Scielo RSS <![CDATA[South African Journal of Science]]> vol. 104 num. 5-6 lang. en <![CDATA[SciELO Logo]]> <![CDATA[<B>A biomass gasification project in the rural Eastern Cape</B>]]> <![CDATA[<b>Lessons from aloes in the Thicket Biome: reconstructing past elephant browsing to understand the present</b>]]> <![CDATA[<B>The contexts of discovery of <I>Australopithecus bahrelghazali</I> (Abel) and of <I>Sahelanthropus tchadensis</I> (Toumaï)</B>: <B>unearthed, embedded in sandstone, or surface collected?</B>]]> <![CDATA[<B>Mechanical nano-resonators at ultra-high frequency and their potential applications</B>]]> Ultrafast nanomechanical resonators have attracted significant attention in research, and have promised a wide range of future applications. Choosing appropriate materials, optimizing fabrication processes, applying ultra-sensitive ultra-high-frequency readout techniques and adapting such devices to a variety of applications are all important subjects to address. This article reviews the current status of this field, and discusses future development directions. <![CDATA[<B>Biomathematics in Africa</B>: <B>an emerging activity</B>]]> Ultrafast nanomechanical resonators have attracted significant attention in research, and have promised a wide range of future applications. Choosing appropriate materials, optimizing fabrication processes, applying ultra-sensitive ultra-high-frequency readout techniques and adapting such devices to a variety of applications are all important subjects to address. This article reviews the current status of this field, and discusses future development directions. <![CDATA[<b>Chaotic linear systems in mathematical biology</b>]]> Chaotic phenomena occurring in the natural sciences have been a focus of interest of both theoreticians and experimentalists for over five decades. Usually associated with nonlinear systems, they can also appear in infinite dimensional linear ones; that is, governed by, for instance, infinite systems of ordinary differential equations or partial differential equations. In this paper we shall provide a survey of mathematical tools which are necessary to analyse chaotic linear systems and discuss several examples of such systems arising in mathematical biosciences, ranging from drift-diffusion processes, through mutations, to the production of blood cells. We shall also attempt to address some questions related to the relevance of the mathematical analysis to properties of real-life systems. <![CDATA[<B>Predator density-dependent prey dispersal in a patchy environment with a refuge for the prey</B>]]> In this article, we examine a two-patch predator-prey model which incorporates a refuge for the prey. We suppose that prey migration is dependent on predator density, according to a general function. We consider two different time scales in the dynamics of the model, a fast one describing patch to patch migration, and a slow one involving local prey and predator interaction. We take advantage of the time scales to reduce the dimension of the model by use of methods of aggregation of variables, and thereby examine the effect of predator density-dependent migration of prey on the stability of the predator-prey system. We establish a simple criterion of viability, namely, the existence of a positive and globally stable equilibrium, and show that density dependence has beneficial effects on both species by providing larger equilibrium densities. <![CDATA[<B>Participatory assessment of the South African abalone resource and its impact on predicted population trajectories</B>]]> Illegal harvesting is a cause for concern in many of the world's fisheries. Over the last decade, the abalone resource in South Africa has come under severe fishing pressure, largely because of increased and unmitigated levels of poaching. The unquantified illegal exploitation of this resource is a major impediment to management, because understanding of abalone population dynamics is affected. Incorrect assessments of population abundance could lead to inadequate attempts by management to stem the decline. Here, population trends along the west coast of South Africa are investigated. A simple discrete-time logistic model was used to estimate parameters within a maximum likelihood statistical framework by fitting to available catch rate data. To address the problem of unknown levels of illegal catch, interview data were collected on non-commercial catch trends and the model was structured to allow this catch to be estimated during the fitting process. The results show that such a participatory approach to stock assessment can lead to an improved understanding of resource dynamics, illustrating the benefit this approach may have for management. <![CDATA[<B>Modelling the dynamics of animal groups in motion</B>]]> Animal groups in motion, examples being fish schools, bird flocks, insect swarms and mammal herds, can exhibit spectacular collective behaviour. Attempts at formalizing the basic individual behaviour that accounts for the complex dynamics of an animal group began over 50 years ago. Numerous models of these dynamics have since been published. We review this information, starting with an overview of various approaches that have arisen from mathematical, physical, and computer-modelling methods. Our focus is on individual-based models of animal groups. Individuals are assumed to exert three types of local interactions with their neighbours in these models, namely attraction, alignment, and repulsion. We review these models according to their main objectives: to compare modelled results with observational data; to analyse the influence of model parameters on simulated group properties; and to investigate group response to any change in environment, or to divergent behaviour of some of the group members. <![CDATA[<B>Relating incidence to 'recent infection' prevalence</B>: <B>application to HIV</B>]]> I present a systematic approach to the derivation of relationships between disease incidence and the prevalence of an experimentally defined state of 'recent infection'. These depend, in general, on details of the population dynamic and epidemiological history, as well as the physiology of early disease progression. The general relations facilitate the evaluation of numerous approximate schemes that could be used for the purpose of estimating incidence from snapshot surveys. Methods for calibrating an incidence/prevalence relation from follow-up studies, for use in subsequent snapshot surveys, are considered. Example data from an ongoing follow-up study are analysed. Statistical power and prospects for practical implementation in southern Africa are also considered. <![CDATA[<B>A time discrete linear model of an age and time of residence structured population in a patchy environment</B>]]> We present a multiregional Leslie matrix population model for an age-structured population in a heterogeneous environment of patches connected by migrations. The aim of the model is to take into account the time of residence of individuals in the patches. We present an application of the model in ecotoxicology. We consider a population located on two patches with a polluted patch whereas the other one is unpolluted. We study the effects of pollutant concentration and of the time of residence in the polluted patch on the global population growth rate. The model could be generalized to the case of N patches (N > 2) for Salmo trutta and cadmium concentration. <![CDATA[<B>Computer construction of species richness maps</B>: <B>testing a new type of multifractal algorithm</B>]]> We show how a new theoretical multifractal model provides means to generate virtual maps of highly variable spatial distributions of species richness. It should allow for various computer experiments in landscape ecology and the study of biodiversity. In this paper, the explicit distribution of species-representative individuals over a large range of scale leads to an original algorithm for the estimation of the Renyi dimensions of a multifractal measure. The method is successfully tested for simulated (S, A) data sets, where the variableS is simply the number of species found in a given domain of area A. This easy tool will help to characterize the spatial variability of multiscale density distributions in many fields, requiring only randomly sampled data at different locations and scales. <![CDATA[<B>A model of HIV infection with two viral strains and cytotoxic T-lymphocyte response under structured treatment interruptions</B>]]> We develop a model of HIV infection with two viral strains, a cytotoxic T-lymphocyte (CTL) response and structured treatment interruptions. We derive new analytical relations characterizing the dynamics between drug-sensitive and drug-resistant variants of the virus, and the strength of the CTL response. <![CDATA[<B>A law of time dilation proportionality in Keplerian orbits</B>]]> We examine the Lorentz factor and the Schwarzschild solution in relation to the estimation and verification of time dilation in particular from Global Positioning System data. As a result, we detect the possible occurrence of a proportionality between the time dilation effects of special and general relativity in free-fall motion in Keplerian orbits. This observation is then proved mathematically. The results show that gravitational time dilation during free fall in Keplerian orbits must be exactly double that caused in special relativity due to linear velocity. We propose that a law be enunciated in view of the proof provided, and of the experimental and technological verification of time dilation effects during the past six to seven decades. The importance of this finding as a universal phenomenon and in the further development of stable clocks and satellite technology is highlighted. <![CDATA[<B>Palaeobiological implications of the bone microstructure of South American traversodontids (Therapsida: Cynodontia)</B>]]> We document the bone microstructure of traversodontids from Argentina and Brazil and make detailed comparisons of the bone microstructure of these cynodonts with that of other traversodontids, nonmammaliaform therapsids (including cynodonts), and Mesozoic mammalian taxa. Our analysis provides information on traversodontid ontogenetic age, growth patterns and life-style adaptations. The osteohistological data derived from the current study, as well as data on other nonmammaliaform cynodonts, provide an understanding of the evolution of life-history patterns among the therapsids. We show that many of the nonmammaliaform cynodonts (including some traversodontids) formed bone at rapid rates but retained flexible developmental growth patterns. In addition, although we observe a trend of increasingly rapid osteogenesis among the traversodontids, our findings do not discern a particular bone tissue type as a synapomorphy of the clade. This reinforces earlier hypotheses that the microscopic structure of bone is influenced by several intrinsic and extrinsic factors, and is not constrained by phylogeny alone. <![CDATA[<b>Middle Miocene vertebrate fauna from Pemba Island, Tanzania</b>]]> This paper records the presence of Middle Miocene terrestrial and marine vertebrates in sedimentary deposits on Pemba Island in the Indian Ocean, offshore Tanzania. Although the fossil assemblage is restricted, the affinities of the terrestrial mammals are typical of African continental faunas of basal Middle Miocene age, c. 16.5 million years ago. The Pemba Series has been reported to be of Lower-Middle Miocene age on the basis of marine invertebrates, in agreement with the age estimates based on a mandible of Libycochoerus massai from the deposits. The discovery is important because it provides a potential means of checking correlations between marine and continental biochronological time scales in a part of the world that has previously yielded no evidence of terrestrial palaeontology. <![CDATA[<B>HIV stigma and mental health status among women living with HIV in the Western Cape, South Africa</B>]]> Since the beginning of the epidemic, people living with HIV and the social groups to which they belong have been stigmatized worldwide. This cross-sectional study, conducted between July and November 2003, investigated the association between HIV stigma and mental health status among black women living with HIV in the Western Cape province of South Africa. Eligible participants completed a questionnaire that assessed HIV stigma, sociodemographic, and mental health status measures. Participants were recruited from one of five primary health care clinics in the rural Western Cape. Recruiters screened 177 women to assess their eligibility. Of those screened, 68% (n = 120) were eligible because they were black South Africans, between the ages of 18 and 45, were living with HIV/AIDS, sought primary health care from one of the five study clinics, spoke Xhosa, and provided written informed consent. A priori hypotheses postulated that women reporting more HIV stigma would experience more consequences for mental health sequelae. The main outcome measures were mental health status variables, including depressive symptomatology, stress of HIV discrimination, quality of life, post-traumatic stress, suicidal ideation and fear of HIV disclosure. In linear regression models, more reports of HIV stigma were associated with significantly more depressive symptomatology (P = 0.03) and a lower quality of life (P = 0.00). The findings from this study indicate that HIV stigma is associated with adverse mental health sequelae among black Xhosa women living with HIV. Educational and legal efforts are required to reduce HIV stigma. Moreover, a public health infrastructure that integrates HIV/AIDS treatment and mental health services may be a promising strategy for reducing the adverse mental health consequences of HIV stigma.