The behavioural thermoregulation and ecophysiology of the leopard tortoise (Geochelone pardalis) in the Nama-Karoo.

Abstract

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.