Body oxygen stores, aerobic dive limits, and the diving abilities of juvenile and adult muskrats (Ondatra zibethicus)

Intraspecific variability in body oxygen reserves, muscle buffering capacity, diving metabolic rate, and diving behavior were examined in recently captured juvenile and adult muskrats. Allometric scaling exponents for lung (b=1.04), blood (b=0.91), and total body oxygen storage capacity (b=1.09) did not differ from unity. The concentration of skeletal muscle myoglobin scaled positively with mass in 254-600-g juveniles (b=1.63) but was mass-independent in larger individuals. Scaling exponents for diving metabolic rate and calculated aerobic dive limit (ADL) were 0.74 and 0.37, respectively. Contrary to allometric predictions, we found no evidence that the diving abilities of muskrats increased with age or body size. Juveniles aged 1-2 mo exhibited similar dive times but dove more frequently than summer-caught adults. Average and cumulative dive times and dive&rcolon;surface ratios were highest for fall- and winter-caught muskrats. Total body oxygen reserves were greatest in winter, mainly due to an increase in blood oxygen storage capacity. The buffering capacity of the hind limb swimming muscles also was highest in winter-caught animals. Several behavioral indicators of dive performance, including average and maximum duration of voluntary dives, varied positively with blood hemoglobin and muscle myoglobin concentration of muskrats. However, none of the behavioral measures were strongly correlated with the total body oxygen reserves or ADLs derived for these same individuals.     

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.     

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.     

Use of DNA fingerprinting to determine parentage in muskrats (Ondatra zibethicus).

The detection of high levels of genetic variability by DNA fingerprinting probes has allowed researchers to accurately assess relatedness. Multiple-mating strategies are characteristic of the mating systems of small mammals. As such, techniques that provide an accurate indication of how individuals are related genetically is of great importance to assess the mating system of a species. In this study, we applied the DNA fingerprinting technique to captive and wild muskrats (Ondatra zibethicus) to determine its usefulness for parentage analysis in wild populations. We found that DNA digested with the restriction enzyme Hae III and probed with Jeffrey's minisatellite 33. 15 identified a large amount of polymorphism in both groups of muskrats. The DNA fingerprinting technique correctly assessed parentage within the captive group. In the wild population, paternity was assigned between two adult males based on diagnostic fragments and similarity of banding patterns. The likelihood that paternity could be misassigned to a full sibling was high in this free-ranging population. However, because natal dispersal in muskrats is male biased, it is unlikely that two brothers would associate with the same female.     

Immunohistochemical evidence: testicular and scented glandular androgen synthesis in muskrats (Ondatra zibethicus) during the breeding season

In order to elucidate the relationship between androgens and the function of the muskrat (Ondatra zibethicus) scented glands during the breeding season, we investigated immunolocalization of steroidogenic enzymes P450scc, 3βHSD and P450c17 in the muskrat testes and scented glands. Nine adult muskrats were obtained in March (n=3), May (n=3) and July (n=3) 2010. Steroidogenic enzymes were immunolocalized using polyclonal antisera raised against bovine adrenal P450scc, human placental 3βHSD and porcine testicular P450c17. Histologically, all types of spermatogenic cells including mature-phase spermatozoa in seminiferous tubules were observed in all testes. Glandular cells, interstitial cells, epithelial cells and excretory tubules were identified in scented glands during the breeding season. P450scc, 3βHSD and P450c17 were only identified in Leydig cells during the breeding season; P450scc and P450c17 were observed in glandular cells of scented glands, however, 3βHSD was not found in scented glands during the breeding season. These novel findings provide the first evidence showing that scented glands of the muskrats are capable of locally synthesizing androgens and androgens acting via an endocrine, autocrine or paracrine manner may play an important role in scented gland function during the breeding season.     

Presence of Giardia spp. and absence of Salmonella spp. in New Jersey muskrats (Ondatra zibethicus)

Of 220 muskrat fecal specimens collected from 12 sites in southwestern New Jersey, 154 (70%) were found to contain cysts of the protozoan parasite Giardia spp. Cysts from selected muskrat fecal specimens infected Mongolian gerbils, but attempts to cultivate trophozoites removed from these gerbils were unsuccessful. Salmonella spp. were not detected in any of the muskrat fecal specimens.     

Presence of Giardia spp. and absence of Salmonella spp. in New Jersey muskrats (Ondatra zibethicus).

Of 220 muskrat fecal specimens collected from 12 sites in southwestern New Jersey, 154 (70%) were found to contain cysts of the protozoan parasite Giardia spp. Cysts from selected muskrat fecal specimens infected Mongolian gerbils, but attempts to cultivate trophozoites removed from these gerbils were unsuccessful. Salmonella spp. were not detected in any of the muskrat fecal specimens.     

Palaeodemographics of small‐bodied mammals in Pleistocene environments: a case study of muskrats (Ondatra zibethicus: Rodentia: Muridae) from north Florida

The effects of environmental change on fecundity and mortality rates of ancient populations are likely to have influenced extinction patterns and biogeographical range shifts. To test for a relationship between environmental change and palaeodemographical change, the mortality profiles of late Pleistocene muskrats (Ondatra zibethicus) from north Florida were compared with recent populations of the same species to assess the effect of the Pleistocene–Holocene transition on the demographics of this species, as well as a potential role in the extirpation of O. zibethicus in Florida during the Pleistocene–Holocene transition. At the older locality (Latvis‐Simpson: approximately 32 ¹⁴C Ky BP), there is strong sedimentological evidence for late summer or autumn seasonal deposition. Most of the individuals in the youngest cohort were adults approximately 7 or 8 months old at death, suggesting that the breeding season had occurred in the fall or winter. This breeding schedule is similar to recent southern populations where breeding is most intense in the fall and winter, and unlike northern populations where breeding occurs in the spring. The inferred breeding seasonality is consistent with other evidence suggesting that the south‐east was warm and equable in the late Pleistocene. At the younger locality (Sloth Hole: approximately 12 ¹⁴C Ky BP) muskrats exhibit faster dental wear and lower life expectancy, suggesting harsher conditions near the time of extirpation. Cooler temperatures, aridity, water table fluctuation, and human presence all comprise potential factors leading to a lower life expectancy during this time interval. The fossil record shows a potential for investigating links between climate change and the demographics of palaeopopulations. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 41–56.     

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.     

Habitat suitability in muskrats: a test of the food limitation hypothesis

Data on habitat selection, overwinter survival, and litter survival rate of muskrats indicate that fitness differs among habitats in marsh environments. We measured nine nutrient-dependent phenotypic variables of muskrats and one independent assessment of forage quality during the ice-free period (May-October) from 1992 to 1994. Data were used to test the hypothesis that differences in food resources limit the demographic performance of muskrats in marginal versus prime habitats (food limitation hypothesis, FLH). None of the null predictions relating habitat quality with reproductive parameters could be rejected statistically. Adult body mass and length were greater in prime habitats, but fat content did not differ, which suggested that the difference was associated with larger structural size of dominant animals in prime habitats. Growth rate of weaned juveniles was not different among habitats, which represented the strongest evidence for rejecting the FLH. Faecal crude protein content, which was used as an index of quality of food ingested, was marginally different between the two most suitable habitats, but the direction of the difference did not support our prediction. Overall, the temporal variation in faecal crude protein content was similar among habitats. We conclude that food is not a key factor limiting the demographic performance of muskrats among habitats during the ice-free season.     

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.     

Muskrat (Ondatra zibethicus) decline after the expansion of American mink (Neovison vison) in Poland

Field survey data in Central Poland revealed that the proportion of sites inhabited by muskrats decreased from 44% to 7% over one decade. This corresponded to the decline in hunting bags of muskrat over the whole of Poland. The largest hunting harvest of muskrat was recorded in 1987/1988 (66,416 individuals), the smallest in 2007/2008 (4,567 individuals). The decline in hunting bags occurred in all regions analysed; however, it was most rapid in the north and north-east. Before the expansion of mink, which started in northern Poland at the beginning of the 1980s, muskrat densities in particular regions depended on the availability of aquatic habitats. A comparison of hunting bags of muskrat and American mink in years 2002–2008 indicated a significant negative correlation between the numbers of these two species harvested in seven regions of Poland. The negative correlation between numbers of muskrat and mink suggests that mink predation is one of the most important factors in the decline of the muskrat population in Poland.     

Immunolocalization of androgen receptor, aromatase cytochrome P450, estrogen receptor alpha and estrogen receptor beta proteins during the breeding season in scent glands of muskrats (Ondatra zibethicus)

Aromatase cytochrome P450 (P450arom) is an enzyme that catalyzes the conversion of androgen to estrogen. Expression of P450arom in extra-gonadal sites and locally-synthesized estrogen play an important role in physiological conditions. The purpose of this study was to investigate the cellular immunolocalization of androgen receptor (AR), P450arom, estrogen receptor alpha (ERa) and estrogen receptor beta (ERβ) in muskrat scent glands during the breeding season. Histological observation and immunohistochemistry of AR, P450arom, ERa and ERβ were performed in the muskrat scent glands. In addition, total proteins were extracted from scent glandular tissues in the breeding season and were used for Western blotting analysis for AR, P450arom, ERα and ERβ. Histologically, glandular cells, interstitial cells, epithelial cells of the excretory duct and the excretory tubules were identified in the muskrat scent glands during the breeding season. AR was only observed in glandular cells of scent glands; P450arom was expressed in glandular cells and epithelial cells of the excretory duct; ERα was found in glandular cells, interstitial cells and epithelial cells of the excretory duct, whereas ERβ was present in glandular cells and epithelial cells of the excretory duct. Also, the positive signals of AR, P450arom, ERα and ERβ by Western blotting were all observed in scent glandular tissues. These results suggested that the scent gland is the target organ of androgens and estrogens, and that estrogens may play an important autocrine or paracrine role in glandular function of the muskrats.     

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     

Genetic structure of muskrat (Ondatra zibethicus) and its concordance with taxonomy in North America

Extrinsic factors such as physical barriers play an important role in shaping population genetic structure. A reduction in gene flow leading to population structuring may ultimately lead to population divergence. These divergent populations are often considered subspecies. Because genetic differentiation may represent differences between subspecies, patterns of genetic structure should reflect subspecies groupings. In this study, we examine the contemporary population genetic structure of muskrat (n = 331) and assess the relevance of 4 geographically distinct subspecies designations across northern North America using 9 microsatellite loci. We predicted that patterns of gene flow and genetic structure would reflect the described subspecies. We found evidence of genetic differentiation between western and eastern regions, and muskrats from Newfoundland (NF) showed significantly lower genetic diversity than central regions. A strong isolation by distance pattern was also detected within the eastern cluster. Our results did not differentiate Ondatra zibethicus spatulus (northwest) from O. z. albus (central), but they suggest a distinction between O. z. obscurus (NF) and O. z. zibethicus (east). This study highlights the need for more phylogenetic studies in order to better understand intraspecific divergence and the genetic characterization of subspecies.     

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.     

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.