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Accueil du site → Doctorat → Afrique du Sud → The ecology of the world’s smallest tortoise, Homopus signatus signatus : effects of rainfall.

University of the Western Cape (2008)

The ecology of the world’s smallest tortoise, Homopus signatus signatus : effects of rainfall.

Loehr, Victor J. T.

Titre : The ecology of the world’s smallest tortoise, Homopus signatus signatus : effects of rainfall

Auteur : Loehr, Victor J. T.

Université de soutenance : University of the Western Cape

Grade : Philosophiae Doctor – PhD 2008

Résumé partiel
Tortoises appear to be successful in arid ecosystems, where they depend on primary production for their predominantly herbivorous diets. The low primary production of arid regions is exacerbated by periodic droughts, so that iteroparous species such as chelonians require mechanisms to overcome resource shortages. The smallest of all tortoises, Homopus signatus signatus, occurs in a dry winter rainfall area in northwestern South Africa that is threatened with aridification due to regional climate change. Homopus s. signatus is listed in the South African Red Data Book and IUCN Red List of Threatened Species, yet its morphology and ecology, including traits that help cope with its dry environment, have been studied little. The conservation status of the taxon requires ecological data to take sound conservation measures. This study evaluates shell size, shape and colour pattern in a population of H. s. signatus, and reports responses of growth, tick infestations, body condition and reproduction to five years of rainfall variation. The small body size of H. s. signatus probably translates to low resource demands, helping tortoises survive their low-productive environment. However, females were larger than males (absolute or scaled on carapace length) to accommodate clutches of single, relatively large eggs that are characteristic for this species. Egg size correlated to female size, and average egg size was similar in all years. Large eggs may be required to produce large hatchlings capable of surviving the harsh Succulent Karoo environment. To facilitate the production of large eggs at a small body size, female pelvic girdles allowed passage of eggs that were typically wider than the width of the pelvic canal. Furthermore, flexibility of the shell in a dorso-ventral plane enabled females to temporarily increase shell volume to accommodate eggs and follicles. Annual rainfall and timing of rains seem to determine the primary production available to the tortoises. Homopus s. signatus accumulated resources when food was abundant (i.e., spring), contributing to high spring body conditions. In the dry season, tortoises probably reduced activity levels and opportunistically acquired resources to maintain relatively stable body conditions throughout the year. Opportunistic foraging may help females increase their body condition towards spring and, subsequently, produce large eggs or multiple clutches. Females may require more feeding time than do males, facilitated by the sexually different shell colour patterns.

Mots clés : Aridity and arid regions ; Tortoises ; Herbivores ; Ecology.


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