Thesis (MSc (Botany and Zoology))--University of Stellenbosch, 2011. / Includes bibliography. / ENGLISH ABSTRACT: Thermal ecology is a central theme in reptilian biology because of the thermodynamic rate
dependence of virtually all biological processes in these ectothermic animals.
Thermoregulation includes active processes (with associated energetic costs related to
altered behaviour and physiology) functioning to maintain body temperatures within a
preferred temperature range, so that the majority of physiological functions occurs optimally,
despite natural variation in the animal’s thermal habitat. The recent development of
quantitative thermal indices now allows researchers to describe the thermal habitat and
thermoregulatory functioning of an ectotherm within its environment from a cost-benefit
perspective. The use of such quantitative biophysical approaches to reptile thermal ecology
studies is however limited in the African context. Cordylus cataphractus is one of the best
studied cordylids, and exhibits various characteristics atypical for the family, such as
permanent group-living, seasonally lowered surface activity, a low resting metabolic rate and
large fat bodies. These characteristics are generally thought to be associated with groupliving
in a semi-arid habitat, yet, the possible links to thermal ecology remains unexplored.
The objectives of the current study was: firstly, to characterize the preferred
temperature range (Tp) of C. cataphractus through the use of ecologically realistic laboratory
thermal gradients; secondly, to explore seasonal and geographical variation in thermal
preference, by comparing Tp among individuals captured from a coastal and inland
population and during different seasons (autumn and spring); thirdly, to describe the thermal
habitat of a C. cataphractus population during summer, autumn, winter and spring and to
then relate these findings to the seasonal activity patterns reported in literature for the
species; fourthly, to describe the seasonal patterns of thermoregulation (during summer,
autumn, winter and spring) in a C. cataphractus population through quantitative
thermoregulatory indices; fifthly, to assess geographic variation in the thermal habitat and
IV
associated patterns of thermoregulation in C. cataphractus among a coastal population
(western range limit) and an inland population (eastern range limit). The thermal habitat of C.
cataphractus was described by measuring operative environmental temperatures (Te) with
hollow copper lizard models placed around rocks according to the natural surface movement
patterns of the species. Variation in thermal habitat quality was subsequently calculated (de
= |Te – Tp|) and averaged. Field body temperatures (Tb) of lizards were measured with
dorsally attached miniature temperature loggers. Thermoregulatory indices were calculated
from Te, Tb and Tp, describing: thermoregulatory accuracy, the effectiveness of
thermoregulation and thermal exploitation for each population (coastal and inland) for the
respective sampling periods.
The preferred body temperature range of C. cataphractus is the lowest recorded
among cordylids to date (mean Tp = 29.8oC) and was conserved among different populations
and within these populations among seasons, despite the fact that environmental
temperatures are known to vary geographically and seasonally.
Thermal habitat quality varied significantly at micro spatial scale around rocks in the
coastal population. Since C. cataphractus males are territorial, competition for thermal
habitat quality around rocks may therefore occur. Such effects will be a function of the time
of year since the variability in thermal habitat quality among rock aspects (around rocks)
varied seasonally.
Thermal habitat quality of crevices varied among seasons and was typically higher in
the open, outside rock crevices, during the cooler winter and spring periods, whereas in
summer and autumn the crevice environments were more favourable. Thermal habitat
quality was high in crevices during autumn, suggesting that the observed repressed surface
activity of C. cataphractus described for the time is not necessarily, as previously thought,
only due to food constraints. Moreover, in contrast to earlier reports, the current results (Tb
versus Te) indicate that individuals emerged from crevices in summer.
The geographical assessment indicated that lizards from the coastal population, with
generally larger groups, thermoregulated more successfully than those from the inland population. The higher thermoregulatory success in the coastal population occurred in spite
of the fact that thermal habitat quality was significantly lower at the coastal locality. The
higher thermoregulatory success in the coastal population was likely due to reduced
predation risk associated with increased group-size. The seasonal trends in
thermoregulation at the coastal and inland population corresponded to the patterns predicted
by the cost-benefit model of thermoregulation, accuracy of thermoregulation and the
effectiveness of thermal exploitation being higher during the thermally more favourable
autumn. / AFRIKAANSE OPSOMMING: Uittreksel
Termiese ekologie is ‘n sentrale tema in reptiel-biologie as gevolg van die termodinamies
tempo-afhanklikheid van feitlik alle biologiese prosesse in hierdie ektotermiese diere.
Termoregulering sluit aktiewe prosesse (wat lei tot energie-koste in terme van gedrag en
fisiologie) in om liggaamstemperature binne ‘n vasgestelde voorkeurtemperatuur-reeks te
handhaaf sodat fisiologiese prosesse optimaal kan geskied te midde van natuurlike variasie
in die dier se termiese omgewing. Die onlangse ontwikkeling van kwantitatiewe funksionele
termiese indekse stel navorsers nou in staat om die werklike termiese omgewing en die
funksionering van die ektoterm binne sy omgewing te beskryf en uit ‘n koste (energie)-
voordeel oogpunt te verstaan. Die gebruik van hierdie biofisiese koste-voordeel benadering
in reptiel termoreguleringstudies is egter beperk in die Afrika-konteks. Cordylus cataphractus
is een van die bes bestudeerde lede van familie Cordylidae, en vertoon verskeie eienskappe
ongewoon vir hierdie groep akkedisse, soos groeplewendheid, beperkte seisoenale aktiwiteit
buite hul skeure, ‘n relatiewe lae rustende metaboliese tempo en relatiewe groot vetliggame.
Hierdie unieke eienskappe is al deur navorsers gekoppel aan die groeplewe lewensstrategie.
Die potensiële koppeling van die termiese ekologie en die spesifieke lewensstrategie
van C. cataphractus benodig verdere studie. Die doelwitte van hierdie studie was
eerstens: om die voorkeurtemperatuur-reeks (Tp) van C. cataphractus te bepaal deur van
ekologies-realistiese termiese gradiënte in die laboratorium gebruik te maak; tweedens: om
geografiese en seisoenale variasie in Tp te ondersoek deur individue te gebruik wat uit
binnelandse en kus-populasies, tydens verskillende seisoene, herfs en lente versamel is;
derdens: om die termiese omgewing, meer spesifiek die variasie in termiese kwaliteit, binne
die habitat van C. cataphractus populasie in verskillende seisoene, somer, herfs, winter en
lente, te moduleer en met die gedokumenteerde aktiwiteitspatrone in verband te bring;
vierdens: om die seisoenale temoreguleringspatrone (tydens somer, herfs, winter en lente)
van C. cataphractus populasie te beskryf; vyfdens: om geografiese variasie in die termiese
habitat en geassosieerde termoreguleringspatrone tussen kus-populasie (westelike
VII
verspreidingsgrens) en binneland-populasie (oostelike verspreidingsgrens) te bestudeer.
Die kwaliteit van die termiese habitat van C. cataphractus is bepaal deur hol koper-modelle
van akkedisse (operatiewe temperatuur modelle (Te)) te plaas rondom rotse in
ooreenstemming met die natuurlike bewegingspatrone van die akkedisse. Die termiese
kwaliteit is gevolglik afgelei (de = |Te – Tp|) en gemiddeldes bereken. Die
liggaamstemperature (Tb) van vrylopende akkedisse in die veld is met dorsaal-gemonteerde
miniatuur temperatuur “data-loggers” gemeet. Termiese indekse (deur Te, Tb en Tp te
gebruik) is bereken om die akkuraatheid en effektiwiteit van termoregulering, sowel as
termiese benutting van die omgewing vir beide populasies (kus en binneland) tydens
verskillende seisoene te beraam.
Die voorkeurtemperatuur-reeks van C. cataphractus is die laagste gedokumenteerde
temperature vir enige lid van die familie Cordylidae tot op hede bestudeer (gemiddeld van Tp
= 29.8oC), en het ten spyte van die feit dat omgewingstemperature wissel op geografiese en
seisoenale vlakke, min gevarieer tussen die twee populasies asook tydens verskillende
seisoene binne die populasies.
Die termiese kwaliteit het beduidend gevarieer tussen seisoene en binne die mikroruimtelike
omgewing rondom rotse in die kus-populasie. Aangesien C. cataphractus
mannetjies territoriaal is, word die aanname gemaak dat kompetisie vir ‘n ruimtelike posisie
ook ‘n termiese koste mag hê aangesien daar beduidende variasie in de om die rotse was.
Variasie in termiese kwaliteit rondom rotse was verder ook funksie van die tyd van die jaar
(seisoene).
Die termiese kwaliteit van skeure het gevarieer tussen seisoene, en termiese
kondisies/toestande was oor die algemeen meer gunstig buite die rots-skeure tydens die
koeler winter en lente tydperke, terwyl skeure termies meer gunstig was in die somer en
herfs maande. Termiese habitat kwaliteit van skeure was besonders hoog gedurende die
herfs, en die voorspelling is dus dat die verlaagde oppervlak-aktiwiteit wat gedurende hierdie
tyd van die jaar vir C. cataphractus gedokumenteer is nie noodwendig funksie van
beperkte voedselbeskikbaarheid is nie. Teenstrydig met gepubliseerde aktiwiteitsrekords dui die resultate (Tb teenoor Te) verder daarop dat individue wel uit skeure kom tydens die warm
somer seisoen.
Die geografiese ondersoek het gewys dat akkedisse van die kus-populasie (wat
gewoonlik uit groter groepe bestaan), meer akkuraat getermoreguleer het as akkedisse van
die binneland-populasie. Die hoër akkuraatheid van termoregulering in die kus-populasie is
bewerkstellig ten spyte van die feit dat die termiese kwaliteit beduidend laer was as die van
die binneland-populasie. Die hoër termoreguleringsakkuraatheid in die kus-populasie kan
waarskynlik toegeskryf word aan laer predasie-risiko geassosieer met groter groepe. Die
seisoenale variasie-patroon van termoregulering kan verklaar word deur die koste-voordeel
model van termoregulering, waarvolgens die akkuraatheid van termoregulering sowel as
termiese benutting hoër is tydens periodes van hoë termiese kwaliteit (i.e. herfs).
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/6603 |
| Date | 03 1900 |
| Creators | Truter, Johannes Christoff |
| Contributors | Van Wyk, Johannes H., Mouton, P. le Fras N., University of Stellenbosch. Faculty of Science. Dept. of Botany and Zoology. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 177 p. : ill. |
| Rights | University of Stellenbosch |
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