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Colleen T. Downs - One of the best experts on this subject based on the ideXlab platform.
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Movement of Leopard Tortoises in response to environmental and climatic variables in a semi-arid environment.
Movement ecology, 2017Co-Authors: Martyn Drabik-hamshare, Colleen T. DownsAbstract:Tortoises (Testudinidae) occur in a wide range of environments, providing important ecosystem functions, such as seed dispersal and refuge in the form of burrows. Tortoise movement has previously been shown to be related to resource availability, reproductive status and local environmental conditions. However, understanding of the variables that drive their movement remains comparatively low. We investigated aspects of movement in Leopard Tortoises Stigmochelys pardalis—the largest and most abundant Tortoise species in sub-Saharan Africa—in response to environmental, climatic and individual variables in the semi-arid Karoo, South Africa. We used GPS telemetry to calculate bihourly and daily movement and used generalized linear mixed models (GLMMs) to ascertain important predictor variables. Temperature, distance from water sources, and month were important variables for predicting both bihourly and daily movement. Our results showed that movement increased when individuals were close to known water sources, indicating that individuals close to water resources make regular long distance movements. Movement showed a positive relationship for temperature in both models, whilst rainfall was an important predictor for bihourly movement. Our results displayed aspects of seasonality, with movement highest in spring months, likely related to reproductive activities, although no sex differences were observed. We identified temporal and spatial conditions in which Leopard Tortoise movement increased. Our results further support the relationship between water as a resource and movement in Leopard Tortoises. Individuals used one of two basic movement behaviours in relation to water in this water scarce environment. Either an individual’s home range and movements included permanent water resources allowing internal water storage replenishment, or excluded these with reliance on food resources (such as grasses, forbs, and succulents) for water.
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Movement of Leopard Tortoises in response to environmental and climatic variables in a semi-arid environment
Movement Ecology, 2017Co-Authors: Martyn Drabik-hamshare, Colleen T. DownsAbstract:BackgroundTortoises (Testudinidae) occur in a wide range of environments, providing important ecosystem functions, such as seed dispersal and refuge in the form of burrows. Tortoise movement has previously been shown to be related to resource availability, reproductive status and local environmental conditions. However, understanding of the variables that drive their movement remains comparatively low. We investigated aspects of movement in Leopard Tortoises Stigmochelys pardalis —the largest and most abundant Tortoise species in sub-Saharan Africa—in response to environmental, climatic and individual variables in the semi-arid Karoo, South Africa. We used GPS telemetry to calculate bihourly and daily movement and used generalized linear mixed models (GLMMs) to ascertain important predictor variables.ResultsTemperature, distance from water sources, and month were important variables for predicting both bihourly and daily movement. Our results showed that movement increased when individuals were close to known water sources, indicating that individuals close to water resources make regular long distance movements. Movement showed a positive relationship for temperature in both models, whilst rainfall was an important predictor for bihourly movement. Our results displayed aspects of seasonality, with movement highest in spring months, likely related to reproductive activities, although no sex differences were observed.ConclusionsWe identified temporal and spatial conditions in which Leopard Tortoise movement increased. Our results further support the relationship between water as a resource and movement in Leopard Tortoises. Individuals used one of two basic movement behaviours in relation to water in this water scarce environment. Either an individual’s home range and movements included permanent water resources allowing internal water storage replenishment, or excluded these with reliance on food resources (such as grasses, forbs, and succulents) for water.
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Aspects of the home range ecology of the Leopard Tortoise in the semi-arid central Karoo: An area threatened with fracking
Journal of Arid Environments, 2017Co-Authors: Martyn Drabik-hamshare, Colleen T. DownsAbstract:Abstract Whilst fracking is used globally, impact studies on wildlife are limited. The semi-arid Karoo, South Africa, a large ecosystem with a high degree of endemism, is targeted for fracking. We investigated how adult Leopard Tortoises (Stigmochelys pardalis) use their environment by determining individual and seasonal variation in home range and effects of weather factors on these pre-fracking. Data were obtained from Global Positioning System (GPS) transmitters placed on Leopard Tortoises (n = 11) on private livestock farms near Beaufort West, South Africa for a year. Kernel density estimation (KDE) was used to estimate home range. Individuals had a mean (±SE) home range of 121.86 ± 28.12 ha, (range 40.53–258.52 ha) with a core area of 76.55 ± 17.33 ha (range 21.22–83.89 ha). No difference was found between annual male and female home ranges. Two telemetered individuals were excluded from analysis because they exhibited apparent nomadic behaviour. Several individuals did not visit permanent water sources, possibly suggesting that dietary water intake was sufficient. Generalised Linear Mixed Models were used to explain monthly home range estimates (95% KDEhref) in regards to biologically significant predictor variables. A single top model (ΔAICc
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Seasonal and Daily Activity Patterns of Leopard Tortoises (Stigmochelys pardalis Bell, 1828) on Farmland in the Nama-Karoo, South Africa
African Zoology, 2013Co-Authors: M.k. Mcmaster, Colleen T. DownsAbstract:The Leopard Tortoise (Stigmochelys pardalis) is the largest of southern Africa’s 13 Tortoise species, and occurs in a variety of habitats from arid and semi-arid areas to mesic grassveld, savanna and bushveld. Seasonal activity patterns of S. pardalis were investigated as a function of rainfall, sex, time of day, temperature and time after sunrise on farmland in the semi-arid Nama-Karoo, South Africa. We predicted that because of seasonal rainfall, and subsequent increase in the food available, activity patterns of Leopard Tortoises would vary greatly among seasons, but that the primary constraint on activity levels within a season would be ambient temperature. Type of activity, time of day that the activity was performed, and the frequency that each activity was performed, differed among seasons. There was no overall seasonal difference in the level of activity with sex, but in certain seasons and with regard to specific activities, there were significant differences between the sexes. Diel activity was ...
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Home Range and Daily Movement of Leopard Tortoises (Stigmochelys pardalis) in the Nama-Karoo, South Africa
Journal of Herpetology, 2009Co-Authors: Megan K. Mcmaster, Colleen T. DownsAbstract:Seasonal shifts and overlap in home range are poorly known in the Leopard Tortoise (Stigmochelys pardalis). Consequently, these, together with daily distance moved, were investigated in the semiarid Nama-Karoo, as a function of season, sex, and body mass. Annual home-range size was significantly greater for telemetered compared with nontelemetered Tortoises stressing the importance of telemetry in spatial ecological studies of Tortoises. Consequently, only telemetered Tortoises were analyzed further in terms of variation between seasons and sexes. Home ranges differed significantly with season and were smallest in winter, when Tortoises were less active. Seasonal home-range size did not differ between sexes. Considerable individual variation existed in the amount of home-range overlap both within and between sexes. Daily distance moved in each season was highly variable for each individual and between individuals. However, the least movement occurred in winter in all individuals. Home-range size and daily distances moved in the active seasons were larger than for Leopard Tortoises found in more mesic habitats, suggesting a resource availability effect. Increased home-range size, in conjunction with their low density and population size, has implications for Leopard Tortoise conservation in semiarid regions in terms of planning and designating conservation areas. Variation in home-range size with regard to habitat type must be considered when planning terrestrial Tortoise conservation efforts.
Daryl Codron - One of the best experts on this subject based on the ideXlab platform.
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Structural density of the Leopard Tortoise (Stigmochelys pardalis) shell and its implications for taphonomic research
Journal of Archaeological Science: Reports, 2019Co-Authors: Sharon Holt, Liora Kolska Horwitz, Jakobus Hoffman, Daryl CodronAbstract:Abstract We investigated bone mineral density values of the shell (carapace and plastron) of a modern Leopard Tortoise ( Stigmochelys pardalis Bell, 1828) to assess its impact on survivorship of the different skeletal elements. We found significant differences among elements for bone mineral density and density volume, but not for bone mineral content. We then compared these findings with those obtained using computed tomography (CT) scans of shells of three modern Tortoise species that differ in size and shape; the Leopard Tortoise, greater padloper ( Homopus femoralis ) and angulate Tortoise ( Chersina angulata ). Results indicate that the patterning of bone density is similar across shell elements and scan sites in all species, although all values were lower in the greater padloper, which we attributed to shape differences – a flat rather than domed shell. Finally, we correlated the frequency of shell elements from the Holocene Leopard Tortoises recovered at the site of Wonderwerk Cave (South Africa) using, as a proxy, the densitometry values obtained from the modern Leopard Tortoise. For most strata there was a significant association with skeletal element survivorship demonstrating the role played by bone mineral density in determining the skeletal composition of this assemblage.
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Bone mineral density in the Leopard Tortoise: Implications for inter-taxon variation and bone survivorship in an archaeozoological assemblage
Quaternary International, 2018Co-Authors: Sharon Holt, Daryl Codron, Liora Kolska HorwitzAbstract:Abstract In this study we present data on bone mineral density (BMD) values for the cranial and post-cranial skeleton of the Leopard Tortoise (Stigmochelys pardalis). We found significant inter and intra-skeletal variation in different values of BMD within this species. The intra-skeletal, but not inter-skeletal pattern, was comparable to that of terrestrial mammals of similar size, despite differences in bone structure. Overall, the Tortoise has bone density that is intermediate between that of leporids (rabbits/hares) on the one hand and canids (dogs/wolves etc.) and marmots on the other. In this study we have used random effects to test regression models of BMD and demonstrate their value over the simple linear regression models that are currently in use. Finally, we applied the modern Leopard Tortoise proxy data to test whether different parameters of BMD impact survivorship of Tortoise bones in the Holocene strata from Wonderwerk Cave (Northern Cape Province, South Africa). Results indicate that this assemblage has undergone some degree of bone density-mediated attrition, probably influenced by burning and deposition time rather than animal agents.
Laura Jane Fielder - One of the best experts on this subject based on the ideXlab platform.
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Seasonal abundance of the Tortoise tick Amblyomma marmoreum (Acari: Ixodidae) on the Leopard Tortoise, Geochelone paradalis.
Journal of Medical Entomology, 1995Co-Authors: Yigal Rechav, Laura Jane FielderAbstract:A survey of Amblyomma marmoreum Koch ticks recorded from the Leopard Tortoise, Geochelone paradalis Bell, in the National Zoological Gardens, Pretoria, South Africa, was carried out over a 19-mo period. No significant differences were found between the tick burdens on male and female Tortoises. A. marmoreum showed a clear seasonal pattern of abundance. Larvae were present on the Tortoise host during late summer, with a peak in February and March; nymphs were abundant during winter, with a peak in June and July. Male ticks were found throughout the year, but females were present only during spring and early to midsummer with a peak in October. It was demonstrated that each developmental stage of A. marmoreum. showed only one peak of activity per year.
Sharon Holt - One of the best experts on this subject based on the ideXlab platform.
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Structural density of the Leopard Tortoise (Stigmochelys pardalis) shell and its implications for taphonomic research
Journal of Archaeological Science: Reports, 2019Co-Authors: Sharon Holt, Liora Kolska Horwitz, Jakobus Hoffman, Daryl CodronAbstract:Abstract We investigated bone mineral density values of the shell (carapace and plastron) of a modern Leopard Tortoise ( Stigmochelys pardalis Bell, 1828) to assess its impact on survivorship of the different skeletal elements. We found significant differences among elements for bone mineral density and density volume, but not for bone mineral content. We then compared these findings with those obtained using computed tomography (CT) scans of shells of three modern Tortoise species that differ in size and shape; the Leopard Tortoise, greater padloper ( Homopus femoralis ) and angulate Tortoise ( Chersina angulata ). Results indicate that the patterning of bone density is similar across shell elements and scan sites in all species, although all values were lower in the greater padloper, which we attributed to shape differences – a flat rather than domed shell. Finally, we correlated the frequency of shell elements from the Holocene Leopard Tortoises recovered at the site of Wonderwerk Cave (South Africa) using, as a proxy, the densitometry values obtained from the modern Leopard Tortoise. For most strata there was a significant association with skeletal element survivorship demonstrating the role played by bone mineral density in determining the skeletal composition of this assemblage.
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Bone mineral density in the Leopard Tortoise: Implications for inter-taxon variation and bone survivorship in an archaeozoological assemblage
Quaternary International, 2018Co-Authors: Sharon Holt, Daryl Codron, Liora Kolska HorwitzAbstract:Abstract In this study we present data on bone mineral density (BMD) values for the cranial and post-cranial skeleton of the Leopard Tortoise (Stigmochelys pardalis). We found significant inter and intra-skeletal variation in different values of BMD within this species. The intra-skeletal, but not inter-skeletal pattern, was comparable to that of terrestrial mammals of similar size, despite differences in bone structure. Overall, the Tortoise has bone density that is intermediate between that of leporids (rabbits/hares) on the one hand and canids (dogs/wolves etc.) and marmots on the other. In this study we have used random effects to test regression models of BMD and demonstrate their value over the simple linear regression models that are currently in use. Finally, we applied the modern Leopard Tortoise proxy data to test whether different parameters of BMD impact survivorship of Tortoise bones in the Holocene strata from Wonderwerk Cave (Northern Cape Province, South Africa). Results indicate that this assemblage has undergone some degree of bone density-mediated attrition, probably influenced by burning and deposition time rather than animal agents.
Yigal Rechav - One of the best experts on this subject based on the ideXlab platform.
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Seasonal abundance of the Tortoise tick Amblyomma marmoreum (Acari: Ixodidae) on the Leopard Tortoise, Geochelone paradalis.
Journal of Medical Entomology, 1995Co-Authors: Yigal Rechav, Laura Jane FielderAbstract:A survey of Amblyomma marmoreum Koch ticks recorded from the Leopard Tortoise, Geochelone paradalis Bell, in the National Zoological Gardens, Pretoria, South Africa, was carried out over a 19-mo period. No significant differences were found between the tick burdens on male and female Tortoises. A. marmoreum showed a clear seasonal pattern of abundance. Larvae were present on the Tortoise host during late summer, with a peak in February and March; nymphs were abundant during winter, with a peak in June and July. Male ticks were found throughout the year, but females were present only during spring and early to midsummer with a peak in October. It was demonstrated that each developmental stage of A. marmoreum. showed only one peak of activity per year.