Literaturdatenbank
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Loehr, V. J. T. (2008). The ecology of the world's smallest tortoise, homopus signatus signatus: effects of rainfall. Unpublished thesis Doctor of Philosophy, University of the Western Cape, South Africa.
Added by: Admin (21 Nov 2009 11:59:59 UTC) Last edited by: Beate Pfau (22 Nov 2009 16:08:39 UTC) |
| Resource type: Thesis/Dissertation BibTeX citation key: Loehr2008c View all bibliographic details  |
Categories: General Keywords: Habitat = habitat, Homopus, Homopus signatus, Schildkröten = turtles + tortoises, Südafrika = Southern Africa, Testudinidae Creators: Loehr Publisher: University of the Western Cape (South Africa) |
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| URLs http://www.homopus.org/ |
| Abstract |
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Testudinidae 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. In a drought year, tortoises had lower spring body conditions than in other years, yet some females produced eggs. Spreading reproductive investments may increase the chance that some offspring hatch when conditions are favourable (i.e., bet-hedging). However, allocations to reproduction during drought appear to occur at the expense of growth, which was often negative. Reproductive costs of males may also be considerable, perhaps as a result of courtship behaviour and male-male agonism, because male growth rates were lower than female growth rates (corrected for body size), and many males shrank during the drought. Shrinking in H. s. signatus seemed to be effected through flexibility of the shell, changing in shape and volume due to starvation, but may also include (reversible) bone resorption. Levels of tick infestations were negatively correlated with rainfall, and H. s. signatus males were particularly vulnerable due to the larger shell openings compared to females. Larger shell openings can facilitate locomotion and courtship behaviours, but tick infestations may counter these advantages by costs through loss of blood and tick-borne diseases. The projected aridification of the Succulent Karoo may increase the frequency of drought years. As a result, frequent low growth rates may increase the time for females to mature from 11-12 to 30 years. This would lead to smaller females (average and maximum sizes), with frequently low body conditions, producing smaller eggs and hatchlings. Furthermore, fewer females would reproduce in drought years so that fewer eggs and offspring would be produced. These results may partly be compensated by conservation measures (e.g., reducing mortality caused by road traffic) that increase survival of adult females and enhance reproduction over many years. However, since aridification may demand larger viable egg and hatchling sizes, the long-term survival of H. s. signatus requires drastic measures allowing populations to migrate to areas that will continue to receive sufficient rainfall in the future.
Added by: Admin Last edited by: Beate Pfau |