Allometry predicts that the 12–17-g American water shrew (Sorex palustris)—the world’s smallest mammalian diver—will have the highest diving metabolic rate coupled with the lowest total body oxygen storage capacity, skeletal muscle buffering capacity, and glycolytic potential of any endothermic diver. Consistent with expectations, and potentially owing to their low thermal inertia, water shrews had a significantly higher diving metabolic rate in 10°C water (8.77 mL O2 g−1 h−1) compared with 30°C water (6.57 mL O2 g−1 h−1). Unlike larger-bodied divers, muscle myoglobin contributed minimally (7.7%–12.4%) to total onboard O2 stores of juvenile and adult water shrews, respectively, but was offset by high blood O2 carrying capacities (26.4%–26.9% v/v). Diving was predominantly aerobic, as only 1.2%–2.3% of dives in 10°C and 30°C water, respectively, exceeded the calculated aerobic dive limits at these temperatures (10.8–14.4 s). The mean voluntary dive time of water shrews during 20-min trials in 3°C–30°C water was 5.0±0.1 s (N=25, n=1,628), with a mean maximum dive time of 10.1±0.4 s. However, the average dive duration (6.9±0.2 s, n=257) of radio-telemetered shrews exclusively foraging in a simulated riparian environment (3°C water) for 12–28 h suggests that mean (but not maximum) dive times of water shrews in the wild may be longer. Mean dive duration, duration of the longest dive, and total time in water all decreased significantly as water temperature declined, suggesting that shrews employed behavioral thermoregulation to defend against immersion hypothermia. Additionally, free-diving shrews in the 24-h trials consistently elevated core body temperature by ∼1°C immediately before initiating aquatic foraging bouts and ended these bouts when body temperature was still at or above normal resting levels (∼37.8°C). We suggest that this observed predive hyperthermia aids to heighten the impressive somatosensory physiology, and hence foraging efficiency, of this diminutive predator while submerged.

We tested the hypothesis that immersion hypothermia enhances the diving capabilities of adult and juvenile muskrats by reducing rates of oxygen consumption (V O2). Declines in abdominal body temperature (T(b)) comparable to those observed in nature (0.5-3.5 degrees C) were induced by pre-chilling animals in 6 degrees C water. Pre-chilling did not reduce diving V O2 of any animal tested in 10 degrees C or 30 degrees C water, irrespective of the nature of the dive. Most behavioural indices of dive performance, including average and cumulative dive times, were unaffected by T(b) reduction in adults, but depressed in hypothermic juveniles (200-400 g). Hypothermia reduced diving heart rate only on short (<25s) dives (16% reduction, P=0.01), but did not affect the temporal onset of diving bradycardia. Post-immersion V O2 was higher for pre-chilled than for normothermic muskrats, but the difference became insignificant on longer (>90 s) dives. Our findings suggest that the mild hypothermia experienced by muskrats in nature has minimal effect on diving and post-immersion metabolic costs, and thus has little impact on the dive performance of this northern semi-aquatic mammal.

Rates of O2 consumption and CO2 production, telemetered body temperature (Tb) and activity level were recorded from adult and subadult water shrews (Sorex palustris) over an air temperature (Ta) range of 3–32°C. Digesta passage rate trials were conducted before metabolic testing to estimate the minimum fasting time required for water shrews to achieve a postabsorptive state. Of the 228 metabolic trials conducted on 15 water shrews, 146 (64%) were discarded because the criteria for inactivity were not met. Abdominal Tb of S. palustris was independent of Ta and averaged 38.64±0.07°C. The thermoneutral zone extended from 21.2°C to at least 32°C. Our estimate of the basal metabolic rate for resting, postabsorptive water shrews (96.88±2.93 J g−1 h−1 or 4.84±0.14 ml O2 g−1 h−1) was three times the mass-predicted value, while their minimum thermal conductance in air (0.282±0.013 ml O2 g−1 h−1) concurred with allometric predictions. The mean digesta throughput time of water shrews fed mealworms (Tenebrio molitor) or ground meat was 50–55 min. The digestibility coefficients for metabolizable energy (ME) of water shrews fed stickleback minnows (Culaea inconstans) and dragonfly nymphs (Anax spp. and Libellula spp.) were 85.4±1.3% and 82.8±1.1%, respectively. The average metabolic rate (AMR) calculated from the gas exchange of six water shrews at 19–22°C (208.0±17.0 J g−1 h−1) was nearly identical to the estimate of energy intake (202.9±12.9 J g−1 h−1) measured for these same animals during digestibility trials (20°C). Based on 24-h activity trials and our derived ME coefficients, the minimum daily energy requirement of an adult (14.4 g) water shrew at Ta = 20°C is 54.0 kJ, or the energetic equivalent of 14.7 stickleback minnows.

The nature and potential thermoregulatory benefits of the heat increment of feeding (HIF) were investigated in short-tailed shrews (Blarina brevicauda). At thermoneutrality, the postprandial rate of oxygen consumption ([Formula: see text]O2) of shrews increased by an average of 18% beyond fasting levels for ca. 2 h following the consumption of 3.5 g of earthworms. Over the same period, body temperature increased by an average of 0.6 °C. The digesta-retention time calculated from nickel alloy tracer excretion rates (168.1 ± 11.4 min (mean ± SE); n = 7) exceeded the duration of HIF (117.5 ± 10.4 min; n = 6) by 43%. This finding suggests that the mechanical costs of feeding may be a relatively mi nor component of HIF in this species. Regression of resting [Formula: see text]O2on ambient temperature (Ta) below thermo neutrality yielded similar slopes (P = 0.71) and intercepts (P = 0.33) for fed and fasted animals, suggesting that HIF substitutes, at least partially, for facultative thermogenesis at low Ta. We found no evidence that HIF enhanced microclimate warming of an insulated, open-flow metabolic chamber occupied by recently fed shrews. Occupancy of this chamber by shrews increased microclimate Tafrom 5 to 9.0–9.5 °C regardless of their nutritional status.

SUMMARY We tested the hypothesis that the body oxygen stores, aerobic dive limit(ADL) and dive performance of muskrats can be enhanced by dive-conditioning in a laboratory setting. We compared several key variables in 12 muskrats trained to swim a 16 m underwater course to a feeding station (`divers') with those of 12 animals precluded from diving but required to travel identical distances in water to feed (`surface swimmers'). Acclimated muskrats assigned to each group were trained concurrently over a 9–11 week period. We observed significant gains in the haematocrit (P=0.0005) and blood haemoglobin concentration (P=0.015) of `divers', but not `surface swimmers'. The post-training blood O2 store calculated for `divers' (22.9 ml O2 kg–1) was nearly 26% higher than that (18.2 ml O2 kg–1) derived for `surface swimmers'(P=0.03). Dive-conditioning had no apparent effect on lung volume,whole blood and plasma volumes, nor on the glycogen level and buffering capacity of skeletal muscles. Cardiac and skeletal muscle myoglobin levels were also similar in both test groups following training. The mean total body oxygen store of `divers' (37.8ml O2 STPD kg–1) was 13.5% higher (P=0.037) than for `surface swimmers' (33.3 ml O2 STPD kg–1), an increase attributed entirely to the gain in blood O2 storage capacity of the former group. However,owing to a slightly higher estimate of diving metabolic rate in dive-conditioned animals, the calculated ADL for this group (61.3 s) was indistinguishable from that of `surface swimmers' (61.8 s). Few differences were observed in the post-training dive behaviour of `surface swimmers' and`divers', a finding consistent with the strong similarity in their calculated aerobic dive capacities.

The dive performance, oxygen storage capacity and partitioning of body oxygen reserves of one of the world's smallest mammalian divers, the star-nosed mole Condylura cristata, were investigated. On the basis of 722 voluntary dives recorded from 18 captive star-nosed moles, the mean dive duration (9.2+/-0.2 s; mean +/- S.E.M.) and maximum recorded dive time (47 s) of this insectivore were comparable with those of several substantially larger semi-aquatic endotherms. Total body O(2) stores of adult star-nosed moles (34.0 ml kg(-1)) were 16.4 % higher than for similarly sized, strictly fossorial coast moles Scapanus orarius (29.2 ml kg(-1)), with the greatest differences observed in lung and muscle O(2) storage capacity. The mean lung volume of C. cristata (8.09 ml 100 g(-1)) was 1.81 times the predicted allometric value and exceeded that of coast moles by 65.4 % (P=0.0001). The overall mean myoglobin (Mb) concentration of skeletal muscles of adult star-nosed moles (13.57+/-0.40 mg g(-1) wet tissue, N=7) was 19.5 % higher than for coast moles (11.36+/-0.34 mg g(-1) wet tissue, N=10; P=0.0008) and 54.2 % higher than for American shrew-moles Neurotrichus gibbsii (8.8 mg g(-1) wet tissue; N=2). The mean skeletal muscle Mb content of adult star-nosed moles was 91.1 % higher than for juveniles of this species (P<0.0001). On the basis of an average diving metabolic rate of 5.38+/-0.35 ml O(2) g(-1) h(-1) (N=11), the calculated aerobic dive limit (ADL) of star-nosed moles was 22.8 s for adults and 20.7 s for juveniles. Only 2.9 % of voluntary dives by adult and juvenile star-nosed moles exceeded their respective calculated ADLs, suggesting that star-nosed moles rarely exploit anaerobic metabolism while diving, a conclusion supported by the low buffering capacity of their skeletal muscles. We suggest that a high mass-specific O(2) storage capacity and relatively low metabolic cost of submergence are key contributors to the impressive dive performance of these diminutive insectivores.

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 : 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.

Metabolic and body temperature (Tb) responses of star-nosed moles (Condylura cristata) exposed to air temperatures ranging from 0 to 33°C were investigated. The thermoneutral zone of this semi-aquatic mole extended from 24.5 to 33°C, over which its basal rate of metabolism averaged 2.25 ml O2 g−1 h−1 (45.16 J g−1 h−1). This rate of metabolism is higher than predicted for terrestrial forms, and substantially higher than for other moles examined to date. Minimum thermal conductance was nearly identical to that predicted for similar-sized eutherians and may represent a compromise between the need to dissipate heat while digging and foraging in subterranean burrows, and the need to conserve heat and avoid hypothermia during exposure to cold. C. cristata precisely regulated Tb (mean ±SE=37.7±0.05°C) over the entire range of test temperatures. Over three separate 24-h periods, Tb of a radio-implanted mole varied from 36.6 to 38.8°C, and generally tracked level of activity. No obvious circadian variation in Tb and activity was apparent, although cyclic 2–4 h intervals of activity punctuated by periods of inactivity lasting 3–5 h were routinely observed. We suggest that the elevated basal metabolic rate and relatively high Tb of star-nosed moles may reflect the semi-aquatic habits of this unique talpid.

Developmental changes in thermoregulatory ability were followed in 1- to 34-d-old muskrats (Ondatra zibethicus) tested individually and in groups composed of 5 littermates. During their first 10-11 d of life, 20- to 60-g nestlings displayed limited thermogenic ability and could not maintain a stable body temperatures (Tb) during 2 h exposure to an air temperature (Ta) of 10 or 25°C. At 25°C, a Ta approximating nest temperatures in the field, nestlings were homeothermic at ca. 10-11 d of age, when they were fully furred, capable of limited swimming, and within 1-2 d of opening their eyes. The onset of thermoregulation occurred at a body mass (60 g) that was considerably less than predicted on the basis of adult body size. Huddling with littermates reduced the cooling of 20- to 60-g young at a Ta of 10°C, but did not advance the age (mass) at which they became effective homeotherms. Huddling by 20- to 60-g nestlings raised the mean Tb, leading to a substantive (64%) gain in the rate of oxygen consumption (Vo2). Conversely, in older nestlings that were close to or fully homeothermic, huddling with littermates consistently lowered Vo2, the greatest metabolic saving (38%) being realized by 17- to 21-d-old (100-140 g) young. We suggest that the rapid development of thermoregulation may be linked to the semiaquatic habit of muskrats, and that thermoregulatory behavior contributes importantly to the growth and morphological development of this prolific microtine rodent.

ABSTRACT The postprandial increase in metabolic rate associated with consuming, assimilating and excreting a meal is often termed the heat increment of feeding (HIF). The metabolic heat production of star-nosed moles, Condylura cristata, held at thermoneutrality was monitored for 4 h following a single 10 min session of feeding on a ration consisting of 0 g (controls), 3.5 g or 10 g of earthworms. Coefficients for metabolizable energy digestibility and digesta passage rate of earthworms fed to C. cristata were also determined. We then tested whether feeding-induced thermogenesis substitutes partially or completely for thermoregulatory heat production in these animals exposed to sub-thermoneutral air temperatures (9–24 °C). A single feeding on earthworms had both short- and long-term effects on the metabolic rate and respiratory exchange ratio of C. cristata. The observed short-term (0–65 min) rise in metabolic rate, assumed to be associated primarily with the physical costs of nutrient digestion, absorption and excretion, was similar to the calculated mean retention time (66.7±7.8 min; mean ± S.E.M., N=5) of this species. This component of the HIF represented 2.9 % of the food energy ingested by moles fed a single 3.5 g (13.21 kJ) meal of earthworms and 1.4 % of the food energy ingested by moles fed a single 7.5 g (28.09 kJ) meal of earthworms. At all test temperatures, resting metabolic rate typically remained above fasting levels for 1–4 h following ingestion of the high-protein earthworm diet. This protracted rise in metabolic rate, presumably associated with the biochemical costs of amino acid oxidation/gluconeogenesis and ureagenesis, averaged 12.8 % of the metabolizable energy and 8.7 % of the gross energy intake. Despite the potential thermoregulatory benefit, we found no evidence that biochemical HIF substitutes for facultative thermogenesis in star-nosed moles exposed to low air temperatures.

Basal metabolic rate (BMR), serum thyroxine (T4) concentration, lean organ mass, and body composition were measured in 94 captive, seasonally acclimatized muskrats (Ondatra zibethicus) between May 1991 and April 1992. Seasonal measurements of oxygen consumption, body water content, and mass were obtained from an additional 124 captive or free-ranging animals in 1994-1995. Mass-independent BMRs (kJ · kg-0.67· h-1) and serum T4concentrations (nmol · L-1) varied significantly over the year (P << 0.0001), with mean values in February exceeding July values by 31.1 and 77.2%, respectively. These variables tracked seasonal changes in the neutral detergent soluble (NDS) content of broadleaf cattails (Typha latifolia), the dominant food of muskrats in the study population. From July through February, alimentary tract, liver, spleen, and heart masses increased, while kidney mass declined. Body fat stores varied significantly over both years, with peak values measured in February. However, lean body and pelt masses exhibited little seasonal variation (P >> 0.05). Stepwise multiple regression and principal component analyses suggested that variation in BMR was associated most closely with changes in heart and alimentary tract masses. Annual variations in basal energy expenditure, serum T4concentration, and organ masses of wild muskrats appear to be linked to seasonal changes in forage NDS content and energy intake, and may be important factors relating to the annual pattern of fat accretion and mobilization in this semiaquatic rodent.

Knowledge of the seasonal energy and forage requirements of free-ranging muskrats (Ondatra zibethicus) is essential for evaluating the habitat requirements and potential effect of this species on aquatic vegetation. We measured rates of water influx with the deuterated water (D2O) technique and assessed the accuracy of this method for estimating dry-matter intake (DMI) of captive muskrats fed a natural diet of cattail (Typha latifolia) rhizomes. Water influx in laboratory feeding trials was highly variable, ranging from 97 to 430 mL.kg(-1).day(-1) ((x) over bar = 243.5 +/- 24.0). Over this range, DMI estimated from water influx exceeded measured DMI by an average of 52.2%. This error was reduced substantially as water influx increased, with a mean error of only 9.2% when water influx averaged 349 mL.kg(-1).day(-1) (DMI > 40 g.kg(-1).day(-1)). At rates of water influx obtained under field conditions (423-915 mL.kg(-1).day(-1)). the error in estimating DMI is predicted to be <10%. We obtained 33 seasonal estimates of the daily intake of water, fresh vegetation, dry matter (DM), and assimilated energy (AE) of 27 free-ranging muskrats. Water influx and consumption of fresh vegetation were highest from spring through fall. However, because of the lower water content and higher digestibility of the winter diet, daily intake of DM and AE were higher (P < 0.05) in winter (76.9 g/kg(0.75), 713.1 kJ/kg(0.75)) than midsummer (54.9 g/kg(0.75), 438.6 kJ/kg(0.75)). If corrections are made far wastage and use of vegetation for house construction, these food consumption estimates can be used to assess the potential effect of muskrats on the primary productivity of prairie marsh ecosystems.

The rate of 14C-urea hydrolysis was determined in 32 field-acclimatized muskrats maintained on natural diets during spring, summer, fall, and winter. We hypothesized that urea recycling occurs in muskrats during all seasons and that the conservation of tissue nitrogen via this mechanism is most prevalent in fall and winter, when forage protein levels are lowest. Muskrats exhibited higher rates of urea hydrolysis and a lower serum urea nitrogen-to-creatinine ratio in fall and winter than in spring and summer. Even after correcting for seasonal differences in blood urea pool size, the adjusted rate of urea hydrolysis was 67% higher in fall and winter than in spring and summer. There was no evidence that the maintenance nitrogen requirements of muskrats fed natural vegetation were affected by seasonal changes in the amino acid composition of the diet. We suggest that increased levels of urea recycling, coupled with adaptive mechanisms for reducing nitrogen excretion and possibly conserving carbon skeletons of essential amino acids, may allow muskrats to reduce their nitrogen requirements on fall and winter diets. Our finding that 14C-urea hydrolysis occurred during all four sampling periods suggests that nitrogen derived from this source may also be critical to supporting large hindgut microbe populations that enable this rodent to exploit the appreciable fiber content of its aquatic plant diet throughout the year.

The potential benefits of social aggregation and communal grooming to the foraging ecology of muskrats were investigated. We hypothesized that muskrats in groups would be more resistant to hypothermia, would spend more time foraging in cold water, and would rewarm more quickly following emergence than solitary muskrats. Despite social grooming and aggregation behavior following episodes of aquatic activity, the post-immersion rewarming rates of muskrats increased only slightly in the presence of nestmates. The maximum rate of rewarming averaged 0.26 degrees C/min when each animal was tested alone, and 0.30 degrees C/min when it was tested in the presence of four nest-mates (P = 0.096). Providing muskrats with the option to huddle between foraging and exploratory dives in cold water had little impact on the precision of control of body temperature. Although muskrats in groups tended to spend more time in water and expend less energy than solitary animals, these trends were not significant. The greatest-differences were in intake of food, with grouped animals in one experiment ingesting nearly three times as much forage as animals feeding alone (P = 0.015). We attribute the higher intake of food of grouped animals to social facilitation rather than to any gain in foraging efficiency arising from aggregation behavior out of water.

We examined the potential nutritional benefits derived by muskrats ( Ondatra zibethicus ) that supplement their diet of aquatic plants with animal tissue. Digestibility, energy, and nitrogen-balance trials were conducted with adult muskrats fed each of three diets: 100% shoots of cattails ( Typha latifolia ); 95% shoots of cattails, 2.5% fathead minnows ( Pime-phales promelas ), and 2.5% flesh of muskrats; 85% shoots of cattails, 10% fathead minnows, and 5% flesh of muskrats. Muskrats presented with diets containing meat selectively increased the proportion of animal tissue ingested above that offered in the ration ( P 50% intake of dry matter) with no apparent loss of ability to digest fiber. They further suggest that free-ranging muskrats consuming diets containing only 2–3.5% animal tissue can meet their maintenance requirements of daily nitrogen (1.02 g N/kg0.75 body mass) solely from meat. We conclude that consumption of meat, even at low levels, can be nutritionally beneficial to muskrats in nature.

The aim of this study was to determine whether seasonal variability in diet quality or cold stress is accompanied by compensatory changes in nutrient selection, energy intake, and digestive capacity of seasonally acclimatized muskrats. We hypothesized that in summer, muskrats meet their energy and nutrient requirements by selectively consuming high-protein, low-fiber aquatic plants. We also predicted that muskrats use fiber as an important energy source during those periods of the year when the nutritional value and diversity of forage species are lowest. At such times, muskrats should be most dependent on microbial fermentation and should exhibit maximal gut size and digestive efficiency. As predicted, muskrats offered naturalforage increased the fraction of protein while reducing the proportion of fiber in their diet during summer, but not during spring or winter digestibility trials. From July to December, muskrats exhibited increases in dry matter intake, gut mass, and forage digestibility. The increase in hindgut mass was accompanied by an 18.5% rise in neutral detergent fiber digestibility, while the proportion of digestible energy derived from the fermentation of fiber increased from 38.4% in July to 53.2% in December. During winter, muskrats were able to reduce their dietary nitrogen requirements by 26.0%. Our results suggest that changes in the absorptive surface area and volume of the gut are important adaptations for promoting nutrient assimilation during periods when muskrats are challenged by both high maintenance costs and a limited choice of diets.

In his critique of our recent paper (A.P. Dyck and R.A. MacArthur. 1992. Can. J. Zool. 70: 1668–1672), Bovet identifies a potential analytical problem in our treatment of the temporal patterns in activity and body temperature of free-ranging beavers in winter. We show that, while a legitimate concern, this problem reflects a calculated trade-off in the sampling methods employed, but did not prevent us from meeting our stated primary objectives. We further demonstrate, with additional analyses, that this source of error in no way undermined or invalidated our essential conclusion that beavers in this study failed to exhibit seasonal hypothermia.

Knowledge of the forage intake and digestive efficiencies of muskrat (Ondatra zibethicus) is essential for developing an understanding of their habitat requirements and their impact on emergent vegetation. We performed 30 complete digestibility, energy-, and nitrogen-balance trials on 6 adult male muskrat fed 5 diets: (1) sedge (Carex atherodes) shoot, (2) softstem bulrush (Scirpus validus) shoot, (3) hybrid cattail (Typha x glauca) shoot, (4) cattail rhizome, and (5) a combination of cattail shoot and rhizome. Dry matter (DM) digestibilities ranged from 61.2 to 70.6%. Neutral detergent fiber (NDF) digestibilities varied from 40.0 to 59.6% for these emergent plant diets with NDF levels ranging from 44.6 to 62.1%. Microbial fermentation of fiber accounted for 39.4% of digestible energy (DE) intake. Muskrat can digest fiber as well as can many ruminants and pseudoruminants, but can do so more efficiently than other rodents. Apparent digestibility of dietary crude protein (DCP) was highest (P < 0.001) for sedge (73.6%) and lowest (P = 0.001) for the cattail rhizome diet (7.2%). However, the daily nitrogen intake (DNI) required by muskrat to maintain tissue balance on a cattail rhizome diet (0.599 g N/kg0.75/day) was less than half the daily intake required for all other diets combined (1.266 g N/kg0.75/day) (P < 0.001). This implies the existence of a protein conservation mechanism by which muskrat could negate the effects of low dietary crude protein during winter.

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.