male and Female both cover it in their turns, and the young is not hatch'd till at seven Weeks' end: All the while they are sitting upon it, or are bringing up their young one, which is not able to provide itself in several Months, they will not suffer any other Bird of their Species to come within two hundred Yards round of the Place; But what is very singular, is, the Males will never drive away the Females, only when he perceives one he makes a noise with his Wings to call to the Female, and she drives the unwelcome Stranger away, not leaving it till 'tis without her Bounds. The Female do's the same as to the Males, whom she leaves to the Male, and he drives them away. We have observ'd this several Times, and I affirm it to be true. The Combats between them on this occasion last sometimes pretty long, because the Stranger only turns about, and do's not fly directly from the Nest. However, the others do not forsake it till they have quite driven it out of their Limits. After these Birds have rais'd their young One, and left it to itself, they are always together, which the other Birds are not, and tho' they happen to mingle with other Birds of the same Species, these two Companions never disunite. We have often remark'd, that some Days after the young leaves the Nest, a Company of thirty or forty brings another young one to it, and the now fledg'd Bird, with its Father and Mother joyning with the Band, march to some bye Place. We frequently follow'd them, and found that afterwards the old ones went each their way alone, or in Couples, and left the two young ones together, which we call'd a Marriage.[29]

The clutch was described as consisting of a single egg; given the bird's large size, this led to proposals that the solitaire was K-selected, which means it produced a low number of altricial offspring, which required extensive parental care until maturity. The gathering of unrelated juveniles suggests that they formed crèches, which may have followed foraging adults as part of the learning process.[33] A study of subfossil remains found that the carpal knob only developed after the bird reached skeletal maturity.[1]

Skeletons of a female and male collected in 1874

Skulls of a male and female in several views

Tafforet's account confirms Leguat's description of reproductive behaviour, adding that Rodrigues solitaires would even attack humans approaching their chicks:

They do not fly at all, having no feathers to their wings, but they flap them, and make a great noise with their wings when angry, and the noise is something like thunder in the distance. They only ly, as I am led to suppose, but once in the year, and only one egg. Not that I have seen their eggs, for I have not been able to discover where they lay. But I have never seen but one little one alone with them, and, if any one tried to approach it, they would bite him very severely. These birds live on seeds and leaves of trees, which they pick up on the ground. They have a gizzard larger than the fist, and what is surprising is that there is found in it a stone of the size of a henn's egg, of oval shape, a little flattened, although this animal cannot swallow anything larger than a small cherry-stone. I have eaten them: they are tolerably well tasted.[29]

The size difference between sexes has led to the suggestion that the Rodrigues solitaire was not monogamous as stated by Leguat, and that this deeply religious man attributed the trait to the bird for moral reasons.[9] It has been proposed that it was instead polygynous, and the wing-rattling behaviour described for males suggests lek-mating, where males gather for competitive mating display.[33] However, size dimorphism does occur in some monogamous birds; most other pigeons are monogamous as well.[32] A 2015 article proposed that males invited females into their territories as secondary mates, which would result in the resident female acting aggressively towards the newcomer. Similar behaviour is seen in species that practice resource-defence polygyny. The territories probably provided all the food the birds needed in addition to acting as breeding-areas, and there was probably intense competition for favourable territories. The fact that Rodrigues island shrank by 90% at the end of the Pleistocene may also have contributed to such competition over territories, and thereby furthered sexual dimorphism.[41]

The Rodrigues solitaire probably became extinct sometime between the 1730s and 1760s; the exact date is unknown. Its disappearance coincided with the tortoise trade between 1730 and 1750; traders burnt off vegetation, hunted solitaires and imported cats and pigs that preyed on eggs and chicks.[14] In 1755, the French engineer Joseph-François Charpentier de Cossigny attempted to obtain a live specimen, as he had been assured the Rodrigues solitaire still survived in remote areas of the island. Though trying for 18 months, and offering large rewards, none could be found. He noted that cats were blamed for decimating the species, but suspected that it was due to hunting by humans instead.[14] The French astronomer Alexandre Guy Pingré did not encounter any solitaires when he visited Rodrigues to observe the 1761 transit of Venus, although he had been assured they survived. His friend, the French astronomer Pierre Charles Le Monnier named the constellation Turdus Solitarius after the bird to commemorate the journey. Although the Rodrigues solitaire is the only extinct bird to have a former constellation named for it, celestial mapmakers did not know what it looked like and star maps depicted other birds.[15]

By the time the discovery of subfossil Rodrigues solitaire bones from 1786 onwards confirmed Leguat's descriptions, no living residents of Rodrigues remembered having seen live specimens. In 1831, a man who had lived on Rodrigues for 40 years said that he had never seen birds large enough to be Rodrigues solitaires. Rodrigues covers only 104 square kilometres (40 sq mi), making it implausible that the bird would have survived undetected.[30]