Summary
We investigated seasonal changes in diet and distribution of giant tortoises (Geochelone gigantea (Schweigger)) on Aldabra atoll in the Indian Ocean. Animals were counted and their activity and feeding behaviour recorded on transects where vegetation composition and primary production had been studied (Gibson and Phillipson in press a, b).
There were striking seasonal shifts in tortoise distribution, and male, female, and juvenile tortoises were found in different proportions in different vegetation types.
Tortoises are selective grazers, feeding on a wide range of foodstuffs of which the most important (61% of feeding observations) was tortoise turf. Diet varied seasonally, with shrub leaves (mostly litter) overtaking tortoise turf in importance in the late dry season; diet broadened as the dry season progressed. Male and female diets were not significantly different but juveniles fed on herbs and mosaic rock vegetation more often than adults.
Seasonal shifts in distribution are due to movements in response to changes in food availability, measured by the foods' cover abundances and production phenologies. Tortoises concentrate on preferred foods when available, but become less selective as production falls. Some differences in size and sex class distribution between habitats can also be explained by food availability.
In the late dry season density peaks on the coastal Sporobolus virginicus (L.) Kunth sward. A detailed study showed that, while at least 20% of the population uses the sward each year, visit times are short and turnover of tortoises high, as would be expected on a non-preferred food.
The giant tortoise interacts with its food supply similarly to other large herbivores, except that the low maintenance needs of this large poikilotherm allow it to develop unusually high population densities.
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References
Bourn DM (1976) The giant tortoise population of Aldabra (Cryptodira: Testudinidae). Part I: preliminary results. Zoologica Africana II:275–284
Bourn DM, Coe MJ (1978) The size, structure and distribution of the giant tortoise population of Aldabra. Phil Trans R Soc Lond B 282:139–175
Coe MJ, Bourn DM, Swingland IR (1979) The biomass, production and carrying capacity of giant tortoises on Aldabra. Phil Trans R Soc Lond B 286:163–176
Feeny PP (1976) Plant apparency and chemical defense. In: JW Wallace, RL Mansell (eds), Biochemical interaction between plants and insects. Recent Advances in Phytochemistry 10:1–40. Plenum Press, New York & London
Frazier JG (1971) Behavioural and ecological observations on giant tortoises on Aldabra Atoll. Unpublished D Phil thesis University of Oxford
Gaymer R (1973) A marking method for giant tortoises and field trials on Aldabra. J Zool Lond 169:393–401
Gibson CWD, Phillipson J (in press a) The primary production of Aldabra atoll, with special reference to habitats used by giant tortoises. Phil Trans R Soc Lond B
Gibson CWD, Phillipson J (in press b) The vegetation of Aldabra atoll: preliminary analysis and explanation of the vegetation map. Phil Trans R Soc Lond B
Grubb P (1971) The growth, ecology and population structure of giant tortoises on Aldabra. Phil Trans R Soc Lond B 260:237–372
Hamilton J, Coe MJ (in press) Feeding, digestion and assimilation of a population of giant tortoises (Geochelone gigantea (Schweigger)) on Aldabra atoll. Journal of Arid Environments (1982)
Jarman PJ, Sinclair ARE (1979) Feeding strategy and the pattern of resource partitioning in ungulates. In: ARE Sinclair, M Norton-Griffiths. Serengeti: Dynamics of an Ecosystem. University of Chicago Press, p 130–163
Merton LFH, Bourn DM, Hnatiuk RJ (1976) Giant tortoise and vegetation interactions on Aldabra atoll. Part I: inland. Biol Conserv 9:293–316
Rosenthal GA, Janzen DH (1979) Herbivores: their interactions with secondary plant metabolites. Academic Press, New York
Scott JA, French NR, Leetham JW (1979) Patterns of consumption in grasslands. In: NR French (ed), Perspectives in Grassland Ecology. Results and applications of the US/IBP Grassland Biome study. Ecological Studies 32, Springer Verlag, New York Heidelberg Berlin, p 89–105
Sinclair ARE, Norton-Griffiths M (1979) Serengeti: Dynamics of an Ecosystem. University of Chicago Press, Chicago London
Swingland IR, Coe MJ (1978) The natural regulation of giant tortoise populations on Aldabra atoll: Reproduction. J Zool London 186:285–309
Swingland IR, Coe MJ (1979) The natural regulation of giant tortoise populations on Aldabra atoll: Recruitment. Phil Trans R Soc Lond B 286:177–188
Swingland IR, Frazier JG (1979) The conflict between feeding and overheating in the Aldabran giant tortoise. In: CJ Amlaner, DW Macdonald (eds) A handbook on biotelemetry and radio tracking, Pergamon Press, London New York, p 611–615
Swingland IR, Lessells CM (1979) The natural regulation of giant tortoise populations on Aldabra atoll. Movement polymorphism, reproductive success and mortality. J Anim Ecol 48:639–654
Voeltzkow A (1897) Einleitung. Madagaskar. Juan de Nova, Aldabra. Abh senckenb naturf Ges 21:1–76
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Gibson, C.W.D., Hamilton, J. Feeding ecology and seasonal movements of giant tortoises on Aldabra atoll. Oecologia 56, 84–92 (1983). https://doi.org/10.1007/BF00378221
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DOI: https://doi.org/10.1007/BF00378221