Contributions of the kidneys and intestines to water conservation,and plasma levels of antidiuretic hormone,during dehydration in house sparrows (Passer domesticus)

Summary The contributions of the kidneys, the small intestine and the lower intestine (rectum plus cloaca) to water conservation during dehydration in unanaesthetized, unrestrained house sparrows (Passer domesticus) were assessed. Thirty hours of acute dehydration resulted in a 12% loss in body mass and a significant increase in plasma osmolality. Glomerular filtration rate declined by 55%, from 7.7 to 3.5 ml/h, and urine flow rate delined by more than 80%, from 0.2 to 0.03 ml/h. These changes are likely attributable to a large increase in plasma levels of arginine vasotocin during dehydration, from <26 pg/ml in hydrated birds to greater than 200 pg/ml after 30 h dehydration. Flow of water from the ileum to the lower intestine was reduced during dehydration, primarily because of a reduced flow of dry matter (with no significant reduction in water content). The rate of water loss in the excreta declined from 0.2 ml/h in hydrated birds to 0.04 ml/h in dehydrated birds. The rate of water reabsorption in the lower intestine (equal to the rate of water loss in the excreta minus the combined rates of inflow into the lower intestine from the urine and the ileal contents) slightly exceeded the rate of water flow from the ileum in both hydrated and dehydrated birds. We suggest that much of the water reabsorbed in the lower intestine of hydrated birds derives from the urine, but that primarily water from ileal contents is reabsorbed in dehydrated birds. That is, urine undergoes significant post-renal modification in hydrated but not dehydrated house sparrows.     

Effects of dehydration,rehydration, and hyperhydration in the lactating and non-lactating black Moroccan goat

The effects of water deprivation, rehydration and hyperhydration were investigated in the black Moroccan goat (Capra hircus). Mean daily water intake was 46 ± 5 ml/kg in lactating and 36 ± 4 ml/kg in non-lactating black Moroccan goats, and milk production 21 ± 1 ml/kg. Mean urine excretion was 8 ± 2 ml/kg body weight in both groups, and the daily water losses via evaporation and feces were estimated at 23 ± 3 ml/kg during lactation and 28 ± 4 ml/kg during non-lactation. Forty-eight hours of water deprivation caused a body weight loss of 9% and 6% in lactating and non-lactating goats, respectively, and a drop of 28% in milk production with only a slight decrease in food intake. After rehydration, the elevated plasma osmolality as well as Na and total protein concentrations returned to basal values within 2–3 hr, indicating a rapid absorption of the ingested water, but urine excretion did not increase. After hyperhydration (10% of body weight), 46% of the load was excreted by the kidneys within 6 hr. In conclusion, black Moroccan goats have a low water turnover, and they can retain water upon rehydration but not store excess water after hyperhydration.     

Digestive parameters and water turnover of the leopard tortoise

Leopard tortoises (Stigmochelys pardalis) experience wide fluctuations in environmental conditions and unpredictable availability of food and water within the Nama-Karoo biome. It was hypothesised that tortoises fed two diets differing in preformed water and fibre content would have differing food intake, gut transit rate, assimilation efficiency, faecal and urinary water loss, and urine concentrations. It was predicted that tortoises fed these contrasting diets would attempt to maintain energy and water balance by altering their digestive parameters. Leopard tortoises fed lucerne (Medicago sativa) had a low food intake coupled with long gut transit times, which resulted in the lowest amount of faecal energy and faecal water lost. Tortoises fed tomatoes (Solanum lycopersicum) had higher food intake and faster gut transit times, but more energy and water was lost in the faeces. However, daily energy assimilated and assimilation efficiency were comparable between tortoises fed the two diets. Urine osmolality was significantly different between tortoises on the two diets. Results indicate that leopard tortoises can adjust parameters such as transit rate, food intake, water loss and urine osmolality to maintain body mass, water and energy balance in response to a high fibre, low water content and a low fibre, high water content diet. This study suggests that this digestive flexibility allows leopard tortoises in the wild to take advantage of unpredictable food and water resources.     

Kidney concentrating ability of a subterranean xeric rodent,the naked mole-rat (Heterocephalus glaber)

The naked mole-rat (Heterocephalus glaber) is a strictly subterranean mammal inhabiting the arid zones of north-east Africa. These animals have no access to free water and water balance thus might be facilitated by regulating renal water loss. The urinary concentrating ability of the naked mole-rat was determined using five dietary manipulations in which both water and salt content were altered. Control animals (n=7) received a high quality protein cereal mixed to a thin paste with water (1 g cereal: 85 g water). Water stress was induced by reducing the water content of the diet by either 50% (n=7) or 65% (n=7). Salt loading was facilitated by replacing the water with the same volume of either 0.9% salt (n=7) or 3.0% salt (n=4) solutions. Changes in body mass, food consumption and urine volume were measured daily. The effect of diet on osmolality and electrolyte concentrations of urine and plasma were determined on termination of the diet trials. Although energy intake was not reduced, naked mole-rats lost body weight with both water stress treatments. Urine volume voided per day decreased significantly with both water stress treatments (P<0.05), such that the most extreme water stress led to an 80% reduction in urine volume. Mildly salt-loaded animals gained weight, yet underwent a sodium diuresis, as indicated by a 1.3-fold increase in the daily volume of urine voided (P<0.05). Maximum urine concentration (1521±250 mmol·kg^-1) was achieved with mild water stress and was 4.6±0.9 times that of plasma. Neither further water stress nor salt loading further increased urine osmolality (P>0.05). The naked mole-rat exhibits a moderate kidney concentrating ability and cannot maintain plasma osmolality or body mass with either extreme water stress or salt loading. Although this species succesfully inhabits arid zones, survival in these areas is not facilitated by renal water conservation, but rather by their underground existence in a microhabitat where humidities are high and radiant heat loads low. In this milieu a moderate kidney concentrating ability is adequate.Abbreviations Bm body mass - ESL extreme salt load - EWS extreme water stress - MSL mild salt load - MWS mild water stress     

Physiological effects of seawater intake in adult harp seals during phase I of fasting

Previous studies have shown that harp seals may drink considerable amounts of seawater. The current study was undertaken to study the physiological responses to bolus administration of seawater. Adult harp seals (Phoca groenlandica) were fasted without access to water for 48 h and then given 1000 or 1500 ml of seawater by a stomach tube. Changes in urine and plasma parameters were thereafter monitored for another 12-20 h. Urine production and urine excretion rate of Na+ and Cl- increased soon after administration and reached a maximum 3-4 h later. Urine osmolality was kept rather stable and high ( approximately 1500 mOsm x kg(-1)) following seawater administration, due to a drop in urine concentration of urea that was proportional to the simultaneous increase in urine concentration of NaCl. Plasma osmolality remained at approximately 340 mOsm x kg(-1), while plasma concentration of urea decreased some 20-25% due to increased excretion of urea when seawater was ingested. Despite bolus administrations of seawater of up to approximately 2% of body mass, homeostasis was maintained and no ill effects observed. It is concluded that the concentrating abilities of the kidneys of harp seals are sufficient to prevent net loss of body water following seawater ingestion. Seawater ingestion may, moreover, increase urinary osmotic space and thus serve as a mechanism to excrete additional urea produced during phase I of fasting.     

Renal function in red wattlebirds in response to varying fluid intake

Red wattlebirds (Anthochaera carunculata) are among the more nectarivorous of the Australian honeyeaters (family Meliphagidae). As such, they potentially ingest large and dilute fluid loads as food, and they produce copious dilute urine in the field. We examined in the laboratory the renal mechanisms by which such fluid loads are processed. Wattlebirds received one of three liquid diets [a mix of honey, water, and Complan (Boots) complete dietary supplement] of varying concentration (250, 1000, and 1750 mmol/kg, Na⁺/K⁺ concentrations of 4/4, 12/15, and 23/30 mmol/l, respectively). We measured renal function via infusion of a filtration marker (¹⁴C-polyethylene glycol) from osmotic minipumps implanted intraperitoneally. Wattlebirds consumed volumes of the three diets sufficient to provide nearly equal caloric intakes (approximately 200 kJ/day), and as a consequence had water turnover rates varying from 30 to 200 ml/day (approximately 50–335% of total body water per day). Renal function in the morning, before feeding, did not differ among diet groups (glomerular filtration rate =18 ml/h, urine flow rate =0.4 ml/h). In the afternoon, after feeding, urine flow did vary, from 3 ml/h in birds on the most concentrated diet to 6 ml/h on the most dilute. This was accomplished by varying the rate of tubular reabsorption of water (from a high of >90% on the most concentrated diet to a low of just over 70% on the most dilute), with little variation in the rate of glomerular filtration (mean ∼23 ml/h). Comparisons between dietary intakes and urinary outputs of water and electrolytes suggest that not all dietary water was absorbed from the gut, but that there was significant post-renal reabsorption of Na⁺. The reduced fractional water reabsorption on the dilute diet was accompanied by a decrease in the circulating concentration of arginine vasotocin (from >4 pg/ml in birds on the two more concentrated diets to <1 pg/ml on the most dilute diet). In contrast, concentrations of aldosterone (10–20 pg/ml) did not differ among diets. Perhaps in consequence, renal fractional absorption of Na⁺ also did not differ, and so birds on the dilute diet, with their higher urine flows, had higher rates of Na⁺ excretion and suffered a decreased concentration of Na⁺ in the plasma. Accepted: 14 January 1988     

Metabolism during starvation/dehydration stress in two species of galagos

Two species of galagos (G. senegalensis moholi andG. garnettii) were subjected to dehydration and starvation stress in order to determine whether, as is common in other animals, these hypometabolic prosimians would lower their metabolic rate even further. Dehydration was confirmed by losses in body mass, a decrease in fecal water content and a rise in urine osmolality. At the height of dehydration, 20 to 25% reduction in body mass, 30 to 40% reduction in fecal water content and urine osmolality ranging from 1.8 to 3.5 Osmol kg⁻¹ H₂O, were recorded in some of the animals. Basal metabolic rate of 0.536 ml O₂ (g·h)⁻¹ inG. s. moholi and 0.302 ml O₂ (g·h)⁻¹ inG. garnettii were recorded, representing 50 to 42% reduction in metabolic rate, respectively, compared with mass specific values. In none of the tested animals did we observe significant reduction in basal metabolism during dehydration/starvation stress compared with the rates observed during the control period. Basal metabolism in the bushbabies seems to have reached the lowest level and no further adjustment is apparently possible as a strategy for energy saving during starvation and/or dehydration stress.     

Temperature regulation and water metabolism in the elephant shrew Elephantulus edwardi

1. The Cape elephant shrew Elephantulus edwardi maintained a stable body temperature (37.6 C) over a wide range of ambient temperatures. 2. Normal eutherian mechanisms of temperature regulation were employed in the maintenance of homeothermy. 3. Water turnover rate and metabolic rate were lower than predicted for a similar-sized eutherian. 4. Maximal urine concentration was fairly high (3118 +/- 267 mOsm/kg) with urine osmolality largely dependent on the urea concentration. 5. Relative medullary thickness of the kidney (6.61 +/- 0.84) indicated a higher maximum urine concentration than found in the laboratory and reflected an arid habitat. 6. E. edwardi can be classed among the "advanced" homeotherms in view of the physiological adaptations employed for survival.     

Glycerol hyperhydration: physiological responses during cold-air exposure.

Hypohydration occurs during cold-air exposure (CAE) through combined effects of reduced fluid intake and increased fluid losses. Because hypohydration is associated with reduced physical performance, strategies for maintaining hydration during CAE are important. Glycerol ingestion (GI) can induce hyperhydration in hot and temperate environments, resulting in greater fluid retention compared with water (WI) alone, but it is not effective during cold-water immersion. Water immersion induces a greater natriuresis and diuresis than cold exposure; therefore, whether GI might be effective for hyperhydration during CAE remains unknown. This study examined physiological responses, i.e., thermoregulatory, cardiovascular, renal, vascular fluid, and fluid-regulating hormonal responses, to GI in seven men during 4 h CAE (15 degrees C, 30% relative humidity). Subjects completed three separate, double-blind, and counterbalanced trials including WI (37 ml water/l total body water), GI (37 ml water/l total body water plus 1.5 g glycerol/l total body water), and no fluid. Fluids were ingested 30 min before CAE. Thermoregulatory responses to cold were similar during each trial. Urine flow rates were higher (P = 0.0001) with WI (peak 11.8 ml/min, SD 1.9) than GI (5.0 ml/min, SD 1.8), and fluid retention was greater (P = 0.0001) with GI (34%, SD 7) than WI (18%, SD 5) at the end of CAE. Differences in urine flow rate and fluid retention were the result of a greater free water clearance with WI. These data indicate glycerol can be an effective hyperhydrating agent during CAE.     

Volume expansion during NOS substrate donation with L-arginine: regulatory offsetting of renal response?

The responses to infusion of nitric oxide synthase substrate (L-arginine 3 mg.kg(-1).min(-1)) and to slow volume expansion (saline 35 ml/kg for 90 min) alone and in combination were investigated in separate experiments. L-Arginine left blood pressure and plasma ANG II unaffected but decreased heart rate (6 +/- 2 beats/min) and urine osmolality, increased glomerular filtration rate (GFR) transiently, and caused sustained increases in sodium excretion (fourfold) and urine flow (0.2 +/- 0.0 to 0.7 +/- 0.1 ml/min). Volume expansion increased arterial blood pressure (102 +/- 3 to 114 +/- 3 mmHg), elevated GFR persistently by 24%, and enhanced sodium excretion to a peak of 251 +/- 31 micromol/min, together with marked increases in urine flow, osmolar and free water clearances, whereas plasma ANG II decreased (8.1 +/- 1.7 to 1.6 +/- 0.3 pg/ml). Combined volume expansion and L-arginine infusion tended to increase arterial blood pressure and increased GFR by 31%, whereas peak sodium excretion was enhanced to 335 +/- 23 micromol/min at plasma ANG II levels of 3.0 +/- 1.1 pg/ml; urine flow and osmolar clearance were increased at constant free water clearance. In conclusion, L-arginine 1) increases sodium excretion, 2) decreases basal urine osmolality, 3) exaggerates the natriuretic response to volume expansion by an average of 50% without persistent changes in GFR, and 4) abolishes the increase in free water clearance normally occurring during volume expansion. Thus L-arginine is a natriuretic substance compatible with a role of nitric oxide in sodium homeostasis, possibly by offsetting/shifting the renal response to sodium excess.     

Renal excretion of urea in a small east African antelope, the dik-dik (Rhynchotragus kirkii)

1. A study on the renal handling of urea by the dik-dik antelope (Rhynchotragus kirkii) was conducted. 2. Plasma and urine samples were analysed for osmolality, urea and creatinine concentrations during dehydration and intra-ruminal loading of potassium and sodium solutions. 3. The glomerular filtration rate (GFR) of the dik-dik was found to be 182.6 +/- 11.7 ml/min/100 kg body mass. 4. Dehydration was observed to increase tubular urea reabsorption and increase plasma and urine osmolalities, but had no effect on the amount of urea filtered at the glomerulus. 5. Potassium loading increased both GFR and urine flow rate.     

Renal response of euryhaline toad (Bufo viridis) to acute immersion in tap water, NaCl, or urea solutions

Green toads (Bufo viridis) were acclimated to either tap water, 230 mOsmol NaCl kg-1 H2O (saline), 500 mOsmol NaCl kg-1 H2O (high saline), or 500 mmol L-1 urea. Renal functions for each acclimation group were studied on conscious animals that had one ureter chronically catheterized. Reciprocal immersion of tap-water- and saline-acclimated toads in the opposite solution did not stress the animals osmotically, and plasma osmolality increased or decreased by no more than 15%. However, urine osmolality and ionic composition changed immediately and profoundly on exposure to the other solution. Exposure of tap-water-acclimated toads to saline decreased urine flow by 30%, whereas the reciprocal immersion led to an increase of 30%. Immersion of tap-water-acclimated toads in high saline led to immediate cessation of urine flow, whereas immersion of 500 NaCl- or urea-acclimated toads in tap water led to a large increase in urine flow, with an overshoot that lasted 10 h (as a result of either salt or urea diuresis). Urine flow then stabilized at a level 5-6 times higher than the value attained at high-salt environment. On immersion of 500 urea-acclimated toads in 500 NaCl, urine flow doubled, accompanied by a change in ion composition, without change in the osmolality. In all experimental conditions, plasma potassium concentration was maintained within a narrow range. The results show that the toad's kidneys contributed efficiently both to osmo- and ionoregulation in a wide range of ambient solutions.     

The effects of clonidine on the kidney function in the anesthetized dog

Studies were performed to determine the mechanism by which the antihypertensive agent clonidine increased urine flow. The response of the kidney has been examined in four combinations. The parameters of renal function have been compared during volume expansion by 1.5-2.0% body weight Ringer solution. In the control animals, volume expansion by 2% body weight, resulted in a slight increase in sodium excretion and urine flow. In 10 anesthetized dogs 1.0 microgram/kg/min of clonidine infused i.v. during 30 minutes (the total amount of clonidine infused was 30 micrograms/kg) decreased the arterial blood pressure from 136 +/- 13 mmHg to 127 +/- 12 mmHg and elevated urine flow from 2.95 +/- 1.65 ml/min to 4.34 +/- 1.77 ml/min while the urine osmolality diminished from 399 +/- 107 mosm/l to 265 +/- 90 mosm/l and the glomerular filtration remained constant. In 5 animals 0.1 microgram/kg/min of clonidine was infused into the left renal artery (this dose is corresponding to the renal fraction of the cardiac output) without any effects in the left kidney. 1.0 microgram/kg/min of clonidine infused directly into the left renal artery produced vasoconstriction in the ipsilateral kidney, decreased the glomerular filtration rate and the urine flow. By contrast in the right kidney the urine flow rose without hemodynamic changes, and the urine osmolality became hypoosmotic compared to the plasma. In ten dogs 1.0 microgram/kg/min of clonidine and 1 mU/kg/min of arginine-vasopressin were infused intravenously. The vasopressin infusion superimposed on the clonidine could not inhibit the increase of the urine excretion, and the fall of the urine osmolality. The results suggest that the clonidine increases the renal medullary blood flow possibly via a direct mechanism, decreases the sympathetic outflow to the kidney and via an indirect pathway, mediated by the renin-angiotensin system. The renal medullary flow increase produces a washout of the medullary osmotic gradient, and the water reabsorption diminishes.     

Effects of fresh and seawater ingestion on osmoregulation in Atlantic bottlenose dolphins (Tursiops truncatus)

Bottlenose dolphins (Tursiops truncatus) are marine mammals with body water needs challenged by little access to fresh water and constant exposure to salt water. Osmoregulation has been studied in marine mammals for a century. Research assessing the effects of ingested fresh water or seawater in dolphins, however, has been limited to few animals and sampling times. Nine 16- to 25-h studies were conducted on eight adult dolphins to assess the hourly impact of fresh water, seawater, and seawater with protein ingestion on plasma and urine osmolality, urine flow rate (ufr), urinary and plasma solute concentrations, and solute clearance rates. Fresh water ingestion increased ufr. Fresh water ingestion also decreased plasma and urine osmolality, sodium and chloride urine concentrations, and solute excretion rates. Seawater ingestion resulted in increased ufr, sodium, chloride, and potassium urine concentrations, sodium excretion rates, and urine osmolality. Seawater with protein ingestion was associated with increased ufr, plasma osmolality, sodium excretion, and sodium, chloride, potassium, and urea urine concentrations. In conclusion, bottlenose dolphins appear to maintain water and plasma solute balance after ingesting fresh water or seawater by altering urine osmolality and solute clearance. Ingestion of protein with seawater appears to further push osmoregulation limits and urine solute concentrations in dolphins.     

Effects of time of day, gender, and menstrual cycle phase on the human response to a water load

Estrogen and progesterone interference with renal actions of arginine vasopressin (AVP) has been shown. Thus we hypothesized that women will have a higher water turnover than men and that the greatest difference will be during the luteal phase of the menstrual cycle. Seven men (32 +/- 3 yr) and six women (33 +/- 2 yr) drank 12 ml water/kg lean body mass on different days at 0800 and at 2000 following 10 h of fast and a standardized meal at 0600 and 1800. Women participated on days 4-11 and 19-25 of the menstrual cycle. Initial urine and plasma osmolalities and urine flow rates were similar in all experiments. The cumulative urine voided over 3 h following the morning drink was less in men (73 +/- 12% of the water load) compared with women in either the follicular (100 +/- 3%) or luteal phases (102 +/- 10%) of the menstrual cycle. Nighttime values (30-43% of the water load) were lower in all experiments and were not different between sexes or menstrual cycle phases. Plasma AVP was higher at night and may contribute to this diurnal response. The data are generally consistent with the stated hypothesis; however, possibly owing to the greatly reduced urine flow in both sexes at night, a difference between sexes was not observed at that time.     

Age and hypohydration independently influence the peripheral vascular response to heat stress

Seven young (Y, 22-28 yr) and seven middle-aged (MA, 49-60 yr) normotensive men of similar body size, fatness, and maximal oxygen uptake (VO2max) were exposed to a heat challenge in an environmental chamber (48 degrees C, 15% relative humidity). Tests were performed in two hydration states: hydrated (H, 25 ml water/kg body wt 1 h before the test, 2.5 h before exercise) and hypohydrated (Hypo, after 18-20 h of water deprivation). Each test began with a 90-min rest period during which the transiently increased plasma volume and decreased osmolality after drinking in the H condition returned to base line. This period was followed by 30 min of cycle exercise at a mean intensity of 43% VO2max and a 60-min resting recovery period with water ad libitum. Although prior drinking caused no sustained changes in plasma osmolality, Hypo increased plasma osmolality by 7-10 mosmol/kg in both groups. There were no significant age differences in water intake, urine output or osmolality, overall change in body weight, or sweating rate. In the H state, the percent change in plasma volume was less (P less than 0.01) during exercise for the Y group (-5.9 +/- 0.7%) than for the MA group (-9.4 +/- 0.6%). Esophageal temperature (Tes) was higher in the Hypo condition for both groups with no age-related differences. Throughout the 3-h period, mean skin temperature was higher in the Y group and significantly so (P less than 0.05) in the Hypo condition.(ABSTRACT TRUNCATED AT 250 WORDS)     

COX-2 disruption leads to increased central vasopressin stores and impaired urine concentrating ability in mice

It was hypothesized that cyclooxygenase-2 (COX-2) activity promotes urine concentrating ability through stimulation of vasopressin (AVP) release after water deprivation (WD). COX-2-deficient (COX-2(-/-), C57BL/6) and wild-type (WT) mice were water deprived for 24 h, and water balance, central AVP mRNA and peptide level, AVP plasma concentration, and AVP-regulated renal transport protein abundances were measured. In male COX-2(-/-), basal urine output and water intake were elevated while urine osmolality was decreased compared with WT. Water deprivation resulted in lower urine osmolality, higher plasma osmolality in COX-2(-/-) mice irrespective of gender. Hypothalamic AVP mRNA level increased and was unchanged between COX-2(-/-) and WT after WD. AVP peptide content was higher in COX-2(-/-) compared with WT. At baseline, plasma AVP concentration was elevated in conscious chronically catheterized COX-2(-/-) mice, but after WD plasma AVP was unchanged between COX-2(-/-) and WT mice (43 ± 11 vs. 70 ± 16 pg/ml). Renal V2 receptor abundance was downregulated in COX-2(-/-) mice. Medullary interstitial osmolality increased and did not differ between COX-2(-/-) and WT after WD. Aquaporin-2 (AQP2; cortex-outer medulla), AQP3 (all regions), and UT-A1 (inner medulla) protein abundances were elevated in COX-2(-/-) at baseline and further increased after WD. COX-2(-/-) mice had elevated plasma urea and creatinine and accumulation of small subcapsular glomeruli. In conclusion, hypothalamic COX-2 activity is not necessary for enhanced AVP expression and secretion in response to water deprivation. Renal medullary COX-2 activity negatively regulates AQP2 and -3. The urine concentrating defect in COX-2(-/-) is likely caused by developmental glomerular injury and not dysregulation of AVP or collecting duct aquaporins.     

Water homeostasis in desert-dwelling horses

This study set out to investigate tolerance of the body water pool to short-term water deprivation in horses and, in particular, to assess whether feral horses from the Namib Desert showed tolerance to dehydration superior to Transvaal. Hydration status was compared in six feral horses from the Namib Desert and in six Boerperd farm horses under conditions of normal hydration and after 72 h of dehydration. Under normal hydration, the two groups did not differ significantly in water intake, plasma sodium and potassium concentrations, plasma osmolality, hematocrit, total plasma protein, body water content, or water turnover (ml.kg-0.82.day-1). The Namib horses were significantly smaller (P less than 0.0001) and turned over 5 liters less water per day than the Boerperd during normal hydration and 4 liters less during dehydration. Increases in plasma sodium concentration after 72 h of dehydration were greater (P less than 0.05) in the Namib horses. It was concluded that horses can easily tolerate water deprivation that results in a 12% reductions in body mass. The feral horses of the Namib desert were not significantly different per unit mass from domestic horses with regard to indexes of total body water content under conditions of normal hydration and after 72 h of dehydration. Their smaller size and, hence, lower water turnover might be mechanisms they use for survival in the Namib Desert.     

Prey size, prey nutrition, and food handling by shrews of different body sizes

We tested some predictions relating metabolic constraints offoraging behavior and prey selection by comparing food handlingand utilization in four sympatric shrew species: Sorex minutus(mean body mass = 3.0 g), S. araneus (8.0 g), Neomys anomalus(10.0 g), and N. fodiens (14.4 g). Live fly larvae, mealwormlarvae, and aquatic arthropods were offered to shrews as smallprey (body mass <0.1 g). Live earthworms, snails, and smallfish were offered as large prey (>0.3 g). The larvae werethe high-nutrition food (>8 kJ/g), and the other prey werethe low-nutrition food (<4 kJ/g). The smallest shrew, S.minutus, utilized (ate + hoarded) <30% of offered food,and the other species utilized >48% of food. The largerthe shrew, the more prey it ate per capita. However, highlyenergetic insect larvae composed 75% of food utilized by S.minutus and only >40% of the food utilized by the other species. Thus, inverse relationships appeared between shrewbody mass and mass-specific food mass utilization and betweenshrew body mass and mass-specific food energy utilization:the largest shrew, N. fodiens, utilized the least food massand the least energy quantity per 1 g of its body mass. Also,the proportion of food hoarded by shrews decreased with increase in size of shrew. With the exception of S. araneus, the sizeof prey hoarded by the shrews was significantly larger thanthe size of prey eaten. Tiny S. minutus hoarded and ate smallerprey items than the other shrews, and large N. fodiens hoardedlarger prey than the other shrews.     

Seasonal changes of food and water consumption and urine production of the marmot, Marmota flaviventris

In early spring, food and water consumption and the excretion and clearances of urine and solutes reached maximal rates. Water consumption exceeded food intake and urine production and plasma osmolality was lowest. Toward early and late summer, water intake decreased faster than food consumption and urine production. Urea excretion and clearances diminished with food consumption, while creatinine clearance decreased only slightly. Plasma osmolality increased. The data are consistent with rehydration soon after hibernation is completed, followed by a period of weight gain and dehydration in preparation for the next prolonged period of hibernation.