Geochelone

Indian star tortoise Geochelone elegans, copyright Shashidharswamy Hiremath.

Belongs within: Testudininae.

Tortoise sorting
Published 25 October 2014
Indian star tortoise Geochelone elegans, copyright P. G. Palmer.

As a whole group, the ‘true’ tortoises of the family Testudinidae are easily recognised, with a usually terrestrial habitus (though at least one species, the serrated hinge-back tortoise Kinixys erosa, is a capable swimmer), columnar legs with short heavy-clawed feet, and a relatively high carapace. But relationships within the tortoises have seen a bit of shuffling around in recent years, and nowhere has that shuffling been more obvious than in the genus Geochelone.

Many of you may know Geochelone as the genus including the giant tortoises of the Galapagos Islands and islands in the Indian Ocean, as well as species such as the radiated tortoise G. radiata of Madagascar and the geometric tortoise G. geometrica of South Africa. At its broadest, about half the world’s tortoise species have been included in Geochelone. However, the genus has always been poorly defined, marked by its ‘primitive’ skull (Gerlach 2001), and the lack of features of other tortoise genera such as the plastral hinge of the Palaearctic Testudo species, or the rear-carapace hinge of African Kinixys species. For the most part, Geochelone was simply a home for the bigger tortoises.

Burmese star tortoise Geochelone platynota, copyright Kalyar Platt.

So it is hardly surprising that phylogenetic studies, whether morphological (Gerlach 2001) or molecular (Fritz & Bininda-Emonds 2007), have failed to support a broad Geochelone as a monophyletic group. As a result, recent authors have advocated recognising a number of separate genera: the Galapagos tortoises belong to the genus Chelonoidis, the Seychelles giant tortoises to Aldabrachelys (as confirmed by ICZN 2013), the radiated tortoise to Astrochelys and the geometric tortose to Psammobates. From being the largest recognised tortoise genus, Geochelone has been cut down to a mere two or three species. These are the Indian star tortoise Geochelone elegans, the Burmese star tortoise G. platynota, and, maybe, the African spurred tortoise G. sulcata.

The Indian and Burmese star tortoises are two very similar species that get their names from their colour pattern, with radiating star-like markings on the carapace. The individual scutes of the carapace bulge outwards, giving the animal an overall lumpy appearance. They are both medium-sized tortoises, growing to over 30 cm in length. The Indian star tortoise Geochelone elegans is widespread in dry habitats in India, Pakistan and Sri Lanka, though conservation concerns have been raised about the extent of harvesting of wild tortoises for food and the exotic pet trade. The Burmese star tortoise G. platynota, on the other hand, is critically endangered, having been almost wiped out from its original range in the central dry zone of Burma. As long ago as 1863, Edward Blyth (who, offhand, sported one heck of an impressive beard) was complaining that specimens were difficult to find due to the local people’s fondness for eating them (Platt et al. 2011).

African spurred tortoise Geochelone sulcata, copyright Chris Mattison.

The African spurred tortoise Geochelone sulcata is found across the southern part of the Sahara Desert and the Sahel. It is a particularly large tortoise, growing to over 80 cm (Swingland & Klemens 1989); in fact, it is the largest tortoise that is not found on oceanic islands. Spurred tortoises dig burrows that can reach up to 3.5 m in length in which to avoid the full heat of the day. This species is placed by some authors in its own genus Centrochelys, cutting Geochelone down to just the two Asian star tortoises. Molecular studies place G. sulcata as the sister taxon to the Asian species (Fritz & Bininda-Emonds 2007), but they have not been associated in morphological studies. Though widespread, the African spurred tortoise is regarded as vulnerable due to the degradation of its habitat. Concern has also been raised, again, about the collection of wild individuals for the pet trade.

Tortoise resurrection
Published 9 October 2008

In a subsequent portion of this narrative I shall have frequent occasion to mention this species of tortoise. It is found principally, as most of my readers may know, in the group of islands known as the Gallipagos… They are frequently found of an enormous size… They can exist without food for an almost incredible length of time, instances having been known wher they have been thrown into the hold of a vessel and lain two years without nourishment of any kind – being as fat, and, in every respect, in as good order at the expiration of that time as when they were first put in… They are excellent and highly nutritious food, and have, no doubt, been the means of preserving the lives of thousands of seamen employed in the whale-fishery and other pursuits in the Pacific.

–Edgar Allen Poe, The Narrative of Arthur Gordon Pym of Nantucket

For sailors in tropical oceans before the invention of refrigeration, keeping supplies of food was a serious issue. It was a permanent challenge to keep supplies fresh and edible, and indeed, much of the time stores failed at both. Under such conditions, the giant tortoises of the Galapagos islands and the Mascarenes and other islands in the Indian Ocean would have been seen as nothing short of miraculous. Tortoises could be captured easily and kept in the hold of a boat for extended periods without feeding, only slaughtered when they were actually required for eating. As a result, ships that were in a position to do so often took on tortoises in large number, and Charles Darwin apparently recorded single vessels taking up to 700 individuals at a time. By modern standards the idea of seven hundred starving tortoises crammed into a single hull seems unthinkably cruel, but doubtless the sailors who otherwise faced another six months of decomposing ship’s biscuit saw things differently.

Geochelone becki, the Volcano Wolf tortoise. Photo by Joe Flanagan.

Unfortunately, such intense harvesting took an inevitable toll. Tortoise numbers declined rapidly, and many went extinct. Honneger (1981) lists three extinct species of tortoise from the Galapagos (including Geochelone abingdoni from Pinta island, which is technically not yet extinct but which only survives in the form of a single captive male) and at least six extinctions from the Seychelles and Mascarenes. Extinct populations on the Galapagos islands of Rabida and Santa Fe may have represented further undescribed species.

However, a paper published yesterday in the Proceedings of the National Academy of Sciences adds a remarkable coda to the history of one of the “extinct” species, the Floreana tortoise Geochelone elephantopus. Using DNA extracted from museum specimens collected on Floreana before the population disappeared, Poulakakis et al. (2008) have demonstrated that G. elephantopus may not be quite as extinct as previously thought. Instead, anomalous genetic haplotypes previously identified in some living individuals of Geochelone becki, a species found on the Volcano Wolf at the northern end of Isabela, the largest island in the Galapagos, indicate descent from G. elephantopus. These individuals would appear to be descendants of past hybridisations between native Volcano Wolf tortoises and introduced Floreana tortoises.

Such a situation is quite believable. As a result of the widespread transport of tortoises for food, many tortoises ended up on islands to which they were not native*. Tortoises were regularly imported to Réunion in the Mascarenes after the native population became extinct. Living populations of giant tortoises on the Granitic Islands of the Seychelles probably descend from imports from Aldabra rather than representing the species originally found there (Honegger 1981). According to Poulakakis et al. (2008), some 40% of the Volcano Wolf tortoises tested showed evidence of Floreana ancestry, so the genetic legacy of Geochelone elephantopus is alive and well, at least in hybrid form.

*Potentially a serious issue for taxonomy, as researchers cannot assume that species names based on inadequate type material necessarily represent the species native to the island the type was collected on. Honegger (1981), for instance, cast doubt on whether Geochelone gouffei, known from a single specimen found on Farquhar Island in the Seychelles, actually originated there.

This still leaves a significant problem—most conservation policies do not cope well with hybrids. A number of species worldwide, such as the black stilt Himantopus novaezelandiae in New Zealand, are regarded as endangered because of the risk of hybridisation with related species. The red wolf Canis rufus and the Florida panther Puma concolor coryi represent two ‘endangered’ taxa in the United States for which the suggestion that their histories could have been compromised by hybridisation led to the suggestion that they should be abandoned as worthwhile conservation targets. However, the disappearance or decline of a species in its pure form due to hybridisation with another species is a different proposition from its decline due to replacement by that species. The genetic legacy of the declining species may still persist. Overemphasis on species “purity” may actually hinder the conservation of endangered taxa, especially if natural hybrid zones with related taxa exist in the first place (Allendorf et al. 2001). If there are no purebred Florida panthers, should that mean that there is no place for panthers in Florida?

Systematics of Geochelone
<==Geochelone Fitzinger 1835 [incl. Chelonoidis Fitzinger 1835]FB-E07
    |--+--G. sulcata (Miller 1779)FB-E07, JPG04
    |  `--+--*G. elegans (Schoepff 1794)FB-E07
    |     `--G. platynota (Blyth 1863)FB-E07, JPG04
    `--+--+--G. nigra (Quoy & Gaimard 1824)FB-E07, JPG04 [=Chelonoidis nigraFB-E07]
       |  `--+--G. chilensis (Gray 1870)FB-E07, JPG04 [=Chelonoidis chilensisFB-E07]
       |     `--+--G. carbonaria (Spix 1824)PG08, JPG04 [=*Chelonoidis carbonariaFB-E07]
       |        `--G. denticulata (Linnaeus 1766)FB-E07, JPG04 [=Chelonoidis denticulataFB-E07]
       `--+--ImpregnochelysGM88
          `--Kinixys Bell 1827FB-E07
               |--K. belliana Gray 1831JPG04
               |--K. erosa (Schweigger 1812)JPG04
               |--K. homeana Bell 1827JPG04
               |--K. lobatsiana (Power 1927)JPG04
               |--K. natalensis Hewitt 1935JPG04
               `--K. spekii Gray 1863JPG04
Geochelone incertae sedis:
  G. borbonica [=Cylindraspis borbonica]D81
  G. gadowi (Van Denburgh 1914)H81
  G. geometrica (Linnaeus 1758)USDI77, L58 [=Testudo geometricaUSDI77]
  G. graii (Duméril & Bibron 1835) [=G. grayi]H81
  G. leptocnemis (Günther 1877)H81
  G. microtympanum (Boulenger 1890)H81
  G. phantastica (Van Denburgh 1907)PG08, H81 [=G. elephantopus phantasticaH81]
  G. sauzieri (Gadow 1894)H81
  G. sumierei (Sauzier 1893)H81 [=Cylindraspis sumeireiD81]
  0--+--G. beckiPG08 [=G. elephantopus beckiS91]
  |  `--G. darwiniPG08 [=G. elephantopus darwiniS91]
  `--+--+--G. abingdoni (Günther 1877)PG08, H81 [=G. elephantopus abingdoniH81]
     |  |--G. chatamensisPG08
     |  `--G. hoodensisPG08
     `--+--G. ephippiumPG08 [=G. elehantopus ephippiumS91]
        `--+--G. porteriPG08 [=G. elephantopus porteriS91]
           `--+--G. elephantopusPG08 [=Testudo elephantopusUSDI77]
              |    |--G. e. elephantopusH81
              |    `--G. e. galapagoensis (Baur 1889)H81
              `--+--G. guntheriPG08
                 |--G. microphyesPG08 [=G. elephantopus microphyesS91]
                 |--G. vandenburghiPG08 [=G. elephantopus vandenburghiS91]
                 `--G. vicinaPG08 [=G. elephantopus vicinaS91]

*Type species of generic name indicated

References

Allendorf, F. W., R. F. Leary, P. Spruell & J. K. Wenburg. 2001. The problems with hybrids: setting conservation guidelines. Trends in Ecology and Evolution 16 (11): 613–622.

[D81] Day, D. 1981. The Doomsday Book of Animals: A unique natural history of three hundred vanished species. Ebury Press: London.

[FB-E07] Fritz, U., & O. R. P. Bininda-Emonds. 2007. When genes meet nomenclature: tortoise phylogeny and the shifting generic concepts of Testudo and Geochelone. Zoology 110: 298–307.

Gerlach, J. 2001. Tortoise phylogeny and the ‘Geochelone’ problem. Phelsuma 9 (Supplement A): 1–24.

[H81] Honegger, R. E. 1981. List of amphibians and reptiles either known or thought to have become extinct since 1600. Biological Conservation 19: 141–158.

ICZN. 2013. Opinion 2316: Testudo gigantea Schweigger, 1812 (currently Geochelone (Aldabrachelys) gigantea; Reptilia, Testudines): usage of the specific name conserved by maintenance of a designated neotype, and suppression of Testudo dussumieri Gray, 1831 (currently Dipsochelys dussumieri). Bulletin of Zoological Nomenclature 70 (1): 61–65.

[JPG04] Joyce, W. G., J. F. Parham & J. A. Gauthier. 2004. Developing a protocol for the conversion of rank-based taxon names to phylogenetically defined clade names, as exemplified by turtles. Journal of Paleontology 78 (5): 989–1013.

Platt, S. G., T. Swe, W. K. Ko, K. Platt, K. M. Myo, T. R. Rainwater & D. Emmett. 2011. Geochelone platynota (Blyth 1863)—Burmese star tortoise, kye leik. Chelonian Research Monographs 5: 057.1–057.9.

[PG08] Poulakakis, N., S. Glaberman, M. Russello, L. B. Beheregaray, C. Ciofi, J. R. Powell & A. Caccone. 2008. Historical DNA analysis reveals living descendants of an extinct species of Galápagos tortoise. Proceedings of the National Academy of Sciences of the USA 105 (40): 15464–15469.

[S91] Schatz, H. 1991. Arrival and establishment of Acari on oceanic islands. In: Dusbábek, F., & V. Bukva (eds) Modern Acarology: Proceedings of the VIII International Congress of Acarology, held in České Budĕjovice, Czechoslovakia, 6–11 August 1990 vol. 2 pp. 613–618. SPB Academic Publishing: The Hague.

Swingland, I. R., & M. W. Klemens (eds). 1989. The conservation biology of tortoises. Occasional Papers of the IUCN Species Survival Commission 5.

[USDI77] United States Department of the Interior. 1977. Endangered and threatened wildlife and plants—republication of list of species. Federal Register 42: 36420–36431.

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