Extending the quasi-steady state approximation by changing variables

The parameter domain for which the quasi-steady state assumption is valid can be considerably extended merely by a simple change of variable. This is demonstrated for a variety of biologically significant examples taken from enzyme kinetics, immunology and ecology.     

Carl F. Craver, Explaining what? Review of explaining the brain: mechanisms and the mosaic unity of neuroscience

Carl Craver’s recent book offers an account of the explanatory and theoretical structure of neuroscience. It depicts it as centered around the idea of achieving mechanistic understanding, i.e., obtaining knowledge of how a set of underlying components interacts to produce a given function of the brain. Its core account of mechanistic explanation and relevance is causal-manipulationist in spirit, and offers substantial insight into casual explanation in brain science and the associated notion of levels of explanation. However, the focus on mechanistic explanation leaves some open questions regarding the role of computation and cognition.     

The Expensive Brain: A framework for explaining evolutionary changes in brain size

To explain variation in relative brain size among homoiothermic vertebrates, we propose the Expensive Brain hypothesis as a unifying explanatory framework. It claims that the costs of a relatively large brain must be met by any combination of increased total energy turnover or reduced energy allocation to another expensive function such as digestion, locomotion, or production (growth and reproduction). Focusing on the energetic costs of brain enlargement, a comparative analysis of the largest mammalian sample assembled to date shows that an increase in brain size leads to larger neonates among all mammals and a longer period of immaturity among monotokous precocial species, but not among the polytokous altricial ones, who instead reduce their litter size. Relatively large brained mammals, altricial and precocial, also show reduced annual fertility rates as compared to their smaller brained relatives, but allomaternal energy inputs allow some cooperatively breeding altricial carnivores to produce even more offspring in a shorter time despite having a relatively large brain. Thus, the Expensive Brain framework explains why brain size is linked to life history pace in some, but not all mammalian lineages. This framework encompasses other hypotheses of energetic constraints on brain size variation and is also compatible with the Brain Malnutrition Risk hypothesis, but the absence of a mammal-wide correlation between brain size and immature period argues against the Needing-to-Learn explanation for slower development among large brained mammals.     

Confirmation and explaining how possible

Confirmation in evolutionary biology depends on what biologists take to be the genuine rivals. Investigating what constrains the scope of biological possibility provides part of the story: explaining how possible helps determine what counts as a genuine rival and thus informs confirmation. To clarify the criteria for genuine rivalry I distinguish between global and local constraints on biological possibility, and offer an account of how-possibly explanation. To sharpen the connection between confirmation and explaining how possible I discuss the view that formal inquiry can provide a kind of confirmation-theoretic support for evolutionary models, and offer an example of how-possibly explanation interacting with testing practice.     

Extending the mirror neuron system model,I

The paper introduces mirror neuron system II (MNS2), a new version of the MNS model (Oztop and Arbib in Biol Cybern 87 (2):116–140, 2002) of action recognition learning by mirror neurons of the macaque brain. The new model uses a recurrent architecture that is biologically more plausible than that of the original model. Moreover, MNS2 extends the capacity of the model to address data on audio-visual mirror neurons and on the response of mirror neurons when the target object was recently visible but is currently hidden.     

Hypoxemia lowers cerebrovascular resistance without changing brain and blood [H+]

We designed the present study to see whether, during acute moderate isocapnic hypoxemia, changes in cerebral vascular resistance (CVR) and brain extracellular fluid (ECF) [H+] can or cannot be dissociated from each other. In seven anesthetized and paralyzed dogs we measured brain ECF pH with surface electrodes (n = 4) or double-barreled microelectrodes (n = 3) with tip diameters of less than 30 micron inserted 5 mm below the surface. Cerebral blood flow (CBF) was measured by radioactive microspheres during normoxemia and moderate hypoxemia, whereas brain ECF pH was measured continuously. In six of the seven dogs brain pH did not change during moderate hypoxemia of 4-20 min duration. In these six animals the mean arterial O2 partial pressure decreased from 84.8 +/- 12.9 (SD) to 46.7 +/- 10.2 Torr during hypoxic gas breathing, resulting in a significant drop in CVR from 3.88 +/- 1.88 to 3.27 +/- 1.97 Torr X ml-1 X min X 100 g and a rise in CBF from 31.7 +/- 12.7 to 47.8 +/- 31.5 ml X min-1 X 100 g-1. The mean brain ECF [H+] was 57.4 +/- 8.2 nmol/l (pH = 7.24) during normoxemia and did not change significantly during hypoxic gas breathing [56.6 +/- 7.7 nmol/l (pH = 7.25)]. Furthermore, arterial and sagittal venous blood and cisternal cerebrospinal fluid (CSF) pH did not change significantly during hypoxic gas breathing. We conclude that during acute moderate hypoxemia reduction in CVR can occur independently from increases in brain ECF, cisternal CSF, and arterial and sagittal venous blood [H+] and PCO2.     

Bushmeat,over-fishing and covariates explaining fish abundance declines in the Central Congo Basin

Fish stocks are declining at alarming rates in the Central African forests but little is known about patterns of fishing pressure for different areas. To contribute to the understanding of covariates that could explain these trends, this study documented the relative abundances of fish in eleven sites in the western Democratic Republic of Congo in relation to human density, distance from the nearest markets and mean numbers of fishing nets. Linear regression indicated that, taken alone, 88 % of observed differences in fish relative abundance correlated with the mean number of fishing nets and 60 % of these differences were attributable to distance from the nearest market. Human density was marginally associated with fish abundance. The study also shows that fish and wildlife relative abundances followed the same trends: where more wildlife species remain, there are more abundant fish stocks. The two conservation messages of this paper are that, as is the case for bushmeat, fishing becomes a problem when the commercial market is its prime driver and a solution to the bushmeat crisis could also reduce over-fishing.     

Extending the quasi-neutral concept

In caricature, the equilibrium paradigm of community ecology states that plant communities are stable entities consisting of competing species - and that such species coexist because each has its “niche”. This paradigm, in its extreme, has been dead for some time. Nevertheless, it has yet to be replaced by a credible “non” — equilibrium paradigm. The quasi-neutral concept of plant communities, proposed by Kristjan Zobel,Folia Geobot. 36: 3–8, 2001, possesses some of the key ingredients of a nonequilibrium theory of diversity. It recognizes that there are inescapable relationships between diversity at different scales, that similarity can influence the rate of competitive exclusion, that successional change is typically associated with changes in life form, and that rarefaction (i.e. “sampling artifacts”) can have strong effects on fine-scale diversity. However, the current formulation of the quasi-neutral concept is incomplete in that it relies on an unrealistic definition of community, it assumes that random sampling means that species richness at one scale will be linearly related to richness at finer scales, it ignores the possibility of fine-scale processes producing broader-scale patterns, and it avoids the subject of fine-scale environmental heterogeneity. But the most serious limitation of the quasi-neutral concept is that similarity of species alone is not sufficient to allow indefinite coexistence. I present the results of a simple simulation to demonstrate that: (1) identical species will eventually be lost from communities due to stochastic “drift”, (2) slight variations in reproductive rates accelerate this loss, but (3) adding a miniscule “cost of commonness” to the model allows the indefinite coexistence of species. I conclude that the quasi-neutral model cannot work without some kind of trade-off.     

Family ideation,immigration, and the racial state: explaining divergent family reunification policies in Britain and the US

Why do family reunification policies differ across liberal democracies? Established literature explains differentiation through reference to the logics of distinct “national models” of immigration policy-making. Drawing on critical race and feminist scholarship, this paper finds consistent racial logics in the political histories of family reunification policies in Britain and the US during the mid-twentieth century. In a context where the geopolitical power of each country was conditioned by an assertive antiracist internationalism, “family” provided a colourblind, ideational platform to rearticulate the state-based racial project of white supremacy. Despite an original convergence, family reunification diverged because of the context-specific racialization of the immigrant family in each case. In rooting divergent reunification policies in the logics of the racial state, this paper contributes to efforts to incorporate race into the sociology of immigration. The paper also develops recent feminist scholarship on the role of “family ideation” in building liberal-democratic immigration systems.     

青鳉与食蚊鱼竞争排斥的生态形态学解释

生态形态学理论指出,形态相似的物种生态位相似,是导致种间竞争排斥的关键性因素。在鱼类入侵生态学研究中引入生态形态学理论,对于加深理解入侵种与土著种的种间相互关系有着重要意义。本文利用形态分析方法,对广东怀集燕都国家湿地公园入侵种食蚊鱼对土著种鳍斑青鳉和弓背青鳉的影响机制进行研究。结果表明: 弓背青鳉和鳍斑青鳉在研究区域常同域分布,但两者的空间分布格局差异显著。两者形态高度相似,符合生态形态学“形态相似者竞争排斥”理论。与群落其他鱼类相比,食蚊鱼与2种青鳉鱼类形态更为相似。聚类分析发现,食蚊鱼和青鳉鱼类聚合在同一分枝,且其空间生态位的重叠度极低。种群相对密度调查显示,食蚊鱼与青鳉鱼类种群数量呈显著的负相关关系。食蚊鱼的入侵是导致2种青鳉鱼类种群数量明显下降的关键原因。形态特征的相似性能够初步解释食蚊鱼与青鳉鱼类的竞争排斥关系,尚需从不同角度就食蚊鱼入侵对青鳉鱼类的作用机制进行深入探索。