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Cortical spreading depression (CSD) produces propagating waves of transient neuronal hyperexcitability followed by depression. CSD is initiated by K+ release following neuronal firing or electrical, mechanical or chemical stimuli. A triphasic (30–50 s) cortical potential transient accompanies localized transmembrane redistributions of K+, glutamate, Ca2+, Na+, Cl− and H+. Accumulated K+ in the restricted interstitial space can cause both further neuronal depolarisation and inward movement of K+ into astrocytes that buffers this increased extracellular K+ concentration ([K+])o. However, astrocyte interconnections may then propagate the CSD wave by K+ liberation through an opening of remote K+ channels by volume, Ca2+ or N-methyl-D-aspartate receptor activation. Changes in cerebral blood volume and in apparent water diffusion co-efficient (ADC) accompanying CSD were first studied using magnetic resonance imaging (MRI) in whole lissencephalic brains. Diffusion-weighted echoplanar imaging in gyrencephalic brains went on to demonstrate CSD features that paralleled classical migraine aura. The ADC activity persisted minutes/hours post KCl stimulus. Pixelwise analyses distinguished single primary events and multiple, spatially restricted, slower propagating, secondary events whose detailed features varied with the nature of the originating stimulus. These ADC changes varied reciprocally with T2*-weighted (i.e. referring to spin-spin relaxation times) waveforms reflecting local blood flow. There followed prolonged decreases in cerebral blood flow culminating in late cerebrovascular changes blocked by the antimigraine agent sumatriptan. CSD phenomena have possible translational significance for human migraine aura and other cerebral pathologies such as the peri-infarct depolarisation events that follow ischaemia and brain injury.
The pattern of the evolutionary radiation of modern birds (Neornithes) has been debated for more than 10 years. However, the early fossil record of birds from the Paleogene, in particular, the Lower Eocene, has only recently begun to be used in a phylogenetic context to address the dynamics of this major vertebrate radiation. The Cretaceous-Paleogene (K-P) extinction event dominates our understanding of early modern bird evolution, but climate change throughout the Eocene is known to have also played a major role. The Paleocene and Lower Eocene was a time of avian diversification as a result of favourable global climatic conditions. Deteriorations in climate beginning in the Middle Eocene appear to be responsible for the demise of previously widespread avian lineages like Lithornithiformes and Gastornithidae. Other groups, such as Galliformes display replacement of some lineages by others, probably related to adaptations to a drier climate. Finally, the combination of slowly deteriorating climatic conditions from the Middle Eocene onwards, appears to have slowed the evolutionary rate in Europe, as avian faunas did not differentiate markedly until the Oligocene. Taking biotic factors in tandem with the known Paleogene fossil record of Neornithes has recently begun to illuminate this evolutionary event. Well-preserved fossil taxa are required in combination with ever-improving phylogenetic hypotheses for the inter-relationships of modern birds founded on morphological characters. One key avifauna of this age, synthesised for the first time herein, is the Lower Eocene Fur Formation of Denmark. The Fur birds represent some of the best preserved (often in three dimensions and with soft tissues) known fossil records for major clades of modern birds. Clear phylogenetic assessment of these fossils will prove critical for future calibration of the neornithine evolutionary timescale. Some early diverging clades were clearly present in the Paleocene as evidenced directly by new fossil material alongside the phylogenetically constrained Lower Eocene taxa. A later Oligocene radiation of clades other than Passeriformes is not supported by available fossil data.
As field determinations take much effort, it would be useful to be able to predict easily the coefficients describing the functional response of free-living predators, the function relating food intake rate to the abundance of food organisms in the environment. As a means easily to parameterise an individual-based model of shorebird Charadriiformes populations, we attempted this for shorebirds eating macro-invertebrates. Intake rate is measured as the ash-free dry mass (AFDM) per second of active foraging; i.e. excluding time spent on digestive pauses and other activities, such as preening. The present and previous studies show that the general shape of the functional response in shorebirds eating approximately the same size of prey across the full range of prey density is a decelerating rise to a plateau, thus approximating the Holling type II (‘disc equation’) formulation. But field studies confirmed that the asymptote was not set by handling time, as assumed by the disc equation, because only about half the foraging time was spent in successfully or unsuccessfully attacking and handling prey, the rest being devoted to searching.
A review of 30 functional responses showed that intake rate in free-living shorebirds varied independently of prey density over a wide range, with the asymptote being reached at very low prey densities (<150/m−2). Accordingly, most of the many studies of shorebird intake rate have probably been conducted at or near the asymptote of the functional response, suggesting that equations that predict intake rate should also predict the asymptote.
A multivariate analysis of 468 ‘spot’ estimates of intake rates from 26 shorebirds identified ten variables, representing prey and shorebird characteristics, that accounted for 81% of the variance in logarithm-transformed intake rate. But four-variables accounted for almost as much (77.3%), these being bird size, prey size, whether the bird was an oystercatcher Haematopus ostralegus eating mussels Mytilus edulis, or breeding. The four variable equation under-predicted, on average, the observed 30 estimates of the asymptote by 11.6%, but this discrepancy was reduced to 0.2% when two suspect estimates from one early study in the 1960s were removed. The equation therefore predicted the observed asymptote very successfully in 93% of cases.
We conclude that the asymptote can be reliably predicted from just four easily measured variables. Indeed, if the birds are not breeding and are not oystercatchers eating mussels, reliable predictions can be obtained using just two variables, bird and prey sizes. A multivariate analysis of 23 estimates of the half-asymptote constant suggested they were smaller when prey were small but greater when the birds were large, especially in oystercatchers. The resulting equation could be used to predict the half-asymptote constant, but its predictive power has yet to be tested.
As well as predicting the asymptote of the functional response, the equations will enable research workers engaged in many areas of shorebird ecology and behaviour to estimate intake rate without the need for conventional time-consuming field studies, including species for which it has not yet proved possible to measure intake rate in the field.
There is no logical or theoretical barrier to the proposition that organismal and cell signaling could transduce environmental signals into specific, beneficial changes in primary structure of noncoding DNA via repetitive element movement or mutation. Repetitive DNA elements, including transposons and microsatellites, are known to influence the structure and expression of protein-coding genes, and to be responsive to environmental signals in some cases. These effects may create fodder for adaptive evolution, at rates exceeding those observed for point mutations. In many cases, the changes are no doubt random, and fitness is increased through simple natural selection. However, some transposons insert at specific sites, and certain regions of the genome exhibit selectively and beneficially high mutation rates in a range of organisms. In multicellular organisms, this could benefit individuals in situations with significant potential for clonal expansion: early life stages or regenerative tissues in animals, and most plant tissues. Transmission of the change to the next generation could occur in plants and, under some circumstances, in animals.
Among the air-breathing vertebrates, the avian respiratory apparatus, the lung-air sac system, is the most structurally complex and functionally efficient. After intricate morphogenesis, elaborate pulmonary vascular and airway (bronchial) architectures are formed. The crosscurrent, countercurrent, and multicapillary serial arterialization systems represent outstanding operational designs. The arrangement between the conduits of air and blood allows the respiratory media to be transported optimally in adequate measures and rates and to be exposed to each other over an extensive respiratory surface while separated by an extremely thin blood-gas barrier. As a consequence, the diffusing capacity (conductance) of the avian lung for oxygen is remarkably efficient. The foremost adaptive refinements are: (1) rigidity of the lung which allows intense subdivision of the exchange tissue (parenchyma) leading to formation of very small terminal respiratory units and consequently a vast respiratory surface; (2) a thin blood-gas barrier enabled by confinement of the pneumocytes (especially the type II cells) and the connective tissue elements to the atria and infundibulae, i.e. away from the respiratory surface of the air capillaries; (3) physical separation (uncoupling) of the lung (the gas exchanger) from the air sacs (the mechanical ventilators), permitting continuous and unidirectional ventilation of the lung. Among others, these features have created an incredibly efficient gas exchanger that supports the highly aerobic lifestyles and great metabolic capacities characteristic of birds. Interestingly, despite remarkable morphological heterogeneity in the gas exchangers of extant vertebrates at maturity, the processes involved in their formation and development are very similar. Transformation of one lung type to another is clearly conceivable, especially at lower levels of specialization. The crocodilian (reptilian) multicameral lung type represents a Bauplan from which the respiratory organs of nonavian theropod dinosaurs and the lung-air sac system of birds appear to have evolved. However, many fundamental aspects of the evolution, development, and even the structure and function of the avian respiratory system still remain uncertain.
The ecosystem approach to fisheries recognises the interdependence between harvested species and other ecosystem components. It aims to account for the propagation of the effects of harvesting through the food-web. The formulation and evaluation of ecosystem-based management strategies requires reliable models of ecosystem dynamics to predict these effects. The krill-based system in the Southern Ocean was the focus of some of the earliest models exploring such effects. It is also a suitable example for the development of models to support the ecosystem approach to fisheries because it has a relatively simple food-web structure and progress has been made in developing models of the key species and interactions, some of which has been motivated by the need to develop ecosystem-based management. Antarctic krill, Euphausia superba, is the main target species for the fishery and the main prey of many top predators. It is therefore critical to capture the processes affecting the dynamics and distribution of krill in ecosystem dynamics models. These processes include environmental influences on recruitment and the spatially variable influence of advection. Models must also capture the interactions between krill and its consumers, which are mediated by the spatial structure of the environment. Various models have explored predator-prey population dynamics with simplistic representations of these interactions, while others have focused on specific details of the interactions. There is now a pressing need to develop plausible and practical models of ecosystem dynamics that link processes occurring at these different scales. Many studies have highlighted uncertainties in our understanding of the system, which indicates future priorities in terms of both data collection and developing methods to evaluate the effects of these uncertainties on model predictions. We propose a modelling approach that focuses on harvested species and their monitored consumers and that evaluates model uncertainty by using alternative structures and functional forms in a Monte Carlo framework.
Although the mathematical principles underpinning population-level evolution are now well studied, the origin and evolution of morphological novelties has received far less attention. Here, a broad but general theory for how this sort of change takes place is outlined, relying on functional continuity, least-constrained components of morphology, redundancy and preadaptation. At least four distinct sorts of redundancy are identified: (i) redundancy arising through duplication (amplification); (ii) redundancy arising through regionalisation (parcellation); (iii) redundancy arising through functional convergence; and (iv) redundancy arising from shared function (functional degeneracy). Although organisms are here recognised to be functionally constrained (“burdened”, in Riedl's terminology), these constraints can be overcome through the combination of the four principles given above. Contrary to its common treatment, functional constraint is neither an ever-increasing restriction on the scope of evolution, nor does it require drastic events to overcome or “break” it. Rather, it is an evolutionary quantity, subject to selection at some level. The rules that govern morphological evolution are the primary controls on what is allowed to happen in the evolution of the overall genotype-phenotype system, suggesting strong controls on the sorts of developmental changes that might be associated with macroevolution. This model, then, sees organism functionality as the primary control on evolvability, with exact genetic make-up being of secondary importance. It should prove possible to recast traditional notions of body-plan evolution into the formulations of complex system analysis, which in the future may prove a unifying discipline for fields as disparate as palaeontology and gene regulatory networks. In particular, understanding how morphology can evolve may provide the critical link between the ecological and morphological networks that are currently the intense focus of evolutionary investigations.
How much effort to expend in any one bout of reproduction is among the most important decisions made by an individual that breeds more than once. According to life-history theory, reproduction is costly, and individuals that invest too much in a given reproductive bout pay with reduced reproductive output in the future. Likewise, investing too little does not maximize reproductive potential. Because reproductive effort relative to output can vary with predictable and unpredictable challenges and opportunities, no single level of reproductive effort maximizes fitness. This leads to the prediction that individuals possessing behavioural mechanisms to buffer challenges and take advantage of opportunities would incur fitness benefits. Here, we review evidence in birds, primarily of altricial species, for the presence of at least two such mechanisms and evidence for and against the seasonal coordination of these mechanisms through seasonal changes in plasma concentrations of the pituitary hormone prolactin. First, the seasonal decline in clutch size of most bird species may partially offset a predictable seasonal decline in the reproductive value of offspring. Second, establishing a developmental sibling-hierarchy among offspring may hedge against unpredictable changes in resource availability and offspring viability or quality, and minimize energy expenditure in raising a brood. The hierarchy may be a product, in part, of the timing of incubation onset relative to clutch completion and the rate of yolk androgen deposition during the laying cycle. Because clutch size should influence the effects of both these traits on the developmental hierarchy, we predicted and describe evidence in some species that females adjust the timing of incubation onset and rate of yolk androgen deposition to match clutch size. Studies on domesticated precocial species reveal an inhibitory effect of the pituitary hormone prolactin on egg laying, suggesting a possible hormonal basis for the regulation of clutch size. Studies on the American kestrel (Falco sparverius) and other species suggest that the seasonal increase in plasma concentrations of prolactin may regulate both a seasonal advance in the timing of incubation onset and a seasonal increase in the rate of yolk androgen deposition. These observations, together with strong conceptual arguments published previously, raise the possibility that a single hormone, prolactin, functions as the basis of a common mechanism for the seasonal adjustment of reproductive effort. However, a role for prolactin in regulating clutch size in any species is not firmly established, and evidence from some species indicates that clutch size may not be coupled to the timing of incubation onset and rate of yolk androgen deposition. A dissociation between the regulation of clutch size and the regulation of incubation onset and yolk androgen deposition may enable an independent response to the predictable and unpredictable challenges and opportunities faced during reproduction.