1000 resultados para Survival


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Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

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The aim of this study was to investigate the impact of a 24-h cooling period prior to freezing on domestic cat epididymal sperm viability. Fifteen tomcats were submitted to routine orchiectomy and sperm samples were retrieved from both epididymides in a Tris-glucose-20% egg yolk extender. For each tomcat, the diluted sperm was split into two equal volumes and cooled to 5 degrees C at a rate of 0.5 degrees C/min; one sample for 60 min (control) and the other for 24 h (cooled). After the cooling period, samples from both groups were frozen using an identical freezing protocol. Sperm samples were evaluated in three different periods: immediately after harvesting, after cooling at 5 degrees C for 24 h (cooled group) and after freezing thawing of control and cooled groups. Evaluations consisted of sperm motility and progressive status, sperm morphology and plasma membrane integrity (PMI) using two fluorescent probes. After cooling for 24 h, a decrease (p < 0.05) in sperm motility, progressive status and PMI was observed when compared to sperm samples immediately after collection. Comparing the results obtained after thawing, no difference (p < 0.05) was found regarding sperm motility, progressive status, PMI and sperm morphology between control and cooled groups. The results from the present study show that cooling cat epididymal spermatozoa at 5 degrees C for 24 h prior to freezing does not lead to major damage of spermatozoa impairing the freeze-thaw process.

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In this paper, we derive score test statistics to discriminate between proportional hazards and proportional odds models for grouped survival data. These models are embedded within a power family transformation in order to obtain the score tests. In simple cases, some small-sample results are obtained for the score statistics using Monte Carlo simulations. Score statistics have distributions well approximated by the chi-squared distribution. Real examples illustrate the proposed tests.

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In this article, proportional hazards and logistic models for grouped survival data were extended to incorporate time-dependent covariates. The extension was motivated by a forestry experiment designed to compare five different water stresses in Eucalyptus grandis seedlings. The response was the seedling lifetime. The data set was grouped since there were just three occasions in which the seedlings was visited by the researcher. In each of these occasions also the shoot height was measured and therefore it is a time-dependent covariate. Both extended models were used in this example, and the results were very similar.

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The influence of food abundance, larval density and interspecific interactions on the survival and body size of Chrysomya albiceps, Chrysomya megacephala and Cochliomyia macellaria was investigated in pure and mixed cultures, to determine the impact of competition and/or facultative predation on native and introduced blowfly species in South America. In mixed cultures there was complete elimination of C. megacephala and C. macellaria. Chrysomya albiceps exhibited higher survival in mixed compared to pure cultures, suggesting that predation offers more advantages than competition for food. Body size of C. albiceps was significantly affected by food scarcity in pure cultures. However, tibia size in males of all species suffered no significant variation as a function of food scarcity. The implications of these results for population dynamics of introduced and native blowfly species are discussed.

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Ties among event times are often recorded in survival studies. For example, in a two week laboratory study where event times are measured in days, ties are very likely to occur. The proportional hazards model might be used in this setting using an approximated partial likelihood function. This approximation works well when the number of ties is small. on the other hand, discrete regression models are suggested when the data are heavily tied. However, in many situations it is not clear which approach should be used in practice. In this work, empirical guidelines based on Monte Carlo simulations are provided. These recommendations are based on a measure of the amount of tied data present and the mean square error. An example illustrates the proposed criterion.

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Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

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Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

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Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

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Leaf-cutting ants belonging to the tribe Attini are major herbivores and important agriculture pests in the neotropics, these ants being thought to feed on the sap which exudes from the plant material which they cut and also on the mycelium of a symbiotic fungus that grows on plant material inside their nests in what is called 'the fungus garden'. However, we have found that the survival of Atta sexdens worker ants on leaves, on mycelium. of the ants' symbiotic fungus, Leucoagaricus gongylophorus, or on plant polysaccharides was the same as that of starved A. sexdens, while, conversely, significantly longer survival was achieved by ants fed on the fungus garden material or on some of the products (especially glucose) of the hydrolysis of plant polysaccharides. We found that the fungus garden contained glucose at a higher concentration than that found in leaves or fungal mycelium, and that this glucose was consumed by the ant to the extent that it was probably responsible for up to 50% of the nutritional needs of the workers. The fungus garden contained polysaccharide degrading enzymes (pectinase, amylase, xylanase and cellulase) in proportions similar to that observed in laboratory cultures of L. gongylophorus. It thus appears that A. sexdens workers obtain a significant part of their nutrients from plant polysaccharide hydrolysis products produced by the action of extracellular enzymes released by L. gongylophorus. In this paper we discuss the symbiotic nutrition strategy of A. sexdens workers and brood and the role played by plant polysaccharides in the nutrition of attine ants. (C) 2003 Elsevier B.V. Ltd. All rights reserved.

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About 45 palm species occur in the Atlantic forest of Brazil, and most of them are affected by loss of seed dispersers resulting from forest fragmentation and hunting. Here we report the effects of habitat loss and defaunation on the seed dispersal system of an endemic palm, Astrocaryum aculeatissimum. We evaluated seed removal, insect and rodent seed predation, and scatter-hoarding in nine sites, ranging from 19 ha to 79 000 ha. We report the seedling, juvenile and adult palm densities in this range of sites. Endocarps remaining beneath the parent palm had a higher probability of being preyed upon by insects in small, mostly fragmented and more defaunated sites. The frequency of successful seed removal, scatter-hoarding and consumption by rodents increased in the larger, less defaunated sites. Successful removal and dispersal collapsed in small (< 1000 ha), highly defaunated sites and frequently resulted in low densities of both seedlings and juveniles. Our results indicate that a large fraction of Atlantic forest palms that rely on scatter-hoarding rodents may become regionally extinct due to forest fragmentation and defaunation. Current management practices including palm extraction and hunting pressure have a lasting effect on Atlantic forest palm regeneration by severely limiting successful recruitment of prereproductive individuals.(c) 2006 the Linnean Society of London.

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The seed deposition pattern created by a seed disperser is one of the components of the efficiency of a species as seed disperser, and ultimately may influence the recruitment of a plant species. In this study, we used the seeds of a bird-dispersed forest palm, Euterpe edulis, to investigate the effects of two distinct seed deposition patterns created by birds that defecate (clumped pattern) and regurgitate seeds (loose-clumped pattern) on the survival of seeds experimentally set in an E. edulis-rich site, and of seedlings grown under shade-house conditions. The study was conducted in the lowland forest of Parque Estadual Intervales, SE Brazil. Clumped and loose-clumped seeds were equally preyed upon by rodents and insects. Although clumped and isolated seedlings had the same root weight after 1 year, the isolated seedlings survived better and presented more developed shoots, suggesting intraspecific competition among clumped seedlings. Our results indicate that animals that deposit E. edulis seeds in faecal clumps (e.g. cracids, tapirs) are less efficient seed dispersers than those that regurgitate seeds individually (e.g. trogons, toucans). Intraspecific competition among seedlings growing from faecal clumps is a likely process preventing the occurrence of clumps of adult palms. (C) 2001 Editions scientifiques et medicales Elsevier SAS.

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Intraspecific variation in seed size is common in wild plant populations and has important consequences for the reproductive success of individual plants. Multiple, often conflicting evolutionary forces mediated by biotic as well as abiotic agents may maintain such a variation. In this paper we assessed seed size variation in a population of the threatened, commercially important palm Euterpe edulis in southeast Brazil. We investigated (i) how this variation affects the probability of attack by vertebrate and invertebrate post-dispersal seed predators, and (ii) if seed size influences the outcome of seeds damaged by beetles in terms of seed germination and early survival of seedlings. Euterpe edulis seeds varied in diameter from 8.3 to 14.1 mm. Neither insects nor rodents selected the seeds they preyed upon based on seed size. Seed germination and total, shoot and root biomasses of one-year seedlings were significantly and positively affected by seed size. Root biomass and seedling survival were negatively affected by seed damage caused by a scolytid beetle (Coccotrypes palmarum) whose adults bore into seeds to consume part of the endosperm, but do not oviposit on them. Seed size had a marginally significant effect on seedling survival. Therefore, if any advantage is accrued by E. edulis individuals producing large seeds, this is because of greater seed germination success and seedling vigor. If this is so, even a relatively narrow range of variation in seed size as observed in the E. edulis population studied may translate into differential success of individual plants. (c) 2006 Elsevier SAS. All rights reserved.

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Modelling post-release survival probabilities of reintroduced birds can help inform 'soft-release' strategies for avian reintroductions that use captive-bred individuals. We used post-release radiotelemetry data to estimate the survival probabilities of reintroduced captive-bred Red-billed Curassow Crax blumenbachii, a globally threatened Cracid endemic to the Brazilian Atlantic Rainforest. Between August 2006 and December 2008, 46 radiotagged Curassows from the Crax Brazil breeding centre were reintroduced to the Guapiacu Ecological Reserve (REGUA), Rio de Janeiro state, Brazil, in seven different cohorts. Reintroduced birds were most vulnerable during the first 12 months post-release from natural predation, domestic dogs and hunting. Annual post-release survival probability was high (75%) compared with published estimates for other Galliform species. However, when considering survival in all birds transported to REGUA (some birds died before release or were retained in captivity) and not only post-release survival, phi in this study was closer to estimates for other species (60%). The duration of the pre-release acclimatization period within the soft-release enclosure and the size of the released cohorts both positively influenced post-release survival of reintroduced Curassows. Our results are relevant to future Cracid reintroductions and highlight the importance of utilizing post-release monitoring data for evidence-based improvements to soft-release strategies that can significantly enhance the post-release survival of captive-bred birds.

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Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)