Philip Myers

Reproduction by Peromyscus maniculatus: Size and Compromise

Breeding records from a laboratory colony of Peromyscus maniculatus bairdii were used to examine patterns of covariation among variables describing reproduction, size, age, parity, and survival. Litter size was strongly negatively correlated with average weights of young at birth and at weaning, but positively related to the total mass of young. Mothers weight was correlated with the number of young in a litter and the average and total weights of young at birth and weaning, though the strength of the correlation varied according to when young were weighed. Weight gained by young during the nursing period also was positively related to mothers size. Parity strongly affected litter sizes and birth weights; however, this effect was attributable to continuing growth of the mother. Similarly much, but not all, of the effect of parity on weaning weights and growth rates was attributable to mothers weight. Female age (independent of weight and parity) was important in determining the number of young born (but not their birth weight), the rate of growth of young during the nursing period, and the average weights of individual young at laboratory weaning. The size of young at birth and their number of littermates was clearly related to their probability of surviving the nursing period, and size differences among young at weaning persisted well into adult life. Parity had no effect on the survival of young. The duration of gestation also was related to the weight of a mother and to the weight of her previous litter; the size of the litter in utero had no significant effect on gestation length. These relationships are discussed from the points of view of (1) the advantage to females and their young of large size, (2) the compromises a female makes in determining the number, size, and timing of birth of her offspring, and (3) the implications of these patterns of covariation for investigations of the evolution of reproductive patterns.

A Phylogeny of the Neotropical Nectar-Feeding Bats (Chiroptera: Phyllostomidae) Based on Morphological and Molecular Data

We present a phylogeny of 35 species of nectar-feeding bats based on 119 morphological characters: 62 from the skin, skull, and dentition and 57 soft tissue characters (the latter from Wetterer et al., 2000). These data support monophyly of the subfamilies Brachyphyllinae, Phyllonycterinae, and Glossophaginae, and the tribes Glossophagini and Lonchophyllini. Our analysis contradicts the phylogeny estimated from the RAG-2 gene, which does not support a monophyletic Glossophaginae (Baker et al., 2000). Parsimony analysis of a combined matrix, containing morphological characters and RAG-2 sequences, results in a phylogeny that includes Brachyphyllinae and Phyllonycterinae in Glossophaginae. Support for most clades is stronger than in the morphological tree, but support for basal nodes of the phylogeny remains weak. The weak support at these basal nodes underscores the historical disagreements regarding relationships among these taxa; combining morphological and molecular data has not improved support for these nodes. Uncertainty regarding basal relationships complicates description of morphological change during the evolution of nectarivory in the Phyllostomidae.

Phenotypic plasticity in skull and dental morphology in the prairie deer mouse (Peromyscus maniculatus bairdii)

Morphologists and systematists have long suspected that dietary consistency can affect skull and dental form in mammals. We examined plasticity of skull shape and tooth morphology in prairie deer mice (Peromyscus maniculatus bairdii) by feeding mice diets that differed in consistency but not nutritional quality. Shape differences were analyzed qualitatively and quantitatively, using both landmark‐based morphometrics and traditional distance measurements. Mice fed a gruel made of laboratory chow soaked in water differed from those fed hard blocks of chow by a slight anterior shift in the incisor tips a narrowed zygomatic plate, a reduction in size of the masseteric tubercles, an overall decrease in skull size in lateral view, and an increase in overall size in ventral view. Disparities between our results and previous studies may be due to the differences in behavior between the inbred, relatively inactive laboratory strains commonly used in experimental studies and the outbred, constantly active species used here. Also, in contrast to previous studies, the statistical analysis employed here took into account both family relationships of the animals and the large number of statistical comparisons performed. Failure to consider these factors would have resulted in an exaggerated estimate of the effects of diet on skull form and may taint other studies that have explored the same aspects of plasticity.

Ambient Temperature and Rainfall: An Effect on Sex Ratio and Litter Size in Deer Mice

Short-term fluctuations in temperature and rainfall affect the reproduction of prairie deer mice, Peromyscus maniculatus bairdii . Wild, pregnant females were captured and brought to the laboratory, where the size, weight, and sexual composition of their litters were recorded. Females that experienced unusually warm springtime temperatures in the field during early pregnancy gave birth to approximately 0.4 more young per litter than other females, and female embryos appeared to be most strongly affected. Females experiencing warm temperatures in the autumn gave birth to approximately 0.5 fewer young than other females in that season, with male embryos most affected. Exposure to heavy rainfall (over 0.1″ in 24 h) during early pregnancy in any season also cost females approximately 0.5 young per pregnancy. Young born to mothers exposed to rainfall weighed 7–9% less than young in other litters. Unusually cold temperatures did not affect the mean number of young born per litter, but reduced the variance in litter size by selecting against especially large and small litters. Laboratory exposure of newly impregnated females to 12 h of high temperatures (33–35°C) led to decreased litter size, but the embryonic sex most affected depended on the timing of exposure. Male embryos suffered the greatest losses when pregnant females were exposed just after implantation of their litters, whereas earlier exposure led to mortality of female embryos.

Sources and Distribution of Organic and Carbonate Carbon in Surface Sediments of Pyramid Lake, Nevada

ABSTRACT Surface sediment samples from 32 sites in Pyramid Lake, Nevada, have been studied to investigate the sources and distribution of carbon within a large, terminal lake basin. The origins of organic and inorganic carbon in the sediments of this lake are predominantly from in-lake sources. Dilution of these sedimentary materials by land-derived elastic components occurs near the mouth of the Truckee River, the only perennial river entering the lake. Total organic carbon (TOC) concentrations and CaCO3 concentrations and d18O values increase while organic matter C/N atomic ratios and d13C values decrease with increasing distance from the river mouth as the proportion of river-derived components decreases. Aragonite precipitates from lake water and dominates CaCO3 deposition in most parts of the lake, except near underlake springs, where calcite precipitates. TOC concentrations increase as water depth increases, reflecting grain sorting as smaller particles are resuspended and focused toward the deep basin center.

A role for suppressed incisor cuspal morphogenesis in the evolution of mammalian heterodont dentition.

Changes in tooth shape have played a major role in vertebrate evolution with modification of dentition allowing an organism to adapt to new feeding strategies. The current view is that molar teeth evolved from simple conical teeth, similar to canines, by progressive addition of extra “cones” to form progressively complex multicuspid crowns. Mammalian incisors, however, are neither conical nor multicuspid, and their evolution is unclear. We show that hypomorphic mutation of a cell surface receptor, Lrp4, which modulates multiple signaling pathways, produces incisors with grooved enamel surfaces that exhibit the same molecular characteristics as the tips of molar cusps. Mice with a null mutation of Lrp4 develop extra cusps on molars and have incisors that exhibit clear molar-like cusp and root morphologies. Molecular analysis identifies misregulation of Shh and Bmp signaling in the mutant incisors and suggests an uncoupling of the processes of tooth shape determination and morphogenesis. Incisors thus possess a developmentally suppressed, cuspid crown-like morphogenesis program similar to that in molars that is revealed by loss of Lrp4 activity. Several mammalian species naturally possess multicuspid incisors, suggesting that mammals have the capacity to form multicuspid teeth regardless of location in the oral jaw. Localized loss of enamel may thus have been an intermediary step in the evolution of cusps, both of which use Lrp4-mediated signaling.

The Seasonal Decline in the Litter Size of Meadow Voles

Variation in litter size was examined in meadow voles ( Microtus pennsylvanicus ) during a 14-month period in 1979–1980 near Ann Arbor, Michigan. Mean litter size declined sharply each autumn, at the end of the breeding season. This pattern of autumnal decline in litter size is widespread in the genus Microtus . We examined four hypotheses that might might explain the seasonal decline in litter size: the decline reflects possibly adaptive changes in the birth mass of offspring; the decline reflects differential investment in offspring of one sex; the decline reflects a decrease in the mass of mothers; and the decline reflects a change in the average age or parity of mothers. To evaluate these hypotheses, we examined changes in litter size, masses of young and mothers, the sex ratio of litters, and associations among these reproductive variables by bringing live-caught pregnant females into the laboratory and observing their litters. Mean neonatal mass, total mass of litters, and mothers mass varied significantly during the breeding season, but preliminary estimates of age and parity of mothers did not. The number of young in a litter was correlated strongly and positively with the postpartum mass of the mother and correlated weakly and negatively with the mass of neonates. No single hypothesis adequately accounted for seasonal changes in litter size. At best, the first and third hypotheses together could account for about half of the seasonal variation in litter size.

Intra- and Interindividual Variations Mask Interspecies Variation in the Microbiota of Sympatric Peromyscus Populations

ABSTRACT Using populations of two sympatric Peromyscus species, we characterized the importance of the host species, physiology, environment, diet, and other factors in shaping the structure and dynamics of their gut microbiota. We performed a capture-mark-release experiment in which we obtained 16S rRNA gene sequence data from 49 animals at multiple time points. In addition, we performed 18S rRNA gene sequencing of the same samples to characterize the diet of each individual. Our analysis could not distinguish between the two species of Peromyscus on the basis of the structures of their microbiotas. However, we did observe a set of bacterial populations that were found in every animal. Most notable were abundant representatives of the genera Lactobacillus and Helicobacter. When we combined the 16S and 18S rRNA gene sequence analyses, we were unable to distinguish the communities on the basis of the animals diet. Furthermore, there were no discernible differences in the structure of the gut communities based on the capture site or their developmental or physiological status. Finally, in contrast to humans, where each individual has a unique microbiota when sampled over years, among the animals captured in this study, the uniqueness of each microbiota was lost within a week of the original sampling. Wild populations provide an opportunity to study host-microbiota interactions as they originally evolved, and the ability to perform natural experiments will facilitate a greater understanding of the factors that shape the structure and function of the gut microbiota.

Climate Change Impacts on Terrestrial Ecosystems in Metropolitan Chicago and Its Surrounding, Multi-State Region

ABSTRACT This paper describes the potential impacts of warming temperatures and changing precipitation on plants, wildlife, invasive species, pests, and agricultural ecosystems across the multi-state region centered on Chicago, Illinois. We examine a geographic area that captures much of Lake Michigan, including a complex mosaic of urbanization and agriculture surrounding southern Lake Michigan. We consider species currently present within this broad region as well as species that are expected to move into or out of the area as climate zones shift northward through the coming century. Our analysis draws upon disparate data sources to compile projections. We conclude that a complex mixture of land use poses particular challenges to natural ecosystems in this region under climate change. Dispersal is likely to be limited for some species, and some populations of native taxa may already be reduced due to habitat loss. Other species can persist, even thrive, within a mixed landscape mosaic, provided natural areas and green spaces are available. If such spaces are somehow connected, they can provide opportunities for native species to inhabit and move through the metropolitan region (perhaps even better than the landscapes previously dominated by agriculture). Strategies for adapting regional agriculture and minimizing pest outbreaks also call for creative management intervention. With additional research, Chicago and its surrounding environs have an opportunity to provide leadership on effective management of natural resources under climate change.

Karyotypes of Some Harvest Mice, Genus Reithrodontomys

Karyotypes of six species of harvest mice, representing both the sub-genus Reithrodontomys ( humulis, sumichrasti , and fulvescens ) and the subgenus Aporodon ( gracilis, mexicanus , and creper ), are described. The karyotype of R. sumichrasti (2n = 40) resembles those previously reported for R. megalotis and R. raviventris , which possess mainly metacentric chromosomes. Reithrodontomys humulis (2n = 51), R. fulvescens (2n = 50), R. gracilis (2n = 52), R. mexicanus (2n = 52), and R. creper (2n = 52) exhibit a large proportion of acrocentric chromosomes. Karyotypic evidence suggests that intrageneric relationships of Reithrodontomys may not be accurately reflected by use of the subgenera Aporodon and Reithrodontomys .