FSD 2010 - Tortoises

5th International Symposium-Workshop on Frugivores and Seed Dispersal (1985-2010)

Tortoise

Geochelone is a genus of tortoises.

Geochelone tortoises can be found in Africa, America, Asia, and several Oceanic islands. The genus consists of 12 species. They primarily eat plants, specifically dark greens. Measured by the length of their shell, the species in this genus are some of the largest turtles in the world. For instance, Aldabra giant tortoise, Aldabrachelys gigantea (see below) grows up to 1.3 m and 300 kg, but only in good conditions/habitats (Hansen, D. comm. pers.). The role of tortoise as seed dispersers has been studied in the neotropics (Josseaume 2002a,b; Strong & Fragoso 2006) and the paleotropics (Hansen et al. 2008).

In the neotropics, despite their highly frugivorous diet and ability to occur at high densities relative to many bird and mammal species, tortoises have received limited attention as potential seed dispersers. The role of red-footed tortoises (Geochelone carbonaria) and yellow-footed tortoises (G. denticulata) (photo (c) Joel N. Strong) as seed dispersal agents was investigated at Maracá Ecological Reserve in northwestern Brazil by analyzing fecal samples for frequency and viability of seed species and estimating daily displacement of tortoises from recaptured and thread-trailed individuals. Over the course of his study, Strong and Fragoso (2006) recorded nearly 20 species of seeds ingested by tortoises, ranging in size from about 1 mm for tiny fig seeds to over 5 cm for a large palm seed, and most seed species were viable and abundant in fecal clumps. The diversity and proportion of viable seeds consumed by tortoises, combined with the seed retention times and daily movements, suggest they may be effective dispersal agents.

In Mauritius, Syzygium mamillatum is a critically endangered endemic upland sub-canopy tree of 2.5–9 m in height. The fruitless search for natural seedlings and saplings of away from adult trees suggests that there are currently no frugivorous animals acting as efficient seed dispersers of S. mamillatum. All extinct Mascarene giant tortoise species have been reported to eat fruits and leaves. As stand-in for the two extinct giant tortoise species of Mauritius, Hansen et al. (2008) used giant Aldabra tortoises, Aldabrachelys gigantea. The main population of the Aldabra Giant Tortoise resides on the islands of the Aldabra Atoll in the Seychelles. The atoll has been protected from human influence and is home to some 60-80,000 giant tortoises, the world's largest population of the animal. Another isolated population of the species resides on the island of Zanzibar. The tortoises exploit many different kinds of habitat including grasslands, low scrub, mangrove swamps, and coastal dunes. Primarily herbivores, Aldabra Tortoises will eat grasses, leaves, and woody plant stems. They occasionally indulge in small invertebrates and carrion, even eating the bodies of other dead tortoises. Hansen et al. (2008) demonstrated that Aldabra giant tortoises could be used as ecological analogues to provide seed dispersal services, and thus resurrect the functional component of some of the extinct endemic frugivores in Mauritius.

Note about taxonomy (Dennis Hansen, pers. comm..)

For the mainland species, Geochelone has almost ceased existing, as it is 'rampantly polyphyletic' according to recent molecular studies (Le et al. 2006). Thus, for example, the two mainland South American species, G. carbonaria and G. denticulata are now firmly placed in the genus Chelonoidis (as are the Galapagos tortoises). The Aldabra Giant Tortoise (Aldabrachelys gigantea), from the islands of the Aldabra Atoll in the Seychelles, is one of the largest tortoises in the world. (it is sometimes referred to as various taxa in the invalid genus Dipsochelys see Frazier 2006 for review of taxonomy).

References

Journal Cheloniens

Website Chelonian

Wikipedia Geochelone

Geochelone denticulata

Aldabra Tortoise

Seed dispersal in Mauritius

Giant Tortoise at Galapagos Islands

Literature

Carlson, J.E., Menges, E.S. & Marks, P. 2003. Seed dispersal by Gopherus polyphemus at Archbold Biological Station, Florida. Florida Scientist 66, 147-154.

Frazier, J. 2006 Giant Tortoises of the Indian Ocean. Herpetological Review 37, 368-373.

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

Hansen, D.M., Kaiser, C.N. & Müller, C.B. 2008. Seed dispersal and establishment of endangered plants on Oceanic islands: The Janzen-Connell model, and the use of ecological analogues. PLoS ONE 3(5): e2111 FREE on line

Le, M., Raxworthy, C. J., McCord, W. P. & Mertz, L. 2006. A molecular phylogeny of tortoises (Testudines: Testudinidae) based on mitochondrial and nuclear genes. Molecular Phylogenetics And Evolution 40, 517-531.

Josseaume, B. 2002. Rôle des Chéloniens dans le régénération des écosystèmes forestiers tropicaux : exemple de la tortue denticulée (Chelonoidis denticulata) en Guyane française. Thèse de Doctorat de l’Université de Paris 6, Paris.

Josseaume, B. 2002. Faecal collector for field studies of digestive responses in forest tortoises. Herpetological Journal 12:169-172.

Rick, C.M. & Bowman, R.I. 1961. Galápagos tomatoes and tortoises. Evolution 15, 407-417.

Strong, J.N. 2005. Seed dispersal and the ecological implications of hunting Geochelone carbonaria and G. denticulata in Northwestern Brazil. Ms Degree Thesis, State University of New York, Syracuse, New York. pdf

Strong, J.N. & Fragoso, J.M.V. Seed Dispersal by Geochelone carbonaria and Geochelone denticulata in Northwestern Brazil. Biotropica 38: 683–686. Abstract; see also this on-line paper

Strong, J.N. & Andrew D.W. 2006. Geochelone carbonaria Swimming. Herpetological Review. 37: 457-458.

Aldabra Giant Tortoise (Aldabrachelys gigantea) eating a palm fruit in Mauritius. (c) Dennis Hansen.