Thesis (M.S.)--University of Wisconsin--Madison, 1970. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

The northwestern muskrat (Ondatra zibethicus spatulatus) is nowhere more abundant than in the delta of the Mackenzie River in northern Canada. An investigation was undertaken to assess the adjustments resident animals have made in order to inhabit a region more than one hundred miles inside the Arctic Circle. The rigours of the physical environment demand that these animals live in burrows rather than in lodges of vegetation, as in more southern areas. During summer the breeding pairs may occupy shallow temporary habitat which is not suitable for tenure during winter. These summer sites must be vacated before ice seals the lakes and prevents escape from a rapidly deteriorating environment. That all muskrats do not desert this temporary summer habitat was indicated by the fact that only half as many marked animals from such sites were encountered subsequently. It may be assumed that a depressed survival was the rule in such locations. Only deeper lakes with adequate submerged food plants constituted satisfactory wintering environment. Normal movement of muskrats in the Mackenzie delta is an average distance of about 300 yards in summer and 100 yards in winter. Winter activity is supported by an extensive system of feeding stations or "push-ups" constructed on the lake ice. These structures are a necessary part of the dally life of the individual muskrat because the dispersed nature of the food plants demands a relatively great radius of activity. The number of muskrats using each push-up varies from three to thirteen with an average of six. The relatively short period of open water in this latitude so shortens the breeding season that primiparous females probably produce only one litter of young their first year of life. However, they can, by maturing sexually at an earlier date their second year, produce two litters. Inasmuch as the late winter population is comprised of four yearlings to each adult female, the delay in breeding induced by the late removal of ice on the lakes and channels is significant In reducing the rate of population Increase. The restrictive effects of climate on breeding activity are compensated for by the birth of larger litters (8.3 young) and by a very satisfactory survival of these young to yearling status. Intolerance between adults is noted during the early part of the breeding season but does not persist during the rearing of the young. As a consequence there are few losses from depredations of adults upon young animals as has been reported in other areas. Densities of animals per unit area are low when compared with races of muskrats from other regions. In addition the size of the individual animal is small, and the majority do not survive long past their second year of life. These observations support the view that the Mackenzie delta provides marginal habitat for muskrats. It is suggested that physical factors induced by the severity of the climate represent the major influence limiting population growth. The fur industry is another significant drain on animal numbers but other factors appear to be less important. All mortality factors taken together, however, have suppressed or eliminated any tendency for muskrat numbers to fluctuate in a cyclic manner as has been reported by several authors for other parts of North America. Reference is made throughout the text to races of muskrats inhabiting more southern latitudes. / Science, Faculty of / Zoology, Department of / Graduate

Diving muskrats (Ondatra zibethica) invoke a series of cardiovascular and respiratory adjustments in response to stimulation of the nares with water. This dive response is characterized by apnoea, a decrease in cardiac output and an increase in peripheral resistance. The result is that blood flow is maintained to those organs most susceptible to oxygen deprivation, the heart and the brain. The initiation of the dive response in mammals is primarily the result of nasal stimulation with water. In addition, the baroreceptors acting via the baroreflex have been suggested to be involved in either the initiation or the maintenance of this response. The chemoreceptors, acting via the chemoreflex, have also been implicated in the maintenance of the dive response, although the importance of this contribution is controversial. The purpose of this thesis was to examine the role of the baroreceptors and chemoreceptors in the diving response of the muskrat. Changes in input from these receptors recorded from the cut carotid sinus nerve and their modulation by the carotid sinus efferent activity during nasal stimulation may have important implications for the role of the baroreceptors and chemoreceptors in the diving response. In the initial part of the dive, baroreceptor activity decreased, while chemoreceptor activity did not change. Subsequently, baroreceptor and chemoreceptor activity increased, exceeding pre-dive levels. This increase was not due to a change in receptor threshold or sensitivity induced by the nasal stimulation, but was a reflection of the increase in the usual stimulus modality of both receptor groups. The efferent activity recorded from the central end of the cut carotid sinus nerve was of two types, both of which responded to nasal stimulation. This change in the efferent discharge has the potential to modify afferent activity. Nasal stimulation caused one type of efferent activity (type A) to stop. The second type of efferent activity (type B) responded with an initial increase in discharge, returning to pre-dive levels after 6.6 seconds. Based-on the similar characteristics of these efferents to those of previous work it is postulated that the actions of the efferents would be to inhibit the baroreceptors and chemoreceptors during the initiation of the nasal stimulation, but to be less effective as the dive progressed. It is concluded that there is no contribution from the baroreceptors to the initiation of the diving bradycardia, although the lack of baroreceptor activity may contribute to the increase in peripheral resistance. Later in the dive, both heart rate and arterial blood pressure increase, despite a concomitant elevation in baroreceptor activity. The baroreceptors therefore have no role in the maintenance of the diving response. The initial inhibition of the chemoreceptors may be important to permit the full expression of the dive response, including a decrease in central respiratory output. Later in the dive the chemoreceptors may contribute to the maintenance and termination of the diving response. / Science, Faculty of / Zoology, Department of / Graduate

Heart rate was found to be significantly lower in unrestrained diving muskrats than in those which were forced to dive. The response in the unrestrained animal represents a heart rate of about 9% of the resting rate and is similar to the cardiac responses recorded in freely diving pinnipeds. Apnea and bradycardia were initiated by water lapping the nares of the conscious animal. Anaesthesia abolished this narial reflex to submersion. In anaesthetized muskrats water was drawn into the nasal cavity causing transient apnea and prominent bradycardia by stimulating receptors located principally in the glottal and pharyngeal areas. Nerve blockade by reversible cooling and section demonstrated that these nasal receptors are innervated by the maxillary and inferior, laryngeal nerves. In the conscious animal trigeminal neurotomy failed to affect the course of the response confirming that the muskrat has a number of external sensory mechanisms capable of initiating the diving reflexes. Respiratory activity was shown to have a marked effect on heart rate when the muskrat was at rest and when water was passed through the nares. Cardioacceleration during nasal stimulation resulted from a central component and from neural input originating in fast adapting pulmonary receptors. Artificial ventilation not only increased heart rate but often tended to restore normal respiratory activity. Pulmonary deafferentation by steaming eliminated the Hering-Breuer reflex to maintained lung inflation as well as the cardioacceleration seen in response to artificial ventilation during nasal stimulation. The loss of the Hering-Breuer reflex occurred first suggesting that different receptors are involved. Lung deflation per se caused a reflex bradycardia but it appears that this does not potentiate the narial reflex since nasal bradycardia was not reduced when lung inflation was maintained. Central and peripheral components arising from respiratory activity have their greatest effect during the recovery period. Elimination of the carotid bodies delayed but did not abolish chemoreceptor driven bradycardia demonstrating that these are the most chemosensitive units but not the only ones responding to changes in blood gas tensions. No role however, has been found for the arterial baroreceptors. The barostatic reflex brought on by drug induced hypertension was triggered at a lower pressure than that found in the seal but it appears that this pressure would not be exceeded in the muskrat if heart rate remained low during a dive. It is concluded that the cardiac response to submersion in the muskrat results from at least three reflex arcs. These reflexes originate from the nares, the lungs and from peripheral chemoreceptors. Although the chemoreceptors act to maintain the prevailing diving responses, it is likely that the external narial reflex accounts for almost all of the cardiovascular adjustment brought about in normal foraging dives since these are usually of short duration. The chemoreflex could play a significant role in dives exceeding one minute by prompting the animal to resurface when oxygen stores are depleted. / Science, Faculty of / Zoology, Department of / Graduate

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Understanding the factors and processes that influence intraspecific genetic variation are essential to better understand evolutionary processes. In this research, I examined patterns of gene flow and their effects on the distribution of genetic variation and spatial genetic structuring at different spatial scales. I used a combination of population genetics, spatial analysis, morphometrics and phylogeography in order to understand the patterns of genetic variation and their resulting phenotypic variations in a semi-aquatic species, the muskrat (Ondatra zibethicus).

1916 infördes ett införselförbud för bisamråttan i Sverige, detta i kontext till ökad pälshandeloch inplanteringar av bisam runt om i Europa. En bisamråttekommité tillsattes 1938 för attutreda frågan rörande införselförbudet, därav denna studie. Genom en kvalitativ samt diskursivmetod har införselförbudet, mediediskursen samt utredningen som utgår från kommittén,studerats under perioden 1916–1942. Studien har visat på ett resultat av en varierandeargumentation där tre centrala huvudteman genomsyrat av ett fjärde har framkommit. De tretemana identifieras som tillståndsansökningar, utländska influenser samt ekonomi. Det fjärdetemat som genomsyrar hela studien är argumenten rörande införsel av den främmande arten,bisamråttan. Dessa läggs vidare i en bredare kontext och identifieras utifrån trehuvudargumentet, införselfrågan, skadegörelse samt ekonomi. Där en konstant argumentationrörande bisamråttefrågan visats över tid. Slutsatsen blir synliggörandet av statens makt övernaturen i relation till statens anspråk på att tillfredsställa ekonomiska behov i Norrland. / In 1916, a ban of import was issued on the muskrat in Sweden, in the context of increased furtrade and implantations of muskrat around Europe. A muskrat committee was appointed in1938 to investigate the issue of its prohibition, thereof this study. Through a qualitative anddiscursive method, the intrusion order, media dissertation and the inquiry based on thecommittee have been studied during the period 1916-1942. The study has shown a result of avaried argumentation in which three key main themes permeated by a fourth have beenemerged. The three themes are identified as license applications, foreign influences andeconomics. The fourth theme that permeates the entire study is the arguments concerning theintroduction of the foreign species, the muskrat. These are laid down in a broader context andidentified on the basis of three main arguments, question of import, damage and economics.Where a constant argument about the question of muskrat has been shown over time. Theconclusion becomes the visibility of the state's power over nature in relation to the state'sclaim to satisfy economic needs in Norrland.

Master of Science / Department of Horticulture and Natural Resources / Adam A. Ahlers / The muskrat (Ondatra zibethicus) is an economically and ecologically important furbearer species that occupy wetlands throughout North America. However, populations across the United States (US) are declining and there is little evidence as to the cause of this decline. Wetlands in the upper Midwest, US, are shifting into more homogeneous vegetation states due to an invasive hybrid cattail species, Typha x glauca (hereafter ‘T. x glauca’), outcompeting native vegetation. This hybrid cattail species is now an abundant potential resource for muskrats and has outcompeted native wetland vegetation. I investigated how landscape composition and configuration affected multiscale habitat use by muskrats during the summers of 2016 – 2017. Additionally, I assessed how fetch (impact of wind and wave action), a process dictated by large-scale landscape configuration, influenced muskrat habitat use at a local-scale representing a resource patch. I randomly selected 71 wetland sites within Voyageurs National Park, Minnesota, and used presence/absence surveys to assess site occupancy by muskrats. Each year, multiple surveys were conducted at each site and I used multiseason occupancy modeling to investigate how both local and landscape factors affect site occupancy and turnover. I predicted a positive relationship between local-scale (2 ha) sites, characterized by shallower and less open water, and muskrat occupancy and colonization rates. I also predicted increased occupancy probabilities and colonization rates in wetlands that contain higher amounts of T. x glauca. However, I expected the amount of fetch at each site to negatively influence site occupancy probabilities and colonization rates. At the landscape-scale (2 km), I expected habitat use by muskrats to be positively related to the percentage of T. x glauca and area of wetlands surrounding sites. At the local-scale, muskrats occupied wetlands that contained shallower water depths and less open water. As predicted, site occupancy probabilities were greater in areas with greater amounts of T. x glauca coverage. My results revealed a cross-scale interaction between the severity of fetch impacts and percent of T. x glauca coverage at sites. Muskrats were more likely to colonize areas with greater fetch impacts if there was also greater coverage of T. x glauca at these sites. At the landscape-scale, site-occupancy probabilities were positively influenced by the percent of open water and landscape heterogeneity surrounding each site. My study was the first to document how invasive T. x glauca populations can mitigate negative effects that high wave intensity may have on muskrat spatial distributions. I was also the first to identify multiscale factors affecting the spatial distribution of muskrats in lacustrine ecosystems.

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