Natal philopatry, or delayed dispersal of sexually mature offspring, may be due to ecological constraints on dispersal. In this study, we manipulated the population density of prairie voles (Microtus ochrogaster) living in experimental outdoor enclosures to test a prediction from the habitat saturation hypothesis that philopatry and subsequent group formation in this cooperatively breeding mammal is affected by the availability of suitable territories. We detected a significant, positive relationship between the proportion of offspring remaining philopatric and density, with females being more philopatric than males at all densities. This increase in philopatry led to a significant increase in the proportion of social units that were groups as well as a significant increase in group size. These results provide the strongest evidence of a causal effect of density on dispersal and group formation in a mammal. Our findings suggest that habitat saturation is at least a partial explanation for philopatry in prairie voles. However, we cannot eliminate the possibility that other variables, such as benefits accrued from remaining philopatric, may also be factors contributing to philopatry. Nonetheless, these results show that changes in ecological conditions can influence social structure within a population, leading to group formation and a social milieu conducive to the evolution of cooperative breeding.
Abstract. Changes in woody vegetation were examined over eight years, using a 1.05‐ha permanent plot in which the location of every shrub and tree > 1m height was mapped. There was little change in the overstory vegetation, as expected for an old‐growth forest. Much greater change occurred in the understory, primarily related to a 40 % increase in density. Differences occurred among species in the under‐story, as Acer saccharum and Prunus serotina increased and Fraxinus americana and Fagus grandifolia decreased. Canopy gap dynamics are implicated in differences among species in the establishment and growth of individuals in the understory and their recruitment into the overstory. It is concluded that because understory is temporally variable, overstory recruitment from the understory may take different courses at different times in the same forest.
In the typical repeated measures analysis, the subject effect is assumed to be random and the error terms random and uncorrelated. (One could assume that the subject effect is fixed while the errors are correlated.) In many statistical packages, unless the user specifically requests that otherMean Squares be used for tests, the MSE is used by default. A simulation study investigates the magnitude of the errors that result if the inappropriate tests are performed. It is found that both the Type I and Type II errors can be greatly affected
White-footed Mice (Peromyscus leucopus) were trapped for two years in the exterior matrix, edge, and interior forest habitat sections of six forests patches in a fragmented agricultural landscape. We used data on the capture locations of P. leucopus individuals from the two years, which differed in rainfall (i.e., summer of 2000 with 50% more rain than summer of 1999), to assess how patch size, edge habitat, and surrounding habitat type influence habitat use and movements in populations of this forest habitat generalist. We found that the proportion of individuals subsequently captured in the forest edge from the exterior was 16 times greater in the wet year than in the dry year and approximately twice as many P. leucopus were not subsequently recaptured from the exterior matrix in the dry year compared to the wet year. For each year, captures between habitats did not differ in relation to patch size, edge forest habitat, or exterior matrix type. These results illustrate the generalist habitat preferences of P. leucopus, but emphasize annual variation in their behavior and distribution.
A brief review of the literature includes an outline of the known causes of hypergenitalism and notes on theoretical considerations. Seventeen cases of non-neoplastic hypergenitalism are analyzed with respect to physical, psychological and social findings. A composite description of this endocrine type is presented. No treatment is suggested but understanding of the problem is essential for all who manage juveniles and adolescents.
Fragmentation of large expanses of forests into small stands has isolated local populations of organisms. Sugar maple (Acer saccharum) was used to determine if the degree of forest fragment isolation affects genetic diversity and structure of local populations. Genetic data were collected from canopy (prefragmentation) and juvenile (postfragmentation) individuals in 15 woodlots. Genotypes were inferred from phenotypic enzyme patterns of seven enzymes representing eight loci extracted from cambium and resolved with starch gel electrophoresis. Analyses of allelic data indicated that genetic diversity was not significantly different between juvenile or canopy subdivisions, or between woodlots with low and high degrees of isolation. Genetic differentiation among woodlots was significantly greater for the canopy than for the juvenile subdivision. Estimates of gene flow indicate that postfragmentation gene flow rates are higher than prefragmentation rates. Apparently, sugar maple's high potential for long-distance gene flow is enhanced by altered wind flux across a fragmented landscape. The results also show that forest fragmentation does not always result in greater isolation of local populations. Key words: sugar maple, gene flow, conservation, isolation, allozymes.
It is found that multicollinearity among the independent variables in logistic regression inflates the variances of the maximum likelihood estimator. A Ridge type estimator is proposed that will have smaller total mean squared error than the maximum likelihood estimator under certain conditions. Empirical study results are presented that evaluate this estimator for different sample sizes and degrees of multicollinearity.
