Molecular and paleontological approaches have produced extremely different estimates for divergence times among orders of placental mammals and within rodents with molecular studies suggesting a much older date than fossils. We evaluated the conflict between the fossil record and molecular data and find a significant correlation between dates estimated by fossils and relative branch lengths, suggesting that molecular data agree with the fossil record regarding divergence times in rodents. Our approach includes a correction for tree hierarchy involving simulating the random appearance of fossils. We also present a ghost lineage approach that attempts to incorporate the potential for the discovery of older fossils into a Bayesian analysis of divergence dates. Applying this approach to a set of Eocene rodent fossils, we estimated the earliest divergence in rodents appears to have occurred at approximately the K/T boundary, but interordinal splits were estimated to have taken place late in the Cretaceous. We propose that some molecular clock studies may overestimate divergence times due to periods of accelerated molecular evolution across multiple lineages or due to saturation of data that is not adequately corrected by the evolutionary model. We have sequenced the complete mitochondrial genomes of three rodent species, Anomalurus beecrofti, Castor canadensis, and Dipodomys ordii, and attempt to resolve phylogenetic relationships within rodents using the mitochondrial genome, a nuclear dataset of comparable size, and a combined analysis containing 26 kbp of sequence data. The combined analysis recovered a Sciuromorpha – Hystricomorpha clade with strong support. Our data suggest that increased character sampling improves resolution at these early nodes while better taxon sampling of mitochondrial genomes has led to better supported clades that converge on conclusions obtained from nuclear datasets. Several molecular studies have concluded that the zokors, genus Myospalax, evolved from within the rodent subfamily Cricetinae. We tested this conclusion using mitochondrial data and determined that Myospalax is sister to a clade containing the subfamilies Spalacinae and Rhizomyinae, and all three of these lineages appear to be basal to the superfamily Muroidea. Based on the position of these three lineages, we suggested that they be placed in a distinct family, the Spalacidae. The murine genera Mus and Rattus are thought to have diverged about 12 million years ago (Ma) based on a series of fossils from the Siwaliks of Pakistan, but assumptions of murid relationships that led to this conclusion have been shown to be false by molecular data. Equally parsimonious hypotheses can be proposed which place the 12 million year old Progonomys fossil at the base of the family Muridae, basal to the subfamily Murinae, or at the Mus - Rattus divergence. We here test the dates of evolutionary divergences in murids. Our results indicate that the family Muridae probably diverged earlier than the Siwalik fossils, but Mus and Rattus diverged at the same time or prior to the 12 Ma fossil date. We also cannot reject the hypothesis that the 12 Ma date represents the oldest split in the Murinae instead of the more derived Mus – Rattus date. We also recovered phylogenetic results suggesting that Taterillus is related to the tribe Gerbillini and not to other genera that are treated as Taterillini and that Gerbillurus evolved from within Gerbilliscus.

No description available.

Thesis (M.S.P.H.)--University of Michigan, 1940.

Thesis (M.P.H.)--University of Michigan, 1947.

Thesis (M.S.P.H.)--University of Michigan, 1940.

Thesis (M.P.H.)--University of Michigan, 1947.

No description available.

Thesis / Master of Science (MS)

Rodents cause significant damage to lowland rice crops across Cambodia and farmers are increasingly turning to chemical rodenticides for pest rodent management. Concern about the potential damage of these chemicals to human and environmental health provides the impetus for a search for alternative rodent management strategies, with particular interest in ecologically-based rodent management (EBRM). EBRM has a strong emphasis on non-chemical methods including the Trap Barrier System (TBS) and uses knowledge of pest rodent ecology to design effective interventions that limit population growth. For Cambodia, almost nothing was known at the start of this study regarding the identity, distribution and ecology of the country’s rodent pests. This research was designed to document this knowledge in parallel with a rodent management trial carried out at Somrong Commune in Kampong Cham Province of Cambodia, with the practical aim of informing on the outcomes of the trial and assisting with future development of EBRM in Cambodia. The large physical scale of the study and the relatively short time frame available for research led me to develop an action research approach that combined traditional positivist biological research with a more constructivist approach and participatory methods to gain access to and utilize local knowledge about rats. This research documents Cambodian pest rodents in lowland rice field cropping systems, through a combination of nation-wide collecting followed by detailed taxonomic assessments, and the use of farmer meetings and interviews to estimate the history and severity of the problem in each area. A total of nine rodent pest species were identified but both the pest rodent community and the severity of the associated problems vary from province to province. One species, the rice field rat (Rattus argentiventer), appears to be actively spreading and is not yet found in the northwestern provinces. A more detailed study in Somrong Commune found seven species and documented aspects of habitat use, breeding biology and movement. The inclusion of local knowledge in the study added a spatial and temporal scale to the study that could not have been obtained through conventional means, and which led to novel hypotheses about pest rodent ecology in the Somrong landscape, presented in the form of a heuristic landscape model. Criteria are developed for assessing the rigour and reliability or trustworthiness of the results. Particular attention is paid to the potential value of ‘expert’ knowledge which is rarely used in participatory research but which holds enormous potential for research into technical issues. The results of the TBS trial carried out in Somrong Commune were analysed to assess whether implementation of this method resulted in lower levels of rodent damage and higher rice yields relative to a single control commune, Lvea, and whether the farming community in Somrong is likely to adopt the method in the future. Although some farmer survey data suggest that yields improved in Somrong during the TBS trial, statistical analysis of the quantitative data set fails to yield compelling evidence of any benefit from use of TBS. The results emphasize the important role of landscape factors, especially the annual flooding cycle of the Mekong River, in determining the spatial and temporal distribution of rodent damage. Analysis of the relationship between rat captures and claimed yield increases also suggests that the TBS was not primarily responsible for the yield increase—too few rats were captured to account for the additional yield, even under extreme scenarios for how individual rodents cause damage to rice crops. Somrong farmers are unlikely to continue with the TBS method after completion of the project due to many perceived technical problems with implementing the method, and other concerns over its high labour and monetary costs. The ecological results of this study are used to suggest various alternative means by which Somrong farmers might combat the problem of rodent pests. Finally, a new approach is recommended for gaining an appreciation of rodent ecology on a landscape scale. This approach uses a combination of generalities about rodent biology and ecology, field observations, and incorporates local knowledge from local ‘experts’. This approach is likely to yield a faster and more complete understanding of the spatial and temporal dimensions of a local rodent community than more conventional approaches and will hopefully lead to more effective and relevant rodent management recommendations.

No description available.