Showing papers in "Philosophical Transactions of the Royal Society B in 1991"
TL;DR: The antagonistic pleiotropy theory proposes that certain alleles that are favoured because of beneficial early effects also have deleterious later effects, and the disposable soma theory suggests that because of the competing demands of reproduction less effort is invested in the maintenance of somatic tissues than is necessary for indefinite survival.
Abstract: In so far as it is associated with declining fertility and increasing mortality, senescence is directly detrimental to reproductive success. Natural selection should therefore act in the direction of postponing or eliminating senescence from the life history. The widespread occurrence of senescence is explained by observing that (i) the force of natural selection is generally weaker at late ages than at early ages, and (ii) the acquisition of greater longevity usually involves some cost. Two convergent theories are the ‘antagonistic pleiotropy’ theory, based in population genetics, and the ‘disposable soma’ theory, based in physiological ecology. The antagonistic pleiotropy theory proposes that certain alleles that are favoured because of beneficial early effects also have deleterious later effects. The disposable soma theory suggests that because of the competing demands of reproduction less effort is invested in the maintenance of somatic tissues than is necessary for indefinite survival.
977 citations
TL;DR: This paper’s purpose is to show that imprinting phenomena in endosperm can provide a coherent explanation of some failures of experimental crosses, and of the prevalence of pseudogamy among apomictic angiosperms.
Abstract: If a mother sometimes has offspring by more than one father and if genes in the offspring are active in acquiring resources from maternal tissues, theory predicts that alleles at some loci in the offspring will evolve different patterns of gene expression depending on the gene’s parent of origin (genomic imprinting). The criteria for the evolution of imprinting are satisfied in many seed plants, and imprinting has been reported from the endosperm of angiosperm seeds. This paper’s purpose is to show that imprinting phenomena in endosperm can provide a coherent explanation of some failures of experimental crosses, and of the prevalence of pseudogamy among apomictic angiosperms. As a consequence of imprinting, seed development comes to depend on a particular ratio of maternal and paternal genomes in endosperm. This ratio is normally two maternal genomes to each paternal genome. Imprinting probably accounts for the failure of crosses between diploids and their autotetraploids, because the 2m: 1p ratio is disturbed in such crosses. Imprinting may also account for the breakdown of endosperm in crosses between related species, if the expression of maternal and paternal genomes in endosperm is out of balance. When a cross fails because of such an imbalance, the reciprocal cross will have the opposite imbalance and a complementary phenotype would be expected. The embryological evidence is consistent with this prediction. For example, many incompatible crosses show delayed wall formation in one direction of the cross, but precocious wall formation in the other direction. Typically, seed development can be classified as showing ‘paternal excess’ or ‘maternal excess’. Paternal excess is associated with unusually vigorous early growth of the endosperm, and maternal excess with the opposite. This pattern is consistent with natural selection on paternal gene expression favouring larger seeds. Genetic evidence from maize confirms an association between paternal gene expression and larger kernel size, and maternal gene expression and smaller kernel size. Genomic imprinting creates a requirement for both maternal and paternal genomes in imprinted tissues. In mammals, imprinting is expressed in derivatives of the zygote. The requirement for a paternal genome has constituted a block to the evolution of parthenogenesis, because all the genes in a parthenogenetic embryo are maternal. In angiosperms, imprinting is primarily expressed in the endosperm rather than the embryo. If the effects of imprinting in the embryo are small, an asexually produced embryo can develop, provided that it is associated with a viable endosperm. Many a Pom^ ts are pseudogamous. That is, the endosperm is fertilized and contains maternal and paternal genes embryo is asexual and contains maternal genes only. Thus, the division of labour between the embryo the endosperm during development of the seed can be seen as a preadaptation for apomixis. Some apomicts are autonomous. That is, the embryo and the endosperm both develop without fertilization. Genomic imprinting in endosperm would seem to constitute a barrier to the evolution of autonomous apomixis. Thus, there is a problem, not previously appreciated, in understanding how autonomous apomixis is possible
372 citations
TL;DR: Experimental data is reported showing that hunters would reduce their mean rates if they included small animals in the array they target, and finding that an exclusive focus on large game with extensive sharing is not the optimal strategy for hunters concerned with maximizing their own chances of eating meat.
Abstract: The assumption that large mamm al hunting and scavenging are economically advantageous to hominid foragers is examined in the light of data collected among the Hadza of northern Tanzania. Hadza hunters disregard small prey in favour of larger forms (mean adult mass ≥ 40 kg). Here we report experimental data showing that hunters would reduce their mean rates if they included small animals in the array they target. Still, daily variance in large animal hunting returns is high, and the risk of failure correspondingly great, significantly greater than that associated with small game hunting and trapping. Sharing large kills reduces the risk of meatless days for big game hunters, and obviates the problem of storing large amounts of meat. It may be unavoidable if large carcasses cannot be defended economically against the demands of other consumers. If so, then large prey are common goods. A hunter may gain no consumption advantage from his own big game acquisition efforts. We use Hadza data to model this ‘collective action9 problem, and find that an exclusive focus on large game with extensive sharing is not the optimal strategy for hunters concerned with maximizing their own chances of eating meat. Other explanations for the emergence and persistence of this practice must be considered
296 citations
TL;DR: It is argued that lack of evolution in most species may be due more to lack of appropriate variability than to other causes, and there are good theoretical and practical reasons for more attention being paid to the mechanisms of supply of new variation.
Abstract: The Darwinian explanation for evolution is that it is the outcome of the interaction between genetic variation and natural selection. There is now good evidence for both the existence of genetic variation and the occurrence of natural selection, the latter potentially at high intensities. The outcome should be rapid evolutionary change; yet in practice very little change is found. Most species are very stable, and in situations where evolution is observed in one species often none is found in others despite equivalent opportunity. Evolutionary failure is commonplace. Despite the occurrence of high levels of protein polymorphism, there is good evidence that the supply of variation making a major contribution to fitness is very limited. As a result it is argued that lack of evolution in most species may be due more to lack of appropriate variability than to other causes: a condition for which the term `genostasis' is proposed. In those situations where appropriate genetic variation is available for one reason or another, evolution is found to be very rapid. There are good theoretical and practical reasons for more attention being paid to the mechanisms of supply of new variation and to those situations where evolution appears not to be taking place.
277 citations
TL;DR: It is argued that plants actually contain a very high diversity of mostly inactive secondary compounds, which result because compounds arising via mutation have an inherently low probability of possessing any biological activity.
Abstract: A common-sense evolutionary scenario predicts that well-defended plants should have a moderate diversity of secondary compounds with high biological activity. We contend that plants actually contain a very high diversity of mostly inactive secondary compounds. These patterns result because compounds arising via mutation have an inherently low probability of possessing any biological activity. Only those plants that make a lot of compounds will be well defended because only high diversity confers a reasonable probability of producing active compounds. Inactive compounds are retained, not eliminated, because they increase the probability of producing new active compounds. Plants should therefore have predictable metabolic traits maximizing secondary chemical diversity while minimizing cost. Our hypothesis has important implications to the study of the evolution of plant defence.
254 citations
TL;DR: Consideration of energetic costs of brain allow the prediction of growth rates in hominid taxa and an examination of the implications for life-history strategy and foraging behaviour.
Abstract: Hominid evolution is marked by very significant increase in relative brain size. Because relative brain size has been linked to energetic requirements it is possible to look at the pattern of encephalization as a factor in the evolution of human foraging and dietary strategies. Major expansion of the brain is associated with Homo rather than the Hominidae as a whole, and the energetic costs are likely to have forced a prolongation of growth rates and secondary altriciality. It is calculated here that modern human infants have energetic requirements approximately 9% greater than similar size apes due to their large brains. Consideration of energetic costs of brain allow the prediction of growth rates in hominid taxa and an examination of the implications for life-history strategy and foraging behaviour.
247 citations
TL;DR: Overall, a network representation is a suitable methodology for inter-ecosystem comparisons and there is an inverse correlation between the Finn Cycling Index (FCI) and the normalized internal ascendancy or system maturity.
Abstract: We compare six marine ecosystems worldwide, using a network analysis of carbon flows for the Swartkops and Ems estuaries, Chesapeake Bay, the Baltic Sea and the Peruvian and Benguela upwelling regions. We find that there is an inverse correlation between the Finn Cycling Index (FCI) and the normalized internal ascendancy or system maturity (A $\_{\text{i}}$ :C $\_{\text{i}}$ ). We also show that there is a clear distinction between system maturity (A $\_{\text{i}}$ :C $\_{\text{i}}$ ) and the production: biomass (P:B) ratio; in upwelling systems the P:B and A $\_{\text{i}}$ :C $\_{\text{i}}$ ratios are both high, whereas in estuarine systems the P:B ratio is low. The P:B ratio thus cannot be used as an indicator of marine ecosystem evolution. Contrary to current views on ecosystems, the aggregate amount of cycling of materials, such as carbon, is not seen as an indication of system maturity but rather of stress. The reason that higher stressed systems are associated with a higher throughout or FCI could be because perturbations frequently impact higher-level species to a greater extent than the lower trophic components. Any release of standing biomass from these higher levels could therefore be taken up through increased recycling via short intense loops. Overall, we conclude that a network representation is a suitable methodology for inter-ecosystem comparisons.
247 citations
TL;DR: Herbaceous piths appear to be a vital resource for African forest apes, offering an alternative energy supply when fruits are scarce, and were correlated negatively with fruit abundance and positively with rainfall.
Abstract: Four categories of plant food dominated the diet of chimpanzees in Kibale Forest, Uganda: non-fig tree fruits, fig tree fruits, herbaceous piths and terrestrial leaves. Fruit abundance varied unpredictably, more among non-figs than figs. Pith intake was correlated negatively with fruit abundance and positively with rainfall, whereas leaf intake was not influenced by fruit abundance. Piths typically have low sugar and protein levels. Compared with fruits and leaves they are consistently high in hemicellulose and cellulose, which are insoluble fibres partly digestible by chimpanzees. Herbaceous piths appear to be a vital resource for African forest apes, offering an alternative energy supply when fruits are scarce.
229 citations
TL;DR: Empirical evidence in a wild population of the bluegill sunfish shows that cuckolder will fertilize decreasing proportions of eggs, relative to parental males, as cuckolders increase in frequency in the population, which supports evolutionary models that assume negative frequency-dependent selection between the competing strategies.
Abstract: This study provides empirical evidence in a wild population for frequency-dependent sexual selection between alternative male reproductive strategies. The bluegill sunfish (Lepomis macrochirus) has two male reproductive strategies, cuckolder or parental, used by different males to compete in fertilizing the same eggs. As the density of cuckolders in colonies of parental males increases, the average mating success of cuckolders initially peaks but then declines. The cuckolder density at which their success peaks is determined by ecological characteristics of each colony. A theoretical analysis assuming random and omniscient cuckolder distributions among ecologically different colonies shows that cuckolders will fertilize decreasing proportions of eggs, relative to parental males, as cuckolders increase in frequency in the population. This supports evolutionary models that assume negative frequency-dependent selection between the competing strategies. Cuckolder and parental strategies may therefore have evolved as an Evolutionarily Stable State (ESSt).
227 citations
216 citations
TL;DR: The authors compare the diets of gorillas and chimpanzees and find that gorillas eat more vegetative plant parts than do chimpanzees, but niche separation is most obvious in periods of fruit scarcity when the two species show different strategies that reduce competition for food.
Abstract: Comparison of the diets of sympatric gorillas and chimpanzees allows an analysis of niche separation between these two closely related species. Qualitatively, their diets are similar, being dominated by an equally diverse array of fruit species complemented with vegetative plant parts, seeds and insects. Gorillas eat more vegetative plant parts than do chimpanzees, but niche separation is most obvious in periods of fruit scarcity when the two species show different strategies that reduce competition for food. Their abilities to overcome mechanical and physical plant defences appear to differ, as gorillas are able to subsist entirely on abundant vegetative foods. Chimpanzees show social adjustment, foraging alone or in small groups, to reduce intra-specific competition for scarce fruit resources. Thus it seems that subtle physiological differences have far-reaching repercussions, defining potential evolutionary pathways for social organization and allowing sufficient niche separation between species.
TL;DR: Until European settlement of Australia 200 years ago, Aborigines lived as nomadic hunter-gatherers all over the continent under widely varying geographic and climatic conditions, and consumed a varied diet in which animal foods were a major component.
Abstract: Until European settlement of Australia 200 years ago, Aborigines lived as nomadic hunter-gatherers all over the continent under widely varying geographic and climatic conditions. Successful survival depended on a comprehensive knowledge of the flora and fauna of their territory. Available data suggest that they were physically fit and lean, and consumed a varied diet in which animal foods were a major component. Despite this, the diet was not high in fat, as wild animal carcasses have very low fat contents through most of the year, and the meat is extremely lean. Everything on an animal carcass was eaten, including the small fat depots and organ meats (which were highly prized), bone marrow, some stomach contents, peritoneal fluid and blood. A wide variety of uncultivated plant foods was eaten in the traditional diet: roots, starchy tubers, seeds, fruits and nuts. The plant foods were generally high in fibre and contained carbohydrates, which was slowly digested and absorbed. Traditional methods of food preparation (usually baked whole or eaten raw) ensured maximum retention of nutrients. In general, traditional foods had a low energy density but high density of some nutrients. The low energy density of the diet and the labour intensity of food procurement provided a natural constraint on energy intake.(ABSTRACT TRUNCATED AT 250 WORDS)
TL;DR: Although food selection in some species of primates appears to be correlated with the protein concentration of foods, it is unlikely that high dietary protein levels are required, at least when foods of balanced amino acid composition (such as leaves) are included in the diet.
Abstract: Many studies have examined the proportion of time that primates devote to feeding on various types of food, but relatively little is known about the intake rates associated with each food. However, the nutritional consequences of foraging can only be interpreted by comparing nutrient intakes with estimated nutrient requirements. The energy available to primates from ingested foods will depend both on the composition of the food and the extent to which various constituents, including fibre fractions, are digested. Both human and non-human primates have relatively low requirements for protein as a consequence of slow growth rates, small milk yields and relatively dilute milk. Because the nutrient demands of growth and reproduction are spread out over time, it appears that primates do not need to seek out foods of particularly high nutrient density, except perhaps during weaning. Although food selection in some species of primates appears to be correlated with the protein concentration of foods, it is unlikely that high dietary protein levels are required, at least when foods of balanced amino acid composition (such as leaves) are included in the diet.
TL;DR: Experimental manipulations of the non-neuronal environment of injured RGCs enhance neuronal survival and make possible a lengthy axonal regeneration that restores functional connections with the superior colliculus, suggesting that injured nerve cells in the mature central nervous system (CNS) are strongly influenced by interactions with components of their immediate environment as well as their targets.
Abstract: In adult mammals, the severing of the optic nerve near the eye is followed by a loss of retinal ganglion cells (RGCs) and a failure of axons to regrow into the brain. Experimental manipulations of the non-neuronal environment of injured RGCs enhance neuronal survival and make possible a lengthy axonal regeneration that restores functional connections with the superior colliculus. These effects suggest that injured nerve cells in the mature central nervous system (CNS) are strongly influenced by interactions with components of their immediate environment as well as their targets. Under these conditions, injured CNS neurons can express capacities for growth and differentiation that resemble those of normally developing neurons. An understanding of this regeneration in the context of the cellular and molecular events that influence the interactions of axonal growth cones with their non-neuronal substrates and neuronal targets should help in the further elucidation of the capacities of neuronal systems to recover from injury.
TL;DR: This paper develops and test multivariate models of the basis of diet selection for populations of montane and savannah baboons and indicates convergence on lower degrees of variation than exist in superficial foodstuff profiles.
Abstract: As large-bodied savannah primates, baboons have long been of special interest to students of human evolution: many different populations have been studied and dietary comparisons among them are becoming possible. Baboons' foraging strategies can be shown to combine high degrees of flexibility and breadth with selectivity. In this paper we develop and test multivariate models of the basis of diet selection for populations of montane and savannah baboons. Food selection is positively related to protein and lipid content and negatively to fibre, phenolics and alkaloids. Seasonal changes in dietary criteria predicted by these rules are tested and confirmed. Although nutritional bottlenecks occur at intervals, a comparison between long-term nutrient intakes in four different populations indicates convergence on lower degrees of variation than exist in superficial foodstuff profiles.
TL;DR: It is proposed that non-cellulolytic omnivorous scavengers and detritivores may be preadapted to evolve symbiont-mediated cellulolytic mechanisms because of the prevalence of mutualistic associations between such species and the microorganisms that normally reside in their hindguts.
Abstract: Despite the abundance and diversity of species that include living or dead plant tissue in their diets, the ability to digest cellulose is rare in insects and is restricted to a small number of orders and families. In this paper it is argued that cellulolytic capacity is uncommon in insects simply because it is a trait that is rarely advantageous to possess. Although there is a growing body of evidence for the occurrence of symbiont-independent cellulose digestion in cockroaches and in higher termites from the subfamily Nasutitermitinae, cellulose digestion in insects is usually mediated by microorganisms. It is proposed that non-cellulolytic omnivorous scavengers and detritivores may be preadapted to evolve symbiont-mediated cellulolytic mechanisms because of the prevalence of mutualistic associations between such species and the microorganisms that normally reside in their hindguts. A scenario is proposed for the evolution of symbiont-mediated cellulolytic capacity in roaches and lower termites. Finally, it is suggested that biochemical studies of insect cellulases might provide crucial insights that would greatly advance our understanding of the evolution of cellulose digestion in insects.
TL;DR: The life history favoured by natural selection maximizes fitness, and this implies maximization of fecundity and survival at all ages, but the observed diversity in life histories suggests that there are constraints on what can be achieved in practice.
Abstract: The life history favoured by natural selection maximizes fitness, and this implies maximization of fecundity and survival at all ages. The observed diversity in life histories suggests that there are constraints on what can be achieved in practice. Functional constraints occur if only certain combinations of age-specific fertility and survival are possible, either because of the physiology of the organism or because of the ecological impact of its environment. The resulting constrained optimization means that the organism is involved in making trade-offs between life-history characters. A major task for the future is the measurement of trade-off functions in the environment in which the life-history evolved. Natural variation between individuals and populations, genetic studies and experimental manipulations have all been used to detect trade-offs. The last two methods are the most satisfactory, and can be complementary. Experimental manipulations are at their best when based on sound physiological understanding of the traits under manipulation. Constraints can also operate on the long-term. Local optima, evolutionary lags and irreversible evolution may all have contributed to the diversity of life histories.
TL;DR: The nature of the plant surface provides a great variety of terrains that require specialization for maximizing tenacity and agility, especially for small insects, and it is suggested that natural enemies may provide significant selection for the relevant morphologies.
Abstract: This short review points out some of the major physical problems faced by insects feeding on plants, and some of the kinds of morphological adaptations that have been noted to date. Major emphasis is given to two factors: the nature of the plant surface, and the difficulty of dealing with hard or tough food. The surface provides a great variety of terrains that require specialization for maximizing tenacity and agility, especially for small insects. It is suggested that natural enemies may provide significant selection for the relevant morphologies. The difficulty of feeding upon certain plant tissues is shown to be overcome in different ways by different herbivore groups. In the case of tough leaves for example, grasshopper mandible adaptations appear to have evolved to maximize efficiency of processing. On the other hand, in the case of caterpillars, mandible adaptations for tough food appear to be minimizing handling time with a concomitant reduction in risk of predation. Convergent evolution is shown in both grasshoppers and caterpillars for dealing with leaves of similar design and an example is given of rapid evolution of mouthpart morphology in response to differences in host characteristics. These examples are used to indicate the risks of using such characters in establishing phylogenetic relationships. Finally, it is pointed out that plant chemical qualities and plant ecological factors can influence insect morphological features.
TL;DR: There are many problems associated with estimating reproductive resources, and also with testing how allocation of these resources affects reproductive and other components of fitness, and how important these may be in allowing empirical results to be interpreted are assessed.
Abstract: The study of allocation of resources offers the possibility of understanding the pressures of natural selection on reproductive functions. In allocation studies, theoretical predictions are generated and the assumptions as well as the predictions can be tested in the field. Here, we review some of the theoretical models, and discuss how much biological reality can be included in them, and what factors have been left out. We also review the empirical data that have been generated as tests of this body of theory. There are many problems associated with estimating reproductive resources, and also with testing how allocation of these resources affects reproductive and other components of fitness, and we assess how important these may be in allowing empirical results to be interpreted. Finally, we discuss the relevance of resource allocation patterns to the evolution of unisexual flowers, both at the level of individual plants (monoecy, andro- and gynomonoecy) and at the population level (dioecy).
TL;DR: A detailed account is given of the feeding mechanism, which has been studied both by direct observation and with the aid of high-speed cine-photography, and which has led to erroneous views on such matters as the amount of energy expended in filtration.
Abstract: Of all anomopods, daphniids have been the most successful exponents of life in open water. Many of them are completely independent of the bottom and subsist entirely on seston. A few of them are truly planktonic. Although the family has been intensively studied from many points of view, various morphological attributes have remained either inadequately known or never investigated. Some of these attributes, understanding of which is necessary if functions are to be appreciated, are considered, especially in the genus Daphnia, with which other genera are later compared. They include aspects of general morphology, the exoskeleton, endoskeleton and muscular system. How Daphnia swims is described, antennal movements being analysed from high-speed cine films. Locomotion is clearly derived from a naupliar mechanism, though the nauplius has long been eliminated from the anomopod life cycle. Antennal beat is more versatile than is immediately apparent and the animals are capable of far more complex manoeuvres than the simple \`hop and sink' movements in which they often indulge. The trunk limbs are responsible for collecting and manipulating the food. Their morphology and arrangement are discussed and their armature, especially as revealed by scanning electron microscopy, is considered. The armature of limbs 3 and 4 dominates the trunk limb complex and makes up an extensive filter chamber. The mouthparts and labrum are basically the same as those already described in detail for other anomopods, but the labrum lacks a keel. A wide range of particulate foods is consumed. A detailed account is given of the feeding mechanism, which has been studied both by direct observation and with the aid of high-speed cine-photography. Most of the basic principles involved were elucidated by Cannon, Storch and Eriksson who, however, disagreed on various points. The account now given is more detailed than any previously presented and is supported by numerous illustrations, whose lack has hitherto hindered comprehension. Parts of some of the earlier interpretations are incorrect, sometimes in ways that are not only intrinsically important, but which have led to erroneous views on such matters as the amount of energy expended in filtration. Trunk limb movements follow a regular rhythmic cycle. Water, containing suspended particles, flows into the carapace chamber via the ventral gape to replace that driven out posteriorly by the pumping action of trunk limbs 3 and 4 and their exopodites, is drawn into the filter chamber and through the filters borne on limbs 3 and 4 into interlimb spaces, from which it is finally expelled posteriorly. Trunk limb 5, whose movements initiate both promotion (the suction and filtration phase of the cycle) and remotion (the expulsion phase), seals the posterior interlimb space posteriorly during promotion of the limbs. There is no pressing of water through the filters during remotion of the limbs. Filtration occurs during approximately half the cycle. Notwithstanding claims to the contrary, the filter plates of trunk limbs 3 and 4 are correctly designated as such and serve as filters. Material abstracted by the filter plates is cleaned off by a series of devices, seven in all, passed into the median food groove, and swept forward by mechanical means to the mouthparts. The mandibles display a high degree of both skeletal and muscular asymmetry, which improves their performance. Any excess food material collected in the food groove is discarded. From the anterior end it is removed by the ejector hooks of the first trunk limbs, then swept out by the post-abdominal claws: from the posterior end it is removed by the post-abdominal claws alone. Errors and shortcomings in certain recent accounts that purport to explain the feeding mechanism are discussed. Trunk limbs 1 and 2 are incapable of filtration and are specialized for roles that have nothing to do with this process. The inapplicability of a model of filtration to the daphniid mechanism is noted and the importance of morphology, even in minute details, is emphasized. Contrary to recent suggestions, the function of \`bristles' cannot easily be changed without changes in morphology. The necessity of understanding a mechanism before making calculations is emphasized and examples of misleading calculations, based on erroneous data, are noted. The habits of certain species of Daphnia are described. Both D. magna and D. obtusa are able to settle on their ventral carapace margins and attach themselves to surfaces, over which they can then glide forward, collecting food material by means of scraper-like spines borne distally on the second trunk limbs as they do so. D. magna can also lift accumulations of detritus from the bottom. Such material is then processed in the usual way. Some species sometimes indulge in swarming behaviour, which involves remarkable coordination between individuals. The way in which phenotypic changes in shape occur in Daphnia and the light this throws on phyletic changes in the genus are described, partly by the method of transformation of coordinates, which can be used to show changes in three dimensions, rather than the usual two. The influence of environmental factors is noted. Geographical, ecological and physiological aspects of radiation are considered. Other genera are treated more briefly. Daphniopsis departs little from Daphnia in its functional morphology and may not merit generic separation. Simocephalus attaches itself to a support by means of simple but effective specializations of the antennae and then remains stationary while it filters. This has enabled it to acquire a robust carapace in a way not permitted to Daphnia (of which a few of the more heavily built species sometimes rest on the bottom). Protection is thereby granted. Acquisition of this habit was probably assisted by the way in which Simocephalus swims, predominantly ventral surface uppermost. The feeding mechanism is essentially the same as that of Daphnia. Scapholoberis and Megafenestra have the same orientation during swimming as Simocephalus and have acquired the habit of hanging suspended beneath the surface film by their ventral carapace margins, for which they are highly specialized in morphology and behaviour. Here too the basic daphniid feeding mechanism is employed. Ceriodaphnia has specialized in small size. Although studied in less detail than Daphnia, it clearly has a similar feeding mechanism. Moina and Moinodaphnia are now often separated from the Daphniidae as the family Moinidae, but this seems unjustified. Trunk limb structure and the feeding mechanism are essentially the same as in other daphniids. These two genera, while primitive in certain respects, have a suite of specializations related to the nourishment of eggs and embryos by secretions produced by a Nahrboden, or `placenta'. This necessitates sealing of the brood pouch, by a device involving the post-abdomen, to prevent loss of the secretion. As embryos grow during development by the accretion of material from without, rather than from stored yolk, distortion and distension of the carapace are necessary to accommodate their increasing volume. The Daphniidae clearly arose from benthic ancestors, some indication of whose morphology and habits is given by certain extant macrothricids. Key features in the evolution of the family, which has existed since at least early Cretaceous times and probably originated even earlier than this, are listed. Of prime importance was the expansion of the gnathobasic filter plates of trunk limbs 3 and 4 at the expense of other filters.
TL;DR: Structural evidence is presented that stomatopod crustaceans have the receptors necessary for colour and polarization vision, and that all retinular cells in rows one to four of the mid-band, and the distal most retInular cells over most of the retina, are not sensitive to polarized light.
Abstract: Stomatopod crustaceans possess compound eyes divided into three distinct regions: two peripheral retinae - the dorsal and ventral hemispheres - and the mid-band. Throughout the eye, in particular in the midband, there are many structural adaptations that potentially enable different portions of the eye to perform different visual tasks. A high degree of optical overlap between these eye regions allows the parallel sampling of various parameters of light from one direction in space. In consecutive papers, we present structural evidence that stomatopods have the receptors necessary for colour and polarization vision. The first paper describes the retinal structures that suggest the existence of polarization sensitivity in stomatopods. mid-band rows five and six, together with the hemispheres, are probably involved in this visual process. By using two strategies, rhabdomal modification and varying the orientation of similar ommatidial units in the three eye regions, stomatopods have the capacity to analyse polarized light in a very detailed manner. All the species included in this study live in shallow, tropical waters where polarized light signals are abundant. It therefore seems likely that their eyes have evolved to take advantage of such environmental cues. Structural evidence also suggests that all retinular cells in rows one to four of the mid-band, and the distal most retinular cells (R8) over most of the retina, are not sensitive to polarized light. These mid-band rows are instead adapted for colour detection. This function of the stomatopod retina and structural features concerned with colour sensitivity are described in paper II (Phil. Trans. R. Soc. Lond. B 334, 57-84 (1991)).
TL;DR: The results showed that the effect of the heterosexual transmission risk factor on increasing the risk of HIV infection depends upon the level of IVDU, and suggested that long-term precise estimates of the future number of AIDS cases will only be possible once the values of these key variables have been evaluated accurately.
Abstract: A data-based mathematical model was formulated to assess the epidemiological consequences of heterosexual, intravenous drug use (IVDU) and perinatal transmission in New York City (NYC). The model was analysed to clarify the relationship between heterosexual and IVDU transmission and to provide qualitative and quantitative insights into the HIV epidemic in NYC. The results demonstrated the significance of the dynamic interaction of heterosexual and IVDU transmission. Scenario analysis of the model was used to suggest a new explanation for the stabilization of the seroprevalence level that has been observed in the NYC IVDU community; the proposed explanation does not rely upon any IVDU or sexual behavioural changes. Gender-specific risks of heterosexual transmission in IVDUs were also explored by scenario analysis. The results showed that the effect of the heterosexual transmission risk factor on increasing the risk of HIV infection depends upon the level of IVDU. The model was used to predict future numbers of adult and pediatric AIDS cases; a sensitivity analysis of the model showed that the confidence intervals on these prediction estimates were extremely wide. This prediction variability was due to the uncertainty in estimating the values of the models' thirty variables (twenty biological-behavioural transmission parameters and the initial sizes of ten subgroups). However, the sensitivity analysis revealed that only a few key variables were significant in contributing to the AIDS case prediction variability; partial rank correlation coefficients were calculated and used to identify and to rank the importance of these key variables. The results suggest that long-term precise estimates of the future number of AIDS cases will only be possible once the values of these key variables have been evaluated accurately.
TL;DR: The formation of the maps is an early event, prior to the establishment of the synaptology of the cerebellar cortical circuitry, and the organization of the spinocerebellar projection in adult mutant mice does not depend on the presence of granule cells or normal Purkinje cells, indicating that synaptogenesis with their target neurons is not involved in the process of map formation.
Abstract: Olivo- and spinocerebellar maps in the adult cerebellum of small rodents are discontinuous, with sharp boundaries. Cortical Purkinje cells constitute a heterogeneous population, organized into parasagittal, mutually exclusive compartments. The boundaries of the intrinsic cortical compartments and those of the projectional maps are congruent. During development; (i) The incoming olivary fibres, once they penetrate in the cerebellar parenchyma, are attracted toward their ultimate terminal fields, without passing through a stage of random dispersion. (ii) Migrating Purkinje cells and inferior olivary neurons begin, asynchronously, to express cellular markers in an independent manner, giving rise to a transient compartmentation of the cerebellar cortex and the inferior olivary complex respectively. In both instances, the biochemical heterogeneity disappears during the first postnatal week, simultaneously with the acquisition of adult-like cerebellar maps. (iii) The formation of the maps is an early event, prior to the establishment of the synaptology of the cerebellar cortical circuitry. Moreover, the organization of the spinocerebellar projection in adult mutant mice does not depend on the presence of granule cells (staggerer) but on the presence of normal Purkinje cells (weaver), indicating that synaptogenesis with their target neurons is not involved in the process of map formation. The matching of region specific chemical labels between incoming afferent fibres and heterogeneous sets of Purkinje cells is the most appealing mechanism for the formation of cerebellar maps.
TL;DR: The results of a study on a small number of bones from a mediaeval and a 17th-century cemetery in Abingdon showing the relation between gross preservation, microscopic preservation and DNA recovery are presented.
Abstract: The analysis of DNA from ancient bone has numerous applications in archaeology and molecular evolution. Significant amounts of genetic information can be recovered from ancient bone: mitochondrial DNA sequences of 800 base pairs have been amplified from a 750-year-old human femur by using the polymerase chain reaction. DNA recovery varies considerably between bone samples and is not dependent on the age of the specimen. We present the results of a study on a small number of bones from a mediaeval and a 17th-century cemetery in Abingdon showing the relation between gross preservation, microscopic preservation and DNA recovery.
TL;DR: The earliest known hominid fossils differed little in dental and mandibular morphology from the middle Miocene apes, and the implied dietary similarity, together with ape-like patterns of dental development and retained arboreal adaptations of the postcrania, suggests little change in the foraging strategies of the earliest hominids compared with their ape ancestors and further suggests similarity in evolutionary grade.
Abstract: During the later Palaeocene and early Miocene, catarrhine primates and the evolving hominoids had adaptations for frugivorous diets, with the emphasis on soft foods. Early in the middle Miocene the hominoids underwent a major shift, both in morphology and in habitat, with the morphology characterized by thickened enamel on the molars, enlarged incisors and massive jaws. The diet indicated by this morphology is interpreted as still mainly frugivorous but with changed emphasis, possibly towards harder objects. The thick-enamelled hominoids are found associated with more open forest habitats, and the distribution of food resources in equivalent habitats today is discontinuous both in time and in space, leading to evolutionary pressures particularly affecting locomotion, brain size and social behaviour. The earliest known hominid fossils differed little in dental and mandibular morphology from the middle Miocene apes, and the implied dietary similarity, together with ape-like patterns of dental development and retained arboreal adaptations of the postcrania, suggests little change in the foraging strategies of the earliest hominids compared with their ape ancestors and further suggests similarity in evolutionary grade. This similarity may have extended to other aspects of behaviour, for example to patterns of tool making and use, which may have been similar in the common ancestor of apes and humans to the pattern shared by the earliest australopithecines and chimpanzees.
TL;DR: The results suggest that the maximization of marrow (fat) yields, not flesh (protein) yields) was the criterion shaping decisions about carcass processing, and that the socio-economic function of the earliest archaeological sites was identified.
Abstract: New evidence for the tissue types exploited by early hominids from carcasses possibly acquired through scavenging is derived from the larger mammal bone assemblages from FLK I, level 22 (Zinjanthropus floor), and FLKN levels 1 and 2 from Bed I, Olduvai Gorge, Tanzania. Published skeletal part profiles from the two archaeological sites are evaluated using (i) modern observations on the sequence by which carnivores consume carcass parts in order to assess the timing of hominid access to carcasses, and (ii) measurements of flesh and marrow yields to assess the tissue types sought and acquired. These results suggest that the maximization of marrow (fat) yields, not flesh (protein) yields, was the criterion shaping decisions about carcass processing. Because of evidence for density-dependent destruction of some flesh-bearing parts by scavengers of the hominid-butchered assemblages, however, it is uncertain whether carcass parts were transported and acquired by hominids in a largely defleshed condition. The results on tissue types acquired are combined with a discussion of predation risk, feeding competition and the equipment needs of carcass processing in an attempt to identify archaeological test implications of competing hypotheses for the socio-economic function of the earliest archaeological sites.
TL;DR: Analysis of the mechanism and structure of lactate dehydrogenases is summarized in a map of the catalytic pathway, which shows slow movements of the protein framework that discriminate between closely related small substrates.
Abstract: Analysis of the mechanism and structure of lactate dehydrogenases is summarized in a map of the catalytic pathway. Chemical probes, single tryptophan residues inserted at specific sites and a crystal structure reveal slow movements of the protein framework that discriminate between closely related small substrates. Only small and correctly charged substrates allow the protein to engulf the substrate in an internal vacuole that is isolated from solvent protons, in which water is frozen and hydride transfer is rapid. The closed vacuole is very sensitive to the size and charge of the substrate and provides discrimination between small substrates that otherwise have too few functional groups to be distinguished at a solvated protein surface. This model was tested against its ability to successfully predict the design and synthesis of new enzymes such as L-hydroxyisocaproate dehydrogenase and fully active malate dehydrogenase. Solvent friction limits the rate of forming the vacuole and thus the maximum rate of catalysis.
TL;DR: The synthesis of NO results in downregulation of the aggregatory function of these cells in a manner similar to that previously described for mammalian platelets, indicating that formation of NO from L-arginine is a pathway of early evolutionary origin.
Abstract: Nitric oxide (NO) synthase, the enzyme responsible for the production of NO from L-arginine, is present in haemocytes of the American horseshoe crab (Limulus polyphemus). The synthesis of NO results in downregulation of the aggregatory function of these cells in a manner similar to that previously described for mammalian platelets. These data indicate that formation of NO from L-arginine is a pathway of early evolutionary origin.
TL;DR: In this paper, a complete lifting line theory of flight in ground effect with a fixed wing is developed, and instructions are given so that it may be applied to animals such as skimmers, pelicans and myotid bats which fly and forage close above water.
Abstract: Flight in ground effect above a flat, smooth surface may give an animal considerable performance advantages, including a reduction in cost of transport of up to 15%, and a reduction in mechanical flight power of as much as 35%, compared with values for flight out of ground effect. Previous theories modelling the phenomenon have either been incomplete or marred by typographical errors. A complete lifting line theory of flight in ground effect with a fixed wing is developed, and instructions are given so that it may be applied to animals such as skimmers, pelicans and myotid bats which fly and forage close above water. Several predictions are made about likely flight behaviour in ground effect, and about the appropriate flight morphology for taking advantage of the potential performance improvements. The most important conclusion, differing from previous analyses, is that slow flight performance in ground effect is very poor, owing to the horizontal air velocity induced around the wing in the presence of the ground.
TL;DR: These adaptations include brightly coloured intrarhabdomal filters, apparent lateral filters and a photoreceptor tiering system unique to the crustacea which allow the spectrum of light available to stomatopods to be sampled over a broad spectral range by receptors with narrowly tuned sensitivities.
Abstract: The stomatopod eye is divided into three distinct regions, two peripheral \`hemispheres' and a dividing mid-band Each of these areas has a separate function and it is the six rows of ommatidia in the mid-band which are the main subject of study here Rows one to four of the mid-band are probably not sensitive to polarized light (paper I (Phil Trans R Soc Lond B 334, 33-56 (1991))) and instead possess many structural features which suggest that they are concerned with colour analysis and perhaps colour vision This, the second of two consecutive papers, examines these adaptations in detail They include brightly coloured intrarhabdomal filters, apparent lateral filters and a photoreceptor tiering system unique to the crustacea Cronin & Marshall (J comp Physiol 166, 261-275 (1989b)) have shown that mid-band rows one to four contains at least eight distinct visual pigments These, in combination with the structures described here, allow the spectrum of light available to stomatopods to be sampled over a broad spectral range by receptors with narrowly tuned sensitivities It is the photostable screening and filtering pigments, rather than the visual pigments, which are examined in detail in this paper These have been divided into two categories: (i) the \`standard' retinal pigments: those that are often found in other crustacean eyes; (ii) the `unusual' retinal pigments: some of these are unique to stomatopod eyes and may be involved in colour vision