Browsing by Author "McMaster, Megan Kay."

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The behavioural thermoregulation and ecophysiology of the leopard tortoise (Geochelone pardalis) in the Nama-Karoo.
(2007) McMaster, Megan Kay.; Downs, Colleen Thelma.
The leopard tortoise (Geochelone pardalis) is the largest of the southern African tortoise species and has a wide distribution range. However, there is a lack of ecological and physiological information about the species, especially arid and semi-arid regions. The Nama-Karoo, an arid region of South Africa, is subject to large fluctuations in rainfall, food availability and ambient temperatures (Ta). This study focused on the thermal behaviour, thermoregulatory, digestive and metabolic plasticity of the leopard tortoise within the Nama- Karoo biome. Seasonal changes in activity patterns and body temperature (Tb) were investigated in free ranging leopard tortoises in the Nama-Karoo. Leopard tortoises had unimodal daily activity patterns in winter, bimodal in summer, and there were daily and seasonal differences in the extent to which certain behaviours were practiced. Daily activity behaviours were executed at lower Tb and at lower Ta in winter compared to summer. In summer, core Tb of all tortoises oscillated on a daily basis well below maximum Ta, while core Tb of all tortoises in winter oscillated well above the daily Ta range. Tortoises were therefore able to maintain their Tb independently of Ta. Differences in Tb as measured from various positions on the tortoises body was investigated in relation to Ta. There was a strong seasonal and temporal influence on the relationship between various Tb’s, with the skin and external shell temperatures being more variable in response to fluctuating Ta’s compared with cloacal and core Tb. Cloacal temperatures were significantly different to other Tb measurements suggesting that it should be treated with circumspection as an exclusive measure of Tb. Heating and cooling rates of leopard tortoises were investigated in the field and under controlled laboratory conditions to determine if the tortoises maximise operational daily activity periods, and to determine the effect of behaviour and size on the rate of heat flux. In the laboratory, cooling rates were faster than heating rates in summer and winter for all size classes and decreased with increasing body mass. Leopard tortoises had significantly faster heating and cooling rates in winter than in summer. Free-ranging leopard tortoises had faster heating rates than cooling rates and their heat flux was largely independent of Ta. Heating and cooling rates were dependant on body mass and surface area-to-volume ratio of individuals. Under experimental conditions, tortoises physiologically adjusted their rate of heat flux, while free-ranging tortoises used physiological and behavioural mechanisms to minimise the risk of overheating, to aid thermal inertia and maximise operative activity time. Seasonal climatic cycles and fluctuating daily temperatures influence the oxygen consumption (VO2) of reptiles, however the result of these effects on metabolism in chelonians is poorly understood. The effect of seasonal and daily differences in Ta on VO2 was investigated. Leopard tortoises’ VO2 was slightly higher than reported for other chelonians. There were significant differences in tortoise VO2 at different Ta’s during the day and night and in different seasons. This metabolic plasticity is possibly an adaptive mechanism to cope with unpredictable environmental conditions. Unpredictable climatic conditions lead to unpredictable food and water availability. Little is known how tortoises adjust dietary parameters in response to food type and water availability, and if this affects body mass, energy and water balance. Therefore this study also considered whether leopard tortoises adjusted food transit rate, food intake and water loss to cope with a diet fluctuating in fibre and water content, and whether body mass, energy and water balance were maintained. Leopard tortoises fed a high fibre, low water content diet had lower food intake rates, longer food transit times, but lower daily energy assimilation compared with tortoises fed a low fibre, high water content diet. Tortoises fed a high fibre, low water content diet had lower urine osmolality, but similar total water loss to those fed a high fibre, low water content diet. The results indicate that tortoises can adjust digestive parameters according to diet composition and exercise some control over energy and water balance. It is concluded that leopard tortoises show a high degree of plasticity in their thermal behaviour and physiology which allows survival in an unpredictable environment, particularly where there are fluctuations in rainfall, food availability and Ta’s. Seasonal and daily variation in thermoregulation, metabolic rate and the uptake of energy allows the leopard tortoise to maximise the duration of operative temperature, to minimise energy loss and to use variable and unpredictable seasonal resources.
The status and ecology of the leopard tortoise (Geochelone pardalis) on farmland in the Nama-Karoo.
(2001) McMaster, Megan Kay.; Downs, Colleen Thelma.
The Family Testudinidae (Suborder Cryptodira) is represented by 40 species worldwide and reaches its greatest diversity in southern Africa, where 14 species occur (33%), ten of which are endemic to the subcontinent. Despite the strong representation of terrestrial tortoise species in southern Africa, and the importance of the Karoo as a centre of endemism of these tortoise species, there is a paucity of ecological information for most tortoise species in South Africa. With chelonians being protected in < 15% of all southern African reserves it is necessary to find out more about the ecological requirements, status, population dynamics and threats faced by South African tortoise species to enable the formulation of effective conservation measures. The Leopard Tortoise (Geochelone pardalis) is the largest of the southern African species and has a wide distribution range, occurring in a variety of habitats. There is a paucity of ecological information about Leopard Tortoises in most of these habitats, especially arid and semi-arid regions. The broad aim of the study was to comprehensively investigate the ecology of Leopard Tortoises on farmland in the semi-arid Nama-Karoo biome, and use the ecological information to make recommendations for conservation. An investigation was made to determine the population size, sex and age distributions, density, biomass, and morphometrics of Leopard Tortoises in the Nama-Karoo. It was predicted that Leopard Tortoises would either be similar in size to Leopard Tortoises in other habitats or larger in order to buffer the effects of extreme temperatures by decreasing the surface-to-volume ratio. In addition, it was predicted that Leopard Tortoises would have a smaller population size, and occur at a lower density and biomass per hectare than those tortoises in mesic habitats. A total of 92 tortoises were caught, and 3899 observations made on Leopard Tortoises on 5500 hectares of farmland. Fourteen tortoises were radio tracked which allowed for repeated observations throughout the year. The population was skewed towards adults, and indicates a low hatchling recruitment. Female Leopard Tortoises were significantly larger than males with respect to all morphometric measurements. A male to female sex ratio of 1: 1.6 was obtained, which was not significantly different from 1: 1. A population estimate of 57.64 ± 3.99 tortoises for the 5500 ha area was obtained using a mark-recapture sampling method. Density of tortoises was extremely low at 0.017 tortoises.ha ¯¹, with a biomass of 0.002 kg.ha ¯¹. Population size, density, biomass and morphometrics were compared with Leopard Tortoise populations in other areas. Leopard Tortoises were larger in size in the semi-arid Nama-Karoo compared with Leopard Tortoises in other areas, probably a mechanism to reduce the effects of extreme temperature fluctuations, and were found at a much lower density. Knowledge of the home range size, home range overlap and seasonal change in home range is imperative to the understanding and conservation of the Leopard tortoise. Home range size, percentage overlap and mean daily distances moved were investigated for Leopard Tortoises as a function of season, gender and body mass. Home range and movement data were calculated for 36 Leopard Tortoises (22 females, 14 males), 14 of which were telemetered (8 females, 6 males), and 22 of which were recaptured 10 or more times (14 females and 8 males) over a period of two years. Mean (±SE) home range size for adult Leopard Tortoises was 205.41 ± 45.57 ha. Home range size was not significantly different between males and females, however females had larger home ranges than males. Mean home range size of males was 133.27 ± 32.90 ha, and of females was 251.32± 70.56 ha. There was a significant difference in home range size between telemetered tortoises (413.81 ± 89.46 ha), and those recaptured 10 or more times (72.79 ± 18.89 ha). It is suggested that unacceptable variation in home range size estimations occur when radio telemetry is not used to recapture tortoises throughout the year. No significant relationship between home range size and body mass was found for all tortoises or between sexes. Significant seasonal and gender variation existed in the seasonal home range sizes. Females had larger home ranges than males in all seasons except spring. Mean daily distance moved by Leopard Tortoises was 136.13 ± 8.94 m with males moving further overall daily than females (males: 161.10 ± 11.8 m; females: 117.07 ± 12.87), but not significantly so. Mean daily distance moved per season was significantly different between the sexes. Females covered the largest mean daily distance in autumn and males in spring. Considerable variation existed in the amount of home range overlap both within and between sexes. Overlap percentages ranged from 5% to 90%, with home ranges most frequently overlapping by 20%. Home range size and daily distances moved in the Nama-Karoo are larger than for Leopard Tortoises in other habitats. This has strong implications for the size of reserves needed in conservation efforts with regard to this and perhaps other, species in arid or semi-arid areas. Seasonal activity patterns of Leopard Tortoises were investigated as a function of rainfall, sex, time of day, temperature and time after sunrise. It was predicted that due to seasonal rainfall, and the subsequent increase in food available, the activity patterns of Leopard Tortoises would vary greatly between 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 amount of time spent performing each activity, differed significantly between the seasons. There was no overall seasonal significant difference between the sexes and the level of activity, however, in certain seasons and with regard to specific activities, there were significant differences between the sexes. Activity patterns were primarily bimodal in summer and autumn, and unimodal in winter and spring, with non-thermoregulatory activities, for example walking and feeding, being performed primarily in the afternoon. There was a significant positive correlation between the number of tortoises caught and rainfall per season, but activity levels and the percentage of tortoises walking and feeding was not correlated with seasonal rainfall. The time of day that an activity was first performed in each season, was primarily a function of the time after sunrise and only secondarily of temperature. The response of Leopard Tortoise activity to rainfall, time of day, temperature and time after sunrise, is discussed. With Leopard Tortoises being ectotherms, they rely largely on behavioural thermoregulation to moderate the effects of daily and seasonal fluctuations in ambient temperature on body temperature. Extensive use is made of refuges to facilitate this behavioural thermoregulation. The Nama-Karoo experiences wide temperature fluctuations both daily and seasonally, and therefore the types and seasonal use of refuges by the Leopard Tortoise, in addition to the orientation of the exits and of the tortoises within the refuges, was investigated. A wide variety of refuges were used, but Lycium spp., Eberlanziaferox (Doringvygie), Opuntiajicus (American Prickly Pear) and grass clumps were preferred as refuges. There was seasonal variation in the use of these refuges that further depended on whether the refuges were used as forms or shelters. Leopard Tortoises in spring and winter often remained in the same refuge for the entire season, or returned to the same refuge on consecutive nights. There was seasonal and behavioural variation in a) compass direction that the tortoises were facing within a refuge, b) compass direction that exits of the refuges were open to, and c) portion of the shell of each Leopard Tortoise within a refuge that was exposed to sun radiation. Tortoises in winter and spring used these three factors to maximise the amount of solar radiation received on their carapace, while tortoises in summer and autumn used them to minimise solar radiation received. Therefore, using a combination of refuge type, exit orientation and tortoise orientation, Leopard Tortoises were able to passively thermoregulate and further control temperature fluctuations experienced in an extreme environment. Leopard Tortoises on farmland in the Nama-Karoo had lower densities, larger body sizes and much larger home ranges than Leopard Tortoises in other habitat types. This is an important aspect to take into account when planning for the conservation of Leopard Tortoises in semi-arid areas, and may hold further implications for other arid or semi-arid tortoise species. Activity patterns and patterns of thermoregulation allow for further understanding of the interactions between tortoises and their environment, habitat, and climate in the wild. In addition, it further aids in the understanding of the methods used by ectotherms to thermoregulate and manipulate body temperatures, especially when living in regions of unpredictable rainfall and extreme temperatures.