Heat increment of feeding and its thermoregulatory benefit in the muskrat (Ondatra zibethicus)

The calorigenic effect of feeding and its potential benefit in defraying thermoregulatory costs and attenuating immersion hypothermia of adult muskrats were investigated. A single session of feeding on aquatic vegetation was sufficient to raise the metabolic rate of muskrats for a period of at least 5 h. The peak postprandial rate of oxygen consumption averaged 1.42 times the level established for fasted animals, and the heat increment of feeding accounted for about 40% of the metabolizable energy intake of muskrats. There was no evidence of a postprandial rise in oxygen consumption of muskrats that entered water at 18–19°C after feeding. In aquatic trials, average and minimum steady-state oxygen consumption rates of fed muskrats were similar to, or even lower than values recorded from fasted animals, implying substitution of heat increment of feeding for thermoregulatory heat production. Our data did not support the hypothesis that heat increment of feeding retards body cooling in water. Net body temperature decline in water was actually higher in fed animals than in fasted controls. However, since previously fed muskrats also entered water at an elevated body temperature, the final body temperature (at 30 min immersion) was similar in all groups. These findings suggest that metabolic heat generated incidental to preimmersion feeding could provide a thermoregulatory benefit to muskrats by reducing the need for active thermogenesis in water.     

Metabolic effects of swimming velocity and water temperature in the muskrat (Ondatra zibethicus)

Metabolic rates, VO2, were studied in four muskrats (Ondatra zibethicus) swimming in a water channel at velocities of 0.2 to 0.75 m/s in water at temperatures of 25 and 30 degrees C. At both water temperatures, VO2 increased linearly with increasing swimming velocity. The VO2 was higher for muskrats swimming in water at 25 than 30 degrees C. The metabolic performance of swimming appears to be influenced by the interaction of swimming velocity and water temperature.     

Climatic variability and body size variation in the muskrats (Ondatra zibethicus) of North America

Summary Patterns of geographic variation in nine morphological characters of adult muskrats (Ondatra zibethicus) are investigated by multiple regression and canonical correlation analysis. Three variables describing the annual precipitation regimen account for 16 to 33% of the variance in each of the skeletal measurements. Highly variable precipitation patterns result in low rates of germination and survivorship for preferred aquatic food plants which in turn reduces average total food availability for muskrats. In large individuals nutritive demands may exceed supply, thus selection favors smaller body size in areas of low food availability.After removing the variation attributable to three precipitation variables, the residual variation in the morphological variables is exposed to canonical correlation analysis with a set of 10 environmental and geographic variables. A canonical variate loaded for climatic seasonality accounts for 60% of the variance in a canonical variate of the morphological residuals which load as a general body size variate. It is argued that seasonality is a major factor selecting for large body size in muskrats and other organisms. During seasonal periods of resource abundance natural selection favors individuals with rapid growth to a large size, while concurrently enhancing survivorship through oncoming periods of resource shortage.     

Isolation and characterization of polymorphic microsatellite loci in muskrat, Ondatra zibethicus

We describe the isolation and characterization of 12 highly polymorphic microsatellite loci for the muskrat, Ondatra zibethicus. Microsatellite markers from three other rodent species were cross-amplified in muskrat and one of them was polymorphic. We observed moderate to high levels of genetic variability in these 13 polymorphic loci (five to 22 alleles per locus) with observed heterozygosity ranging from 0.48 to 0.96. These markers will be useful for further studies on population genetic structure in muskrat and potentially in other rodent species.     

Effects of structural connectivity on fine scale population genetic structure of muskrat,Ondatra zibethicus

In heterogeneous landscapes, physical barriers and loss of structural connectivity have been shown to reduce gene flow and therefore lead to population structuring. In this study, we assessed the influence of landscape features on population genetic structure and gene flow of a semiaquatic species, the muskrat. A total of 97 muskrats were sampled from three watersheds near Sudbury, Ontario, Canada. We estimated population genetic structure using 11 microsatellite loci and identified a single genetic cluster and no genetic differences were found among the watersheds as a result of high levels of gene flow. At finer scales, we assessed the correlation between individual pairwise genetic distances and Euclidean distance as well as different models of least cost path (LCP). We used a range of cost values for the landscape types in order to build our LCP models. We found a positive relationship between genetic distance and least cost distance when we considered roads as corridors for movements. Open landscapes and urban areas seemed to restrict but not prevent gene flow within the study area. Our study underlines the high‐dispersal ability of generalist species in their use of landscape and highlights how landscape features often considered barriers to animal movements are corridors for other species.     

Baculum variation and allometry in the muskrat (Ondatra zibethicus): a case for sexual selection

Sexual selection is a powerful force that influences the evolution of a variety of traits associated with female mate choice and male–male competition. Although other factors have been implicated, sexual selection may be particularly important in the evolution of the genitalia. Traits under sexual selection typically have high phenotypic variance and positive allometry relative to non-sexual traits. Here, we test the hypothesis that the baculum (os penis) of the muskrat (Ondatra zibethicus) is under sexual selection by examining phenotypic variance and allometry relative to non-sexual traits. Muskrats were sampled from Ontario, Canada, and a variety of traits measured. Measurements included baculum length and width, and three non-sexual traits (skull length, skull width, hind foot length). We used coefficient of variation (CV) and allometric slopes calculated using reduced major axis regression to test our hypotheses. Baculum traits had significantly higher CV’s relative to non-sexual traits. Baculum traits also showed positive allometry, whereas all non-sexual traits had negative allometric relationships. In addition, baculum width had higher CV’s and steeper allometric slopes than baculum length, indicating that, in muskrat, baculum width may be more influenced by sexual selection than baculum length. Positive allometry of the baculum is consistent with other examples of mammalian genitalia, but contrasts with negative allometry found in many insects. Other examples of positive allometry and high phenotypic variance of the baculum have suggested that females may use the baculum as an indicator of male quality. “Good genes” indicator traits may be particularly important in species that mate in an environmental context that prohibits female assessment of male quality. Muskrats mate aquatically, and thus females may be unable to properly assess males prior to copulation.     

Seasonal changes in the oxygen storage capacity and aerobic dive limits of the muskrat (Ondatra zibethicus)

Summary The oxygen storage capacity and partitioning of body oxygen reserves were compared in summer-and winter-acclimatized muskrats (Ondatra zibethicus). Blood volume, blood oxygen capacity, and skeletal muscle myoglobin content were higher in December than in July (P<0.02). Total lung capacity increased only slightly in winter (P>0.05). The oxygen storage capacity of a diving muskrat was calculated at 25.2 ml O₂ STPD · kg^-1 in July, compared to 35.7 ml O₂ STPD · kg^-1 in December. Blood comprised the major storage compartment in both seasons, accounting for 57% and 65% of the total oxygen stores in summer and winter, respectively. Based on available oxygen stores and previous estimates of the cost of diving, the aerobic dive limit (ADL) increased from 40.9 s in July to 57.9 s in December. Concurrent behavioral studies suggested that most voluntary diving by muskrats is aerobic. However, the proportion of dives exceeding the calculated ADL of these animals was shown to vary with the context of the dive. Only 3.5% of all dives initiated by muskrats floating in the water exceeded their estimated ADL. Provision of a dry resting site and access to a submerged food source increased this proportion to 18–61%, depending on the underwater distance that foraging muskrats were required to swim. Serial dives exceeding the estimated ADL were not accompanied by extended postdive recovery periods.Abbreviations ADL acrobic dive limit - Hb hemoglobin - Hct hematocrit - Mb myoglobin - PaO₂ arterial O₂ tension - STPD standard temperature and pressure, dry     

Muskrats (Ondatra zibethicus) in treatment wetlands

Muskrat grazing can change treatment wetlands from being densely vegetated to a patchwork of open and emergent areas. Muskrats consume a portion of the annual net primary productivity, primarily rhizomes, but their mounds represent a greater share of this production. Densities of 20 or more animals per ha have been found, which can destroy the majority of the macrophyte standing crop in a given year. At such an exacerbated scale, muskrat herbivory may be termed as an “eatout,” and is evidenced by the removal of essentially all emergent plant parts. Destruction of the wetland vegetative infrastructure may create an attendant loss of some water quality functions, but may not harm others. The integrity of berms may be threatened by burrowing. Impacts on wetland hydraulics are also possible. In all cases, loss of the emergent vegetation has been viewed with dismay by owners, wetland practitioners, regulators and the general public. Several case histories are reviewed to illustrate the breadth and severity of muskrat damage. Muskrat control is given scant attention in existing treatment wetland literature, which provides very limited information on potential muskrat problems, or on the means to control them. Controls include trapping, shooting, poisoning, hazing, and exclusion in order to protect the wetland from excessive vegetation destruction by these rodents. This paper summarizes available muskrat controls, as well as their effectiveness. While many of these approaches have had a limited effect on deterring these industrious creatures, there are some methods that have proven to be effective over the long run, and should be considered in wetland design.     

Daily and seasonal activity patterns of the muskrat Ondatra zibethicus as revealed by radiotelemetry

Activity patterns of the muskrat Ondatra zibethicus L. were examined during summer and winter in Delta Marsh, Manitoba, Canada (50°11'N, 98°23'W). Animals exhibited intermittent activity throughout the day with a mean periodicity close to 6 h in both seasons. The 24-h activity pattern in summer was typically bimodal with major peaks occurring between sunset and sunrise. Muskrats were more diurnal in winter when maximal levels of activity occurred in late afternoon and early evening (1500–2000). Pronounced inter- and intra-individual variability in daily activity ensured continuous, or nearly continuous occupation of winter shelters by a variable number of muskrats, and thus enchanced microclimate stability. Daily variation in activity and weather were weakly correlated both summer and winter. Of the environmental variables tested, only photoperiod and wind speed in summer, and photoperiod and air temperature in winter correlated significantly with total time spent in lodge or burrow per day.     

Predator–prey coupling: interaction between mink Mustela vison and muskrat Ondatra zibethicus across Canada

In this paper we explore variation in the predator-prey interaction between mink Mustela vison and muskrat Ondatra zibethicus across Canada based on 25 years of mink (predator) and muskrat (prey) data from the Hudson's Bay Company. We show that predator–prey interactions have stronger signatures in the west of Canada than in the east. In particular, we show that the observed phase plot trajectories of mink and muskrat rotate significantly clock-wise, consistent with predator–prey theory. We also investigate four phases of the mink muskrat interaction sequence (predator crash phase, prey recovery phase, etc.) and show that they are all consistent with a strong coupling in the west, whereas the presence of generalist predators and alternative preys can explain deviations from this pattern in the east.     

Effects of hypoxic factors on cardiac chronotropic reactions in the muskrat Ondatra zibethicus in free behavior

Effects of natural and artificial hypoxic factors on cardiac chronotropic reactions were studied in the muskrat Ondatra zibethicus naturally adapted to underwater hypoxia under conditions of free behavior. To record cardiac activity, original implanted ECG sensors designed in the laboratory were used. Under observations were muskrats in the states of rest, movement, swimming on the water surface, diving, underwater swimming, forced underwater immersion, and artificial apnea, in the low-pressure chamber during changes of pressure from 100 to 25 kPa (ascent to an altitude of 11 km) and in the atmosphere of hypoxic mixtures with 5–10% O₂ as well as under conditions of hemic nitrite hypoxia after injection of 3 mg/kg NaNO₂. Heart rate (HR) in muskrats is labile and can change within the limits from 15 to 360 beats/min. A characteristic feature of hypoxic action is development in muskrats of bradycardia that can appear either instantly—both as a conditional reflex and from the nose lobe receptors—or gradually at a decrease of pO₂ in inhaled air. Before diving and after coming to the surface a brief tachycardia can also be observed. The gradual development of tachycardia takes place in nitrite hypoxia. Development of bradycardia was eliminated at blockade of M-cholinoreceptors by atropine, and of tachycardia—at blockade of β-adrenoreceptors by propranolol. Blockade of α-adrenoreceptors by phentolamine did not affect cardiac chronotropic reactions, which indicates the absence of their connection with vasoconstriction. Analysis of the cardiac rhythm variability has revealed a large spectrum of slow cardiointerval fluctuations connected with animal functional states. Regulation of cardiac chronotropic reactions in muskrats under effect of hypoxic factors operates along both sympathetic and parasympathetic pathways of the autonomic nervous system, the leading role in these processes being played by vagus influences. Original Russian Text. V. I. Shereshkov, T. E. Shumilova, D. A. Kuzmin, I. N. Yanvareva, and A. D. Nozdrachev, 2006, published in Zhurnal Evolyutsionnoi Biokhimii i Fiziologii, 2006, Vol. 42, No. 4, pp. 371–377.     

Morphological features of cellular responses to different rates of trematode Quinqueserialis quinqueserialis (Trematoda: Notocotilidae) invasion in muskrat (Ondatra zibethicus)

The results of investigation of leukocyte morphology and leukocyte contents of blood and caecum depending on the trematode Quinqueserialis quinqueserialis invasion rate in muskrats from natural population are given. At low trematode invasion rates, there was observed systemic activation of lymphopoiesis and neutrophil granulocytopoiesis with a decrease in the monocyte-macrophage response in caecum (trematode localization organ). At the same time, under high invasion rates, there was detected induction of T cell suppressor activity and the absence of a granulocyte response in the tissues under study. Intensification of B lymphocyte blast transformation in caecum tissues as well as the appearance of blast cells in the blood of infected muskrats was observed.     

Occurrence of Campylobacter jejuni and Giardia species in muskrat (Ondatra zibethica)

A total of 189 muskrat fecal samples were surveyed for Campylobacter and Giardia species. Campylobacter jejuni was recovered from 47.5% of these samples, and Giardia species were detected in 82.5%. These findings indicate that muskrat may be of importance to the health both of humans and of domestic animals.     

Changes in aquatic plant communitieson the island of Valaam due to invasion by the muskrat Ondatra zibethicus L.(Rodentia, Mammalia)

Muskrat invaded Valaam Island (Northern part of European Russia) in the 1970s. Aquatic plant communities of 1962 and 1993 were compared on the same plots. Quantitative changes were tested with the help of jack-knifing estimates of most known inventory (-) diversity indicators. Qualitative transformations were assessed using -diversity values. The results demonstrated substantially more discriminant ability of diversity measures than classical methods of mathematical statistics. All of the -diversity values declined synchronously without exception. Species composition also changed greatly and those species which turn out to be more resistant to muskrat grazing became the main dominant plants. The activity of Ondatra became the over-riding ecological factor connecting the littoral plant communities of the Valaam. It is concluded that the sustainability of this ecosystem was damaged by muskrat's invasion and that the role of muskrat should not be underestimated when studying the ecology of freshwater littoral communities.     

Hematologic and blood chemistry reference values for free-ranging muskrats (Ondatra zibethicus)

Baseline hematologic and serum chemistry values are used by veterinarians and wildlife biologists to identify abnormally high or low levels of particular blood parameters in a target species. This vital information can assist animal care providers in making informed decisions on the care of wildlife and help to determine diagnoses for certain illnesses. Published blood parameter values are not available for wild-caught muskrats (Ondatra zibethicus). We measured 27 blood parameter values from 29 free-ranging, riparian muskrats caught from June-November 2008 in east-central Illinois, USA, and compared mean values between adults and juveniles. Adult muskrats had higher levels of globulins (F(1,27)=6.394, P ≤ 0.018) and eosinophils (F(1,25)=6.883, P ≤ 0.015) than did juvenile muskrats, possibly because of increased exposure to parasites and allergens over time.     

Seasonal changes in the expression of PACAP,VPAC1, VPAC2, PAC1 and testicular activity in the testis of the muskrat (Ondatra zibethicus)

Pituitary adenylate cyclase-activating polypeptide (PACAP) plays an important role in the steroidogenesis and spermatogenesis in the testis through its receptors PAC1, VPAC1 and VPAC2. In this study, we investigated the seasonal expressions of PACAP, PAC1, VPAC1, VPAC2, luteinizing hormone receptor (LHR), follicle stimulating hormone receptor (FSHR), steroidogenic acute regulatory protein (StAR), 3β-hydroxysteroid dehydrogenase (3β-HSD) and CYP17A1 in the testis of male muskrat during the breeding season and non-breeding season, respectively. Histologically, we observed the presence of Leydig cells, Sertoli cells and various types of germ cells in the testis during the breeding season, yet only Leydig cells, Sertoli cells, spermatogonia and primary spermatocyte during the non-breeding season. In addition, the immunohistochemical localizations of PACAP and VPAC1 were identified in the Leydig cells, spermatogonia and spermatozoa during the breeding season, while only in the Leydig cells and spermatogonia during the non-breeding season, and PAC1 and VPAC2 were localized in the Leydig cells in both seasons, among which LHR, StAR, 3β-HSD and CYP17A1 were also expressed. Meanwhile, the protein and mRNA expression levels of PACAP, PAC1, VPAC1, VPAC2, LHR, FSHR, StAR, 3β-HSD and CYP17A1 in the testis during the breeding season were significantly higher than those during the non-breeding season. These results suggested that PACAP is involved in the regulation of steroidogenesis and spermatogenesis via an endocrine, autocrine or paracrine manner in the testis of muskrat.Key words: Pituitary adenylate cyclase-activating peptide (PACAP), PACAP receptors, steroidogenesis, testis, Ondatra zibethicus.