Browsing by Author "Esterhuizen, N."
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- ItemThe metabolic costs of sexual signalling in the chirping katydid Plangia graminea (Serville) (Orthoptera: Tettigoniidae) are context dependent: cumulative costs add up fast(The Company of Biologists, 2017) Doubell, M.; Grant, P.B.C.; Esterhuizen, N.; Bazelet, C.S.; Addison, P.; Terblanche, J.S.Katydids produce acoustic signals via stridulation, which they use to attract conspecific females for mating. However, direct estimates of the metabolic costs of calling to date have produced diverse cost estimates and are limited to only a handful of insect species. Therefore, in this study, we investigated the metabolic cost of calling in an unstudied sub-Saharan katydid, Plangia graminea. Using wild-caught animals, we measured katydid metabolic rate using standard flow-through respirometry while simultaneously recording the number of calls produced. Overall, the metabolic rate during calling in P. graminea males was 60% higher than the resting metabolic rate (0.443±0.056 versus 0.279±0.028 ml CO2 h−1 g−1), although this was highly variable among individuals. Although individual call costs were relatively inexpensive (ranging from 0.02 to 5.4% increase in metabolic rate per call), the individuals with cheaper calls called more often and for longer than those with expensive calls, resulting in the former group having significantly greater cumulative costs over a standard amount of time (9.5 h). However, the metabolic costs of calling are context dependent because the amount of time spent calling greatly influenced these costs in our trials. A power law function described this relationship between cumulative cost (y) and percentage increase per call (x) (y=130.21x−1.068, R2=0.858). The choice of metric employed for estimating energy costs (i.e. how costs are expressed) also affects the outcome and any interpretation of costs of sexual signalling. For example, the absolute, relative and cumulative metabolic costs of calling yielded strongly divergent estimates, and any fitness implications depend on the organism’s energy budget and the potential trade-offs in allocation of resources that are made as a direct consequence of increased calling effort.
- ItemNitrogen and phosphorus influence Acacia saligna invasiveness in the fynbos biome(2020) Esterhuizen, N.; Forrester, J.; Esler, K.J.; Wigley-Coetzee, C.; Morcillo, R.J.; Kleinert, A.; Perez-Fernandez, M.; Valentine, A.J.This study attempts to understand how invasive legumes such as Acacia saligna may compete with indigenous legumes such as Virgilia divaricata. The two species are trees with similar growth forms. We studied the competitive ability of invasive and indigenous seedlings under variations in soil phosphorus availability. South African fynbos vegetation is threatened by invasive Acacia. The indigenous tree legume, Virgilia, grows in similar phosphorus soil conditions as Acacia although there is a gap in the knowledge of their physiology. We investigated the utilization of different inorganic P sources by the invasive A. saligna and the native V. divaricata in the presence and absence of root nodules in each species. Plant performance in terms of photosynthesis and biomass production was also analysed. Plants were cultivated in silica sand supplied with Long Ashton nutrient solution, modified to contain either 50 mu M P or 500 mu M P applied as NaH2PO4 center dot 2H(2)O. Rate of growth was estimated as the increase in mass in plants harvested after 4 and 8 weeks of growth. After 4 weeks of growth, the seedlings of Virgilia grew quicker and produced more biomass than Acacia, under both phosphorus conditions. However, this was reversed after 8 weeks of growth, with Acacia out-competing Virgilia. Increased growth of the invasive legumes was achieved by relying on soil nitrogen under high phosphorus conditions and shifting to atmospheric sources under lower phosphorus levels. The strategies of altering photosynthetic carbon balance and nitrogen acquisition under varying soil phosphorus conditions potentially underpin the invasive potential of Acacia in fynbos soils.
- ItemThe Zollinger-Ellison syndrome : a case report(Health & Medical Publishing Group, 1985) Du Toit, D. F.; Louwrens, H.; Esterhuizen, N.; Falck, V. C.; Du Toit, L. B.A vagotomy and antrectomy was performed on a 15-year-old boy with malignant gastrinoma for emergency control of massive upper gastro-intestinal haemorrhage from a large posterior penetrating duodenal ulcer in the presence of jejunal ulceration and liver metastases. Hypergastrinaemia was confirmed by elevated serum gastrin levels. In the short term postoperatively the patient's disease has been controlled with oral cimetidine. Although controversy continues over the efficacy of cimetidine in the management of gastrinomas, medical treatment should be considered as an alternative to total gastrectomy in children with malignant gastrinomas because they are often slow-growing, indolent and compatible with long survival.