Body cooling and the diving capabilities of muskrats (Ondatra zibethicus): a test of the adaptive hypothermia hypothesis.

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.

Bioenergetics and thermal physiology of American water shrews (Sorex palustris).

Rates of O(2) consumption and CO(2) production, telemetered body temperature (T(b)) and activity level were recorded from adult and subadult water shrews (Sorex palustris) over an air temperature (T(a)) range of 3-32 degrees 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 T(b) of S. palustris was independent of T(a) and averaged 38.64 +/- 0.07 degrees C. The thermoneutral zone extended from 21.2 degrees C to at least 32 degrees 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 O(2) g(-1) h(-1)) was three times the mass-predicted value, while their minimum thermal conductance in air (0.282 +/- 0.013 ml O(2) 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 degrees 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 degrees 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 T(a) = 20 degrees C is 54.0 kJ, or the energetic equivalent of 14.7 stickleback minnows.

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.

Postprandial heat increment does not substitute for active thermogenesis in cold-challenged star-nosed moles (Condylura cristata).

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 degrees 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.

Fasting metabolism and thermoregulatory competence of the star-nosed mole, Condylura cristata (Talpidae: Condylurinae).

Metabolic and body temperature (Tb) responses of star-nosed moles (Condylura cristata) exposed to air temperatures ranging from 0 to 33 degrees C were investigated. The thermoneutral zone of this semi-aquatic mole extended from 24.5 to 33 degrees 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 degrees 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 degrees 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.

Urea recycling in muskrats (Ondatra zibethicus): a potential nitrogen-conserving tactic?

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.

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 degrees 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.

Effects of CO2 inhalation on acid-base balance and thermal recovery following cold water dives by the muskrat (Ondatra zibethicus).

Thermoregulatory, cardiac, and blood gas responses of muskrats to CO2 inhalation were examined during recovery from controlled dives in 3 degrees C water. It was shown that CO2 levels previously recorded in the winter lodges of this species are sufficient to reduce postdive oxygen consumption and rate of rewarming in unrestrained animals. Postdive exposure to 5-10% CO2 reduced recovery of abdominal and subcutaneous temperature (Tb) in the middorsal lumbar region, but did not affect interscapular warming overlying brown adipose tissue. Depressed recovery of deep Tb was matched by a prolonged acidosis in restrained muskrats breathing 5-10% CO2 following 90-s submergence in 3 degrees C water. This occurred despite a comparatively high whole body buffer value (delta H+/delta PaCO2) of 0.53 nM X l-1 X mmHg-1. Protracted recovery of arterial pH did not appear to involve retention of CO2 accumulated during diving, since in all trials, PaCO2 returned to the predive baseline within 5 min of surfacing. Perturbations in PaO2, like PaCO2, were restored within 5 min of surfacing, irrespective of inspired gas mixture. Predive exposure to 5-10% CO2 had no discernible effect on establishment of bradycardia in diving muskrats. Compared to predive values, postimmersion heart rate was significantly reduced in 5-10% CO2, but not in control runs.

A biotelemetry system for monitoring heart rates in unrestrained ungulates.

An EKG telemetry system has been designed for use on free-living ungulates. An FM transmitter with a battery life of 2 +/-0.5 months and a range of 2-6 km is mounted on the back of the animal by means of an adjustable harness. The EKG signal is detected by subdermal electrodes overlying the sternum and transmitted to an FM receiver equipped with a frequency down-converter. The output of the receiver is the modulated audio carrier which is recorded on magnetic tape. The EKG waveform is recovered from the tape-recorded signal using an FM demodulator connected to a strip chart recorder.