Browsing by Author "Louw, G N"

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    Behavioural and eco-physiological studies on blue wildebeest Connochaetes taurinus at the Etosha National Park
    (1980) Berry, Hermanus Human; Louw, G N
    An investigation into the decline of blue wildebeest numbers at Etosha National Park was made during the period 1974-1978. Aspects of wildebeest ecology which were investigated were activity patterns, energy and protein budgets, nutrition, nutritional status, disease and parasites, predators and scavengers, and population structure. Activity patterns in the free-ranging population were measured over a period of one year, using marked individuals and by scanning herds. The population spent, on average, 53% of its time resting, 33% grazing, 12% in movement, 1,5% in overt social encounter sand less than 0,5% drinking and suckling. Photoperiod and temperature were the primary correlates of activity/inactivity and a linear relationship existed between increasing temperature and increasing inactivity. The activity data were used to calculate an energy budget for maintenance and activity in the population, while energy demand for growth, gestation, lactation and homeothermy were estimated by formulae based on domestic ruminants. Mean increment over resting metabolic rate for free existence in wildebeest approximated 2,0. This represented a mean annual energy demand of 8,5 gigajoules per wildebeest. Similarly, a protein budget was estimated for the population on a seasonal basis.
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    Cyanogenesis and the feeding preference of Acraea horta (L.) (Lepidoptera : Acraeinae)
    (1987) Raubenheimer, David; Louw, G N
    The statement that similar sorts of plants often have similar medicinal properties (Le. contain similar chemicals) is at least 300 years old, while the concept probably dates back several thousand years (Stace, 1980). Indeed, a knowledge of the secondary chemicals produced by different plant species has played an important role since the early stages of man's cultural, and probably evolutionary, development (e.g. Leopold and Ardrey, 1972). Until recently, however, the biological role of this large group of compounds has remained largely obscure, with many plant physiologists regarding them as waste products and of no possible survival value to plants (Harborne, 1982). In recent years a tremendous increase in attention paid to these compounds has led to the suggestion that plant secondary substances evolved as herbivore deterrents, and diversified with the plants producing them as herbivores evolved various means of coping with their toxicity. In what follows I present an overview of events leading to the establishment of this theory, and a history of the discovery that some insect herbivores have turned the toxicity of plant secondary compounds to their own advantage, thereby becoming exclusively associated with toxic food plants. It will be seen that although this theory of plant-herbivore coevolution has become generally accepted, it has yet to be tested for a wide range of herbivores and chemical types. From this view I have taken a detailed look at a specific group of herbivores and plant toxins: Lepidoptera feeding on cyanide-producing plants.
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    Thermal physiology of selected African ungulates with emphasis on the physical properties of the pelage
    (1981) Hofmeyr, Margaretha Delina; Louw, G N
    The ungulates include some of the largest land animals and, in consequence, they cannot avoid climatic stress as do small mammals. A great deal of information concerning the physiology of thermoregulation in domestic ungulates is available but relatively little is known about thermoregulation in wild ungulate species. Moreover, in the past, thermoregulation studies were conducted mainly in climatic chambers and consequently the effects of certain important environmental parameters were ignored. The pelages of mammals provide a barrier between the animal and its environment, and the physical characteristics of the pelage can feature prominently in the thermal adaptations of animals to specific climatic zones. Scholander et at. (1950), for example, have demonstrated the value of a thick pelage in arctic mammals. The thick pelage provides good protection against heat loss and thus extends the ambient temperature limits which the animal can tolerate without the need to increase metabolic heat production. Schmidt-Nielsen (1959) has shown that a thick pelage can also be advantageous to an animal in a desert environment. The thick, dorsal fur in the camel provides protection against excessive heat gain from the environment and thereby assists with water conservation. The colour of a mammal's pelage is also involved in the thermal adaptations to specific environmental conditions.