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. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2007.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/10785 |
| Date | 30 May 2014 |
| Creators | McMaster, Megan Kay. |
| Contributors | Downs, Colleen Thelma. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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