Browsing by Author "Louw, Gideon"

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    Life history and physiological ecology of the lizard, Cordylus Giganteus
    (1992) Van Wyk, Johannes H; Louw, Gideon; Turner, Scott; Nicolson, Sue W
    Cordylus giganteus is a large, terrestrial, viviparous lizard, endemic to the Highveld grasslands of South Africa. Its distribution is limited and its conservation status is vulnerable. Autopsy and mark-recapture methods were used to study the seasonal aspects of its reproductive cycle, diet, energy reserves, growth, population dynamics, daily activity and thermoregulation. Reproduction is distinctly seasonal in both sexes. Females may reproduce biennially. Vitellogenesis commenced in autumn (March), and continued through hibernation with ovulation in spring (October). Two or three young are born in autumn. A functional placenta is implicated. Seasonal steroid hormone profiles are presented. Males exhibit a postnuptial spermatogenetic cycle. Spermatogenesis commences in spring with peak spermiogenesis in autumn and testicular regression following in late autumn. Spermatozoa are stored in the epididymis and ductus deferens for seven to eight months. A bimodal plasma testosterone profile is reported, consistent with spermiogenesis in autumn and mating behaviour in spring. C. giganteus feeds during 8 months of the year and prefer Coleoptera as prey. Fat bodies are utilized for winter maintenance and reproduction. Hatchlings grow 20-30mm during the first year and maximum growth rates occur in summer. Males and females attain sexual maturity at about 165 mm SVL in the fourth year. Seasonal affects on growth rate resulted in poor fit by either logistic-by-length or von Bertalanffy models and a seasonal oscillating model was introduced. Adult males are smaller than females; head sizes are the same but allometric slopes differed significantly. Population size and structure remained stable in the study area. Densities ranged from 9 - 11 lizards/ha. The age structure is marked by the low relative abundance of juveniles. Survivorship during the first year varied among years. Mortality was highest during summer months rather than winter months. Average annual survival of adults 'was high, but varied with sex and years (ranged from 58%-80%). A life table yielded a net reproductive rate (Ro= 1) sufficient to sustain the population, if the reproductive life of an adult female is at least 12 years. Lizards remain in their burrows during winter. In summer, activity was bimodal on sunshine days but unimodal on cool overcast days. Body temperature is regulated by behavioural means (postural and orientation changes) and by shuttling to the cool burrow microclimate. The life history strategy corresponds partially to that of K-selection.
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    The thermal implications and ecological consequences of coloration in selected species : tenebrionid beetles (Onymacris bicolor and Onymacris ungui cularis), Cape gannets (Morus capensis) and Cape cormorants (Phalacrocorax capensis)
    (1989) Lombard, Amanda T; Louw, Gideon
    The thermal significance of coloration was investigated in two species of Namib Desert tenebrionid beetles and two species of marine birds. Body temperatures and heat fluxes of a black beetle (Onymacris unguicularis) and a beetle with white elytra (Onymacris bicolor) were compared in a wind tunnel in the laboratory. The effects of visible radiation, infrared radiation, conduction, convection, beetle colour and substratum colour on body temperature were analysed. Results showed that body colour has no overall effect on body temperature. Black elytra are warmed more by visible radiation, but colour is not relevant to heat loss by convection, or to heat flux between a beetle and a heated sand substratum, whether by emitted radiation or reflected visible radiation. 0. bicolor absorbs more heat by conduction and free convection from a heated substratum, but differences in shape between the two species may explain this effect. Combining the various modes of heat exchange to simulate natural conditions reveals that the extra heat absorbed by black elytra is readily dissipated by convection, and owing to the increased heating of white beetles from the substratum, heat fluxes between the two beetles balance. Body temperatures of dead and live beetles of both species were also measured in the field. Experiments were conducted on a hot sand substratum in the beetles' natural environment, under both visible (sunny) and infrared (shaded) conditions. Results supported the laboratory experiments, and showed that when temperature differences do occur between black and white beetles, these differences are generally less than 3.5 °C. These differences are small when compared with the ranges of body temperatures experienced by active beetles in the field (± 10 °C). In addition, these temperature differences occur only at low wind speeds (< 2 m s-1). Activity studies in the field showed that beetles choose to be active in high wind speeds, possibly because of the nature of their food source, which is wind-blown detritus. It is concluded that coloration does not have adaptive value in terms of the thermal biology of Namib Desert tenebrionid beetles. Physical properties of the plumages of white Cape gannets (Morus capensis) and black Cape cormorants (Phalacrocorax capensis ) were measured. Black plumages absorb more visible radiation than white plumages in still conditions. However, laboratory experiments with excised plumages showed that at wind speeds of 2 m s-1, cormorant plumages and skins were only 2-3 °C warmer than those of gannets. These differences disappeared at wind speeds of (< 2 m s-1). A biophysical heat transfer model predicted that in still, warm, sunny conditions, cormorants may gain up to 185 of their field metabolic rates, whereas gannets would gain only 42 . Field observations confirmed that nesting cormorants experience greater heat stress than gannets, even though cormorant nests occur in areas of lower micrometeorological temperatures. Cormorants begin to dissipate heat by evaporative water loss (i.e. pant) at lower environmental temperatures than gannets. The thermal consequence of coloration in these two species are that cormorants may have a lower cost of endothermy at temperatures below the thermoneutral zone, but may experience more heat stress during warm conditions; cormorants select cooler and windier nesting sites than gannets; and increased surf ace temperatures of black cormorant plumages may aid evaporative water loss from wet plumages, facilitating wing-drying. However, ptiloerection and wind may interact in the natural environment, negating the differential heating effects of coloration. It is concluded that the thermal implications of colour are negligible in both species, considering the temperate nature of their environment; Colour in both species is best explained by feeding ecology: white coloration is conspicuous to conspecifics and cryptic to prey in plunge divers (gannets), whereas black colour is cryptic to both conspecifics and prey in solitary swimmers (cormorants).
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    Water and energy balance in the Namib sand-dune lizard, Angolosaurus skoogi
    (1992) Clarke, Brian Craig; Louw, Gideon; Nicolson, Sue W
    The field metabolic rate (FMR) and water flux of the herbivorous Namib sand-dune lizard, Angolosaurus skoogi, was measured during the summer non-breeding period (early January) by means of doubly labelled water. FMR was about half that expected for a lizard of this size. Water flux and laboratory measured standard metabolic rates were, however, typical of other desert lizards. It thus appeared a priori that this species was behaviourally reducing metabolic costs through reduced activity. This generated questions with regard to the ecological and physiological significance of low FMRs. Is the reduction forced by intolerable environmental conditions or the need to stay in positive water and energy balance? Alternatively, is activity reduction voluntary and geared towards optimizing growth and storage, possibly in preparation for periods of physiological stress ? The research required to answer these questions forms the basis of this thesis. There has been much discussion about the evolutionary and ecological significance of growth rates. The consensus seems to be that, under most circumstances, animals are adapted in ways that maximize growth or optimize growth in relation to realistically attainable rates of gross energy intake. Animal behaviour may be influenced by factors unrelated to growth considerations such as starvation, involuntary hypo/hyperthermia, predator evasion and social interactions. These factors may be difficult to identify and isolate, making this hypothesis difficult to test. Previous studies on the biology of A. skoogi have shown that this species has a perennially available food source, a wide range of potential body temperatures at its disposal and a low predation risk. Social interaction appear to be confined to the breeding season. This species is thus a particularly suitable subject for a scope for growth optimization study.