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.     

Competition between co-dominant plants of the Serengeti plains depends on competitor identity,water, and urine

Kyllinga nervosa (Steud.) and Sporobolus kentrophyllus (K. Schum.) are co-dominant plants of the Serengeti short-grass plains, Tanzania. The plains are characterized by seasonal and sporadic rainfall and currently support in excess of 1.5 million migratory ungulates. The interactive effect of simulated bovine urine and water availability were tested on the competitive interactions of these species in the laboratory. Sporobolus kentrophyllus was a superior competitor to K. nervosa over the tested treatment levels with respect to growth and reproductive effort. Sporobolus kentrophyllus exhibited rapid growth in response to urine addition, leading to a significant species × urine interaction while reduced growth by K. nervosa in response to low water availability explained the significant species × water interaction and is likely explained by K. nervosa's shallow root system. Kyllinga nervosa, however, appears to be more tolerant of low nitrogen conditions based on its similar growth with and without the urine treatment. The effect of intraspecific competition on total biomass was similar for S. kentrophyllus and K. nervosa. Competition resulted in increased size differences (asymmetry) for K. nervosa and for the interspecific competition treatments compared to the size differences observed for plants grown individually (in absence of competition).Total reproductive biomass was reduced most by competition with S. kentrophyllus, irrespective of target species. The water treatment did not influence reproduction while the urine treatment significantly increased reproductive biomass and interacted with target species, competitor species, and yielded a three-way urine × target × competitor species interaction.Results suggest that codominance of these two species in the Serengeti is regulated by water availability, nitrogen input from grazers, and local neighbor identity.     

Effects of water dilution, housing, and food on rat urine collected from the metabolism cage

The objective of the study reported here was to investigate three factors that may affect the amounts of water consumed and urine excreted by a rat in the metabolism cage: water dilution, housing, and food. Young F344/N rats (eight per group) were used for all experiments. Food was withheld from rats before each 16-h urine collection, then rats were transferred into a metabolism cage. For trial A (water dilution), urine was collected from rats supplied with dyed water (0.05%, vol/vol). This was repeated three times over a 2-week period. Dye in water or urine was quantified, using a spectrophotometer. For trial B (housing), rats were individually housed in wire cages for 3 weeks before the first urine collection. Then they were group housed in the solid-bottom cage (four per cage). After 2 weeks of acclimation, urine collection was repeated. For trial C (food), one group of rats was provided with food, the other was not, during urine collection. About 8% of urine samples of small volume (< or = 3 ml) from trial A were contaminated with drinking water up to 13% of volume. The average urine volume associated with individual housing was approximately twice as large as that associated with group housing. When food was provided during urine collection, rats consumed similar amounts of water but excreted significantly smaller amounts of urine than did rats without food. It was concluded that water dilution of a urine sample from a sipper bottle is relatively small; rats individually housed in wire caging before urine collection can consume and excrete a larger quantity of water, compared with rats group housed in solid-bottom cages; and highly variable urine volumes are, in part, associated with lack of access to food during urine collection.     

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     

Plasma homeostasis and cloacal urine composition inCrocodylus porosus caught along a salinity gradient

Summary Juveniles of the Estuarine or Saltwater Crocodile,Crocodylus porosus, maintain both osmotic pressure and plasma electrolyte homeostasis along a salinity gradient from fresh water to the sea. In fresh water (FW) the cloacal urine is a clear solution rich in ammonium and bicarbonate and containing small amounts of white precipitated solids with high concentrations of calcium and magnesium. In salt water (SW) the cloacal urine has a much higher proportion of solids, cream rather than white in colour, which are the major route for excretion of potassium in addition to calcium and magnesium. Neither liquid nor solid fractions of the cloacal urine represent a major route for excretion of sodium chloride. The solids are urates and uric acid, and their production probably constitutes an important strategy for water conservation byC. porosus in SW. These data, coupled with natural history observations and the recent identification of lingual salt glands, contribute to the conclusion thatC. porosus is able to live and breed in either fresh or salt water and may be as euryhaline as any reptile.     

Identification of crystalline allantoin in the urine of African Cricetidae (Rodentia) and its role in their water economy

Summary All eleven cricetid species, examined in this investigation, produced an off-white crystal-line precipitate in their urine when deprived of water, whereas not one murid examined did so. This crystalline compound was identified as allantoin, a common end product of purine catabolism. The quantity found in the solid precipitate alone accounted for 47% of the total nitrogen excreted and was approximately 14 times greater than the predicted quantity of allantoin from purine degradation. It appears that there is a shift in nitrogen excretion from urea to allantoin in the Cricetidae.Water-deprived cricetids had higher urine osmolalities, urea concentrations and lower daily percentage body water turnovers than the murids. This can be explained by the substantial water savings associated with excreting solid allantoin. The discrepancy in the mode of nitrogen excretion between the two families inhabiting the Namib Desert can be attributed to their different evolutionary histories, the Cricetidae being pre-adapted for survival in deserts.Abbreviations WTR water turnover rate     

Energy and water balance in the lesser double-collared sunbird (Nectarinia chalybea) feeding on different nectar concentrations

The water balance of nectarivores is tightly linked to their energy balance. When nectar is dilute, consumption of a large water excess is inevitable. We investigated energy and water balance in lesser double-collared sunbirds, Nectarinia chalybea (8 g), kept at 20 °C and fed different nectar concentrations (0.4, 0.8 M sucrose or 1.2 M sucrose). The mass of sucrose consumed, body mass, day-time mass gain and night-time mass loss were the same irrespective of diet, the birds compensating energetically for changes in sucrose concentration by drinking greater volumes of the more dilute solutions. Sunbirds consumed between 0.5 times and 1.8 times their body mass in preformed water per day, depending on sucrose concentration, and excreted around 75% of the water. The difference between water gain (preformed and metabolic water) and excreted water is assumed to equal evaporative water loss, and was similar on 1.2 M and 0.8 M sucrose, but was higher on a diet of 0.4 M sucrose. The osmolalities and K⁺ and Na⁺ concentrations of the excreted fluid were extremely low, so that sunbird urine resembled that of hummingbirds and freshwater vertebrates rather than that of typical terrestrial vertebrates. N. chalybea is able to maintain energy and water balance over a range of nectar concentrations by adjusting the volume of solution consumed and by excreting copious, dilute fluid. Accepted: 2 January 1999     

Discontinuous gas exchange in dung beetles: patterns and ecological implications

This study correlates a distinctive pattern of external gas exchange, referred to as the discontinuous gas exchange cycle (DGC), observed in the laboratory, with habitat associations of five species of telecoprid dung beetles. The beetles were chosen from a variety of habitats that would be expected to present different amounts of water stress. All five species exhibited DGC. Sisyphus fasciculatus has been recorded only in woodland areas, and does not have strict spiracular control during its DGC. Anachalcos convexus and Scarabaeus rusticus are associated with open mesic habitats. Both species exhibit a distinct DGC, previously found in some other insect species, but intermediate within this study group. Sc. flavicornis and Circellium bacchus are typically found in arid regions, and have the most unusual form of DGC, with spiracular fluttering during the burst phase. These results support the hypothesis that spiracular fluttering reduces respiratory water loss. From this study we conclude that the DGC is an ancestral adaptation, most probably as a result of anoxic environments in underground burrows, but that spiracular control is enhanced to reduce respiratory water loss in beetle species that live in arid habitats. Received 4 August 1999 / Accepted: 7 October 1999     

The distribution of fluoride in some antarctic seals

Summary The aim of this investigation was to find out why some antarctic seals tolerate fluoride levels in their food (krill) that would be toxic to man. The fluoride content of different soft and hard tissues, urine and feces from 16 L. carcinophagus, 13 L. weddelli and 2 A. tropicalis (urine only) were determined by the fluoride sensitive electrode or by a photometric method (urine). Results and conclusions are following: All soft tissues have low concentrations of fluoride in the same range as soft tissues from non antarctic and non krill eating marine or terrestrial vertebrates. But within the soft tissues there are significant differences in the fluoride content (blood: 0.001 ppm F/dw — brain: 9.1 ppm F/dw). A special tolerance for fluoride of soft tissues can be excluded. Compared with man or weddell-seals the crabeater-seals store two times more fluoride in their skeleton. Hard structures that stop their mineralisation early in the development like teeth have only a relatively low fluoride content. For the same reason only neglectable amounts of fluoride could be detected in hair. Fluoride excretion by the kidneys is efficient and is — besides the storage of fluoride in the skeleton — the second mechanism to guarantee a good blood homoiostasis for fluoride. Until now it is impossible to decide how much fluoride from the food is resorbed by the digestive tract.     

Kidney structure and function of obligate and facultative hibernators: the white-tailed prairie dog (Cynomys leucurus) and the black-tailed prairie dog (Cynomys ludovicianus)

The white-tailed prairie dog is an obligate hibernator that enters a heterothermic phase when maintained in the cold with low intensity light and ad libitum food and water. The black-tailed prairie dog (a facultative hibernator) will not hibernate under similar conditions. It has been suggested that the black tailed prairie dog remains active during the winter because it can conserve water more effectively due to a more efficient kidney. The present study revealed no significant differences between the species in renal morphology: relative medullary thickness, nephron heterogeneity, renal vasculature, or fornix dimensions, all of which are structures associated with the urinary concentrating mechanism. In addition, there was no difference in number of nephrons between the two species. The black-tailed prairie dog does produce a more concentrated urine when food and water deprived. However, this difference was not observed when the animals were salt loaded. The water-deprivation and salt-loading experiments suggest that the higher urine osmolality produced by the back-tailed prairie dog during fasting is a result of a higher urea load due to a greater protein catabolism and not because of a differential capacity to concentrate urine.Abbreviations C cortex - GFR glomerular filtration rate - H height - IS inner stripe - IZ inner zone of medulla - L length - OS outer stripe - PE polythylene - RMT relative medullary thickness - T _a ambient temperature - W width     

Effects of water restriction during growth and adulthood on renal function of bobwhite quail,Colinus virginianus

Renal function and osmoregulation were studied in bobwhite quail (Colinus virginianus) raised with unrestricted water (chronically unrestricted group) or restricted water (chronically restricted group). There was no difference in urine concentrating ability between adult and juvenile (3.5 or 7.5 week-old) quail. A filtration marker (polyethylene glycol) was infused into adult quail via osmotic minipumps and responses to the following regimens studied: ad libitum water intake, short-term (4-day) water restriction, and acute (1-day) dehydration (withdrawal of all drinking water). Chronically restricted quail had higher urine-to-plasma ratios of polyethylene glycol and lower urine flow rates during short-term restriction. A greater proportion of the reduction in urine flow rate during dehydration was attributable to enhanced tubular reabsorption, rather than reduced rates of filtration, in chronically restricted than in chronically unrestricted birds. Chronically restricted birds also had higher maximum urine-to-plasma ratios of polyethylene glycol (but not higher urine osmolality). These differences occurred in the face of arginine vasotocin concentrations that were not different in the two groups of birds (approximately 15 pg·ml^-1 during hydration, and 45 pg·ml^-1 during water restriction or dehydration). These observations suggest that chronically restricted quail have an enhanced responsiveness of tubular reabsorption to dehydration, a finding consistent with previous observations of tubule hypertrophy and hyperplasia in these birds (Goldstein and Ellis 1991). Despite this, no difference was found in medullary cAMP levels, either basal or arginine vastotocin-or forskolin-stimulated, in the two groups. When given water ad libitum, chronically restricted quail drank copiously (more than two times the drinking rate of chronically unrestricted birds rehydrating from acute dehydration or short-term water restriction), but glomerular filtration rate, hematocrit, and plasma osmolality did not differ in the two groups under this condition; chronically restricted quail excreted the excess water consumed during rehydration in a copious urine accomplished by reduced tubular water reabsorption.Abbreviations ADH antidiuretic hormone - AVT arginine vasotocin - m_b body mass - cAMP cyclic adenosine-monophosphate - DEH birds raised with restricted water intake - dpm decays per minute - ECF extracellular fluid - ECFV extracellular fluid volume - E _PEG total rate of polyethylene glycol excretion - GFR glomerular filtration rate - Hct hematocrit - HYD birds raised with unrestricted water intake - PEG polyethylene glycol - P _osm plasma osmolality - P _PEG plasma concentration of polyethylene glycol - U _PEG urine concentration of polyethylene glycol - (U/P)_PEG urine-to-plasma ratio concentration of polycthylene glycol - V urine flow rate     

Devasting effects of river flooding to the ground-nesting bee, <Emphasis Type="Italic">Andrena vaga</Emphasis> (Hymenoptera: Andrenidae), and its associated fauna

Large populations of non-social bees are rarely seen in today's cultural landscapes. One of the few exceptions is the fossorial bee Andrena vaga Panzer, (Andrenidae), a typical insect species of river flood plains, which forms sizeable nesting aggregations along the high water dams of the Upper Rhine, each containing up to several tens of thousands of bees. In May 1999, a record flood of the River Rhine led to extreme water levels at the high water dams of the Upper Rhine. Though natural flooding is often viewed as a management practice with considerable conservation perspectives for river flood plain management, we demonstrate a catastrophic effect of the 1999 inundation on the immature offspring of two affected populations of A. vaga. All immatures in brood cells located low on the landward side of a dam died, very likely because saturation of the soil restricted oxygen availability.The two affected aggregations sited on the high water dam declined in the following year to 23 and 56% of pre-flood size, but not at a neighbouring, non-flooded aggregation. Rare parasites associated with A. vaga also decreased in number following flooding. These results highlight a dilemma on the use of natural flooding of river plains for conservation purposes.     

Seasonal changes in the water metabolism of woodrats

Summary Dusky-footed woodrats (Neotoma fuscipes) and desert woodrats (N. lepida) experience pronounced seasonal variations in the aridity of their habitats. The effects of seasonal aridity upon the water conserving abilities of these species were assessed through measurements of water conserving abilities and kidney structure of animals captured in summer and winter, and through measurements of animals' abilities to acclimate to differing water availabilities in the laboratory.Urine concentrating ability was the water conserving mechanism most responsive to changes in the availability of water. Summer and summeracclimated N. fuscipes (431.7 and 459.4 mEqCl^-/1) demonstrated urine Cl^- concentrating abilities substantially greater than those of winter and winter-acclimated N. fuscipes (245.7 and 337.4 mEqCl^-/1). Summer, winter-acclimated, and winter N. lepida exhibited urine Cl^- concentrations equivalent to those of winter N. fuscipes; summer-acclimated N. lepida exhibited markedly greater values (466.7 mEqCl^-/l) equivalent to those of summer and summer-acclimated N. fuscipes.Measurements of relative thicknesses of renal cortex and medulla yielded no significant differences among the experimental groups of N. fuscipes and N. lepida, thus suggesting that both species possess equal abilities to concentrate urine. These data are confirmed by urine concentrations of summer-acclimated animals of both species.Water conserving abilities of both species correlate well with climatic and dietary plant water content data. Thus, during the dry, warm summer months (when plant moisture is reduced) N. fuscipes conserves water mainly through increased urine concentration. The laboratory acclimation data and differences between summer and winter animals strongly suggest that N. fuscipes undergoes an acclimatization to the seasonal aridity which increases gradually during spring and peaks in late summer, thus enabling this water-dependent species to exist on reduced water requirements. The uniformly low water conserving abilities of winter-acclimated, winter, and summer N. lepida physiologically verify the previous reports that this species satisfies its water requirements through utilization of succulent cactus, thereby avoiding the stress of summer aridity in its habitat.The fact that both species exhibit equal capacities to conserve water indicates that the much greater geographic distribution of N. lepida is not the result of differences in physiological water conserving abilities, but instead may be the result of specific physiological adaptation by N. lepida to utilization of cactus and other plants containing noxious or toxic compounds.     

Nestling provisioning in water pipits (Anthus spinoletta): do parents go for specific nutrients or profitable prey?

In this study, we investigated whether free-living insectivorous water pipits (Anthus spinoletta) choose prey according to biochemical quality as measured by protein, lipid, carbohydrate, energy and water contents and/or according to profitability as measured by density, size and catchability. Food preference – expressed in relation to availability – is estimated for 22 arthropod taxa (families and orders). Uni- and multivariate statistics detected no relationships between food preference and nutrient contents, but revealed that more larger prey items are fed to nestlings than smaller ones, both for all prey taken together and within individual taxa. Furthermore, slow-flying arthropods, which are easier to catch, were usually preferred over walking and fast-flying ones. Combined with results from previous studies on the effects of vegetation, prey density and catchability on search times and energy intake, these findings suggest that water pipits select their prey primarily to maximize profitability, i.e. energy intake per unit time. Qualitative traits seem to be important only for specific taxa. For example, toxins or poor digestibility may be responsible for the avoidance of heteropterans, beetles and ants and for feeding the nestlings fewer tipulids than expected at high tipulid densities. Received: 18 January 1999 / Accepted: 14 June 1999     

Changes in sodium,calcium and magnesium ion concentrations in sturgeon (Huso huso) urine and in kidney morphology

During adaptation to brackish water the young great sturgeon Huso huso is able to regulate its serum osmolarity and ion concentrations. After transfer from fresh water to brackish water the ion concentrations in the urine increase and the urine becomes isoosmotic to the blood serum after 24h. The Na⁺ and K⁺ concentrations in the urine increase during the first 12 h by 4.4 and 7.7 times, respectively, later decreasing again. The Mg²⁺ and Ca²⁺ concentrations in the urine increase by 3.4 and 14 times during the first 72h in brackish water and remain high thereafter. These results suggest that the kidneys play an important part in the regulation of serum osmolarity and in the removal of Ca²⁺; however, in contrast to teleosts, Mg²⁺ must be removed extrarenally. During adaptation to a hyperosmotic medium the diameters of the Malpighian bodies, the glomeruli and the diameter of the tubules initially all decrease, but the distal tubules become morphologically differentiated into two regions and the diameter of the distal section later increases again. It is suggested that this is the site of Ca²⁺ secretion into the urine.     

Urine as another potential source for template DNA in polymerase chain reaction (PCR)

This technical note examines the potential for preparing template DNA in polymerase chain reactions (PCR) from urine in Japanese macaques (Macaca fuscata). Microsatellite band patterns from urine samples showed close agreement with those of blood and fecal samples, and only a few hundred μl of urine yielded a template DNA for PCR. This research will increase the opportunity for scientists to examine the genetic backgrounds of their target animals by using non‐invasive sample collection in the wild. Am. J. Primatol. 48:299–304, 1999. © 1999 Wiley‐Liss, Inc.     

Water retention and plasma and urine composition in toads (Bufo viridis Laur.) under burrowing conditions

Summary Osmoregulation in the terrestrial toad,Bufo viridis, was studied under burrowing conditions in the laboratory. The toads can live for over 3 months burrowed in soil containing 9–10% moisture, maintaining constant body volume due to a large increase in the plasma osmolality, contributed mainly by urea. Water content of the tissues remains constant. Relatively large volumes of urine are stored in the urinary bladder during water restriction. The osmolality of the urine does not exceed that of the plasma. Urea uptake across the skin was measured in vitro and was greatly elevated in skins from the burrowed toads. The increase in plasma osmolality enables greater water absorption from the soil under water restricted conditions while the water content of the tissues is maintained constant since cell membranes are highly permeable to urea. It is concluded that the urea accumulating ability and urea tolerance form the basis for both the terrestriality and salt adaptability of this and other amphibian species.     

Fate of sheep urine-N applied to an upland grass sward

Summary Sheep urine was applied once in August to 1 m² plots of a N-deficientLolium perenne-dominated sward at a rate equivalent to a single urination (48 g N m^–2) at an upland site. After 17 days herbage dry matter (DM) and total N were increased 19- and 63-fold respectively compared with a control receiving water only. Soil mineral N (NH ₄ ⁺ and NO ₃ ^– ) levels in the top 20 cm were greater in urine plots until 30 days after urine application when cumulative yields of herbage DM and N were 10 and 21 times greater than those of the control. Maximum recovery of urine N by herbage was only 16% of that applied, and, although swards responded rapidly to urination there were substantial losses of N, perhaps via leaching and/or volatilisation, from the soil-plant system.     

Olfactory Predator Discrimination in Yellow‐Bellied Marmots

The mechanism underlying olfactory predator identification may be relatively experience‐independent, or it may rely on specific experience with predators. A mechanism by which prey might identify novel predators relies on the inevitable creation of sulfurous metabolites that are then excreted in the urine of carnivorous mammals. We tested whether free‐living, yellow‐bellied marmots (Marmota flaviventris) and mid‐sized herbivores that fall prey to a variety of carnivorous mammals could discriminate herbivore (elk—Cervus elephas) urine from predator (red fox—Vulpes vulpes, coyote—Canis latrans, mountain lion—Felis concolor, wolf—Canis lupus) urine, a novel herbivore (moose—Alces alces), and a distilled water control. We further asked how specific this assessment was by testing whether marmots responded differently to predators representing different levels of risk and to familiar vs. unfamiliar predators. We found that marmots responded more to urine from coyotes (a familiar predator on adults), mountain lions (a potentially unfamiliar predator that could kill adults) and wolves (a locally extinct predator that could kill adults) than to elk urine (a non‐predator). Red fox (a predator that poses a risk only to recently emerged marmot pups) urine elicited a less substantial (but not significantly so) response than coyote urine. Marmots can identify predators, even novel ones, using olfactory cues, suggesting that experience with a specific predator is not required to identify potential threats.     

Osmoregulatory mechanisms of the Australian freshwater crocodile, Crocodylus johnstoni, in freshwater and estuarine habitats

The estuary of the Limmen Bight River in Australia's Northern Territory is home to an unusual salt water-adapted population of the Australian `freshwater' crocodile, Crocodylus johnstoni. Crocodiles were captured from tidal reaches of the estuary ranging in salinity from 0.5–24‰ and from several permanent fresh water reaches more or less remote from saline waters. C. johnstoni is an effective osmoregulator in moderately saline waters and has osmoregulatory mechanisms very similar to its more marine-adapted relative, the estuarine crocodile Crocodylus porosus. Fasted C. johnstoni in brackish water appear to lose little sodium in cloacal urine, relying on their lingual salt glands for excretion of excess sodium chloride. The lingual glands show clear evidence of short-term and long-term acclimation to salt water. Like estuarine crocodiles, C. johnstoni drinks fresh water and will not drink sea water. Gross sodium and water fluxes in brackish water are very similar to those in other crocodilians, suggesting differences in integumental permeability are not a major influence on osmoregulatory differences between crocodilians. The data reinforce the hypothesis that crocodylids differ fundamentally from alligatorids in the structure and function of the renal-cloacal-salt gland complex and are of interest in current debate over the evolutionary and zoogeographical history of the eusuchian crocodilians. Accepted: 25 February 1999