Gas exchange in the lung of a dasyurid marsupial: morphometric estimation of diffusion capacity and blood oxygen uptake kinetics

Placental and marsupial mammals differ in the balance of their cardio-respiratory systems. To determine if differences also occur in lung structure, a morphometric study of the lungs of a dasyurid marsupial, Dasyuroides byrnei, was undertaken. The total lung volume was large, but a comparatively smaller proportion was devoted to gas exchange tissue. Eighty-seven percent of lung parenchyma was air space. The volume of capillary blood was lower than would be predicted on a body mass basis, but the hematocrit was high (55%). The rate of oxygen uptake by the blood of D. byrnei was not different from that of recent determinations on human blood. However, a large oxygen capacity resulted in a high theta O2. Morphometric data were used, with theta O2 calculated for D. byrnei, to estimate the pulmonary diffusion capacity. Mass specific DLO2 was high by comparison to those reported for placentals. The erythrocytes contributed 70% of the diffusion resistance to oxygen transfer, with the large theta O2 counteracting the low Vc. We conclude that, similarly to placentals, lung structure is unlikely to limit oxygen transport in this animal, although the structural emphasis appears to be geared towards maximising alveolar ventilation.

Thermogenic capabilities of the opossum Monodelphis domestica when warm and cold acclimated: similarities between American and Australian marsupials

1. Monodelphis domestica is a small marsupial mammal from South America. Its thermogenic abilities in the cold were determined when the opossums were both warm (WA) and cold (CA) acclimated. Maximum heat production of M. domestica was obtained at low temperatures in helium-oxygen. 2. Basal metabolic rate (BMR) in the WA animals was 3.2 W/kg and mean body temperature was 32.6 degrees C at 30 degrees C. These values were lower than those generally reported for marsupials. Nevertheless, these M. domestica showed considerable metabolic expansibility in response to cold. Sustained (summit) metabolism was 8-9 times BMR, while peak metabolism was 11-13 times BMR. These maximum values were equal to, or above, those expected in small placentals. 3. Cold acclimation altered the thermal responses of M. domestica, particularly in warm TaS. However, summit metabolism was not significantly increased; nor did M. domestica show a significant thermogenic response to noradrenaline, which in many small placentals elicits non-shivering thermogenesis. The thermoregulatory responses of this American marsupial were, in most aspects, similar to those of Australian marsupials. This suggests that the considerable thermoregulatory abilities of marsupials are of some antiquity.

Osmotic and ionic regulation during dehydration in a large bird, the emu (Dromaius novaehollandiae): an important role for the cloaca-rectum

The emu, a large (40 kg) flightless bird, has low water requirements yet it has a limited ability to produce concentrated urine. To clarify the factors involved in excretion and water conservation by the emu the patterns of excretion during water restriction were examined. Additionally, kidney function was studied, as also was the role of the cloaca and rectum in the final modification of excreta. Following water restriction the osmolality of the excreta fluid increased to a maximum level at 6 d of 428 +/- 39(8) mosmol (mean +/- S.E.M. (n)), the urine to plasma ratio being 1.35 +/- 0.12(8). In normally hydrated birds almost all Na and Cl were excreted in the fluid fraction but a third of the K was associated with the solid fraction of the excreta. After water restriction for 9 d the pattern of excretion of K did not change but the majority of Na (79%) and Cl (73%) were excreted in the solid phase, possibly associated with urates. In ureteral urine most of the nitrogen was excreted as uric acid or urates; a large fraction, 36 +/- 4(4)%, was not identified. The glomerular filtration rate (G.F.R.) was 41.3 +/- 2.2(6) ml/kg.h. This value is low when compared with other birds and may be related to the large size of the emu. The G.F.R. decreased by 30% after 7 d water restriction. The cloaca-rectum appears to play a major role in the modification of excreta. In vivo perfusion studies showed transepithelial net transport rates of water and major electrolytes to be large. Transport of water from an isosmotic solution, 3.36 +/- 0.17 ml/kg.h, was approximately 10 times the maximum value reported for birds previously. The Na and Cl transport rates were also comparatively high.

Use of helium-oxygen to examine the effect of cold acclimation on the summit metabolism of a marsupial, Dasyuroides byrnei

To determine whether marsupial mammals increase their metabolic capabilities during cold acclimation, the metabolism of both warm and cold acclimated Dasyuroides byrnei was examined by exposure to cold in a helium-oxygen atmosphere. Mean values of heat production and conductance were significantly higher in a helium-oxygen atmosphere than in air. Body temperature did not change until metabolic capacity was exhausted. Both cold and warm acclimated groups could maintain a metabolic scope of 10-11 times the basal or standard level for this species. Such a metabolic scope is much higher than levels recorded for placental mammals. At very low ambient temperatures cold acclimated D. byrnei could sustain a high level of heat production longer than could warm acclimated animals. While there are some similarities between marsupial mammals and placental mammals in their responses to cold acclimation, an increase in maximum metabolism, as reported for placentals, does not seem to occur in marsupials.

In vitro studies of sodium transport across the lower intestine of a desert parrot

The lower intestine (coprodeum and colon) of the Australian parrot, the galah, was mounted in Ussing chambers. Short-circuit current (SCC), electrical potential difference (PD), and unidirectional fluxes of Na and Cl were measured in birds that were fed mixed seeds or were NaCl loaded. The net Na transport of both coprodeum and colon was nearly equal to the SCC, and the flux ratio for Cl was unity. In birds which received mixed seeds, average coprodeal Na transport was 7.8 mu eq . cm-2 . h-1, and PD was 19 mV. The Km for Na was 5.7 meq/l. In colon, Na transport was reduced by 67% and PD by 70%. The ratio between unidirectional Na and Cl fluxes in the serosa-mucosa direction was 0.7. Salt loading suppressed coprodeal, but increased colonic Na transport. The coprodeal and colonic SCC and NA transport of birds receiving mixed seeds were inhibited by amiloride on the mucosal side. Colonic SCC of NaCl-loaded birds was only slightly reduced by amiloride (by 17%), but stimulated by amino acids (by 18%).

Body Temperature Variability in the Australian Water Rat, Hydromys chrysogaster, in Air and Water

Body and skin temperatures were recorded from water rats exposed to a range of air temperatures and also immersed in water at various temperatures. They were able to maintain stable core temperatures (mean 36.1�C, SD 0.65, n=61) at air temperatures up to 30�C. At 35�C they were unable to avoid hyperthermia despite their use of saliva spreading to enhance heat loss. Regional heterothermia was observed both in air and in water, but water rats were unable to maintain deep body temperatures at water temperatures lower than 25�C. Variations in the level of activity in cold water affected the rate of heat loss. Examination of the vascular system revealed the presence of various networks adapted for heat loss, but no major heat-conserving vascular retia. It is suggested that the poor thermoregulatory performance of water rats during aquatic excursions is largely due to the lack of heat-conserving retia, and to the poor insulative capacity of the fur when wet. The platypus, which possesses excellent fur insulation and a highly developed heat-conserving system, is a very competent homeotherm in cold water. The behavioural responses displayed by water rats which enable them to exploit the aquatic environment in south-eastern Australia are discussed.

Standard Metabolism of Monotremes and the Evolution of Homeothermy

The standard metabolic rates (SMR) of species from all extant monotreme genera were determined together with body temperatures. The echidnas, Zaglossus bruijni and Tachyglossus aculeatus, of the Family Tachyglossidae, both had an SMR 25-30% of predicted eutherian levels. The platypus Ornithorhynchus anatinus, however, had a much higher SMR. The weight-independent values (watts, kg*-075) were 0.86, 0.98 and 2.21 respectively for Z. bruijni, T. aculeatus and 0. anatinus. Body temperatures were similar, however, with means in the range 31.3-32�C. The data are suggested to support the idea of a gradual evolution in metabolic capability rather than marked jumps in metabolic levels.

Comparison of the Diets of Yellow-Footed Rock-Wallabies and Sympatric Herbivores in Western New South Wales

Diet of the rare yellow-footed rock wallaby (Petrogale xanthopus) was found by study of faeces along the escarpment of the Coturaundee Range in western New South Wales, Australia. Normally 42 to 52% of the diet (estimated as the occurrence of recognizable particles within a microscope field) was of forbs, mostly small herbaceous ephemeral species. During drought the most important component was browse, 44% of diet. Grass was important. There was considerable overlap of species eaten by the rock wallabies and the introduced goats and rabbits, 75% in the case of goats during drought; rabbits were not seen in the higher parts of the escarpment. There was less competition from the euros (Macropus robustus). Control of introduced herbivores may be essential if the rock wallaby populations are to have a chance of recovery.

Kidney structure and function of desert kangaroos

The structure and function of the kidneys of two species of desert kangaroos, the red kangaroo (Megaleia rufa) and the euro (Macropus robustus), were examined. Both kangaroos had glomerular filtration rates (GFR), renal plasma flows, and urine flow rates, when hydrated and dehydrated, which were lower than those of similarly sized eutherian mammals. The differences observed between the two species of marsupials were both structural and functional. The GFR of the red kangaroo was higher than that of the euro, under hydrated and dehydrated conditions, and this was correlated with the occurrence of larger and more numerous glomeruli, particularly juxtamedullary glomeruli, in the red kangaroo. Although the kidney of the euro had a greater relative medullary thickness than that of the red kangaroo, the latter had better urine-concentrating abilities. As opposed to this the euros reabsorbed significantly more urea from the filtrate when dehydrated (89.0%) than did the red kangaroos (69.2%). This ability of the euro to resorb more urea may be related to their tendency to overgraze their restricted home ranges during drought.

Seasonal Changes in Diet Preferences of Free-Ranging Red Kangaroos, Euros and Sheep in Western New South Wales

Stomach contents of free-ranging animals were analysed botanically for low Atriplex shrubland in western New South Wales, Australia, during 1972-74 in good seasons producing abundant growth and diversity of vegetation. Red kangaroo (Megaleia rufa), euro (Macropus robustus) and sheep (Ovis aries) selected grass and forbs when those were readily available. In poorer pasture sheep selected mainly flat-leaved chenopods (saltbush) and kangaroos selected mainly grass with different amounts of flat- or round-leaved chenopods. Euros were the most selective, eating grass even when there was little grass present. Potential overlaps in diet between kangaroos and sheep were greatest in good pasture and least in the poorest conditions. The other groups of plants considered were non-chenopod shrubs and browse. Extent of overlap was not clear, because animals may have eaten different species within the groups of plants. The study period did not include any severe drought, in which overlap in diet and competition between animal species would have been most significant.

Effects of dehydration on body-water distribution in desert kangaroos

The effect of dehydration on the distribution of water in the bodies of two species of desert kangaroos, the red kangaroo Megaleia rufa and the euro Macropus robustus, has been examined. The volumes of various body-fluid compartments were determined in normally hydrated animals and then after the kangaroos had been dehydrated until body weight declined to 80% of the initial weight. The fluid compartments examined were total body water, plasma volume, intracellular volume (cellular and gut water), and extracellular volume. Both species were camel-like in their response to dehydration in that plasma volume was maintained in both species, falling by only 8.3% in red kangaroos and 7.4% in euros. The pattern of water loss from other compartments differed between species, particularly gut water loss. This compartment, which includes the large rumenlike fore stomach, contributed 56% of the total water loss of red kangaroos but only 22% of the loss from euros. The ecological implications of the preferential maintenance of gut water by the sedentary, cave-dwelling euros have been discussed.

Comparative metabolism of tritiated water by macropodid marsupials

The total body-water content (TBW) and rate of water turnover were measured usingtritiated water in five species of macropodod marsupials (kangaroos), which ranged in weight from 1 to 50 kg. Animals fitted with rumen cannulas were used to estimate the time required for tritiated water to equilibrate within the body of large kangaroos. In hydrated kangaroos this was 6 h, during the time 2.7% of the injected tritiated waterwas lost from the body. During dehydration, the equilibrium time was extended to 10h. Values up to 78% of body weight were found for TBW in the larger species of kangaroo, and these values were similiar to those found for other ruminantlike mammals, particularly those with a low body-fat content. The smaller macropodids had a TBW (about 60% of body weight) similiar to that of most laboratory mammals. The rates of waterturnover of the macropodids were related to body weights by the expression 1/day = 0.09kg-0.80. Macropodid marsupials have a daily water usage shich is about two-thirds ofthat found for eitherians and this may be related to the lower metabolic rate of marsupials.

Role of the sweating from the tail in the thermal balance of the rat-kangaroo Potorous tridactylus

Among the marsupials the thermoregulatory response of sweating is uncommon and has only been described in the larger macropodids. Sweating in kangaroos is very unusual in that it only occurs in response to an exercise heat load. The thermoregulatory responses of a smaller, more generalized rat-kangaroo Potorous tridactylus were therefore examined to obtain a more general appreciation of sweating in macropodids. The pattern of heat balance at low and neutral temperatures was characteristic of that previously found for macropodids; body temperature was 35.9 � 0.52 (mean � se). Standard metabolism was only slightly higher than the predicted level for marsupials and minimal conductance was low, c. 1.3 W m-2 per degree Celsius. At moderate air temperatures heat was primarily lost by vasodilation and panting. The thermoregulatory responses at high air temperatures (near or above body temperature) also included copious sweating from the tail, but not from the body generally. Sweating rates of 600-650 g water per m2 per hour were obtained; these are about twice the generally reported rates for eutherians such as cows and horses.

Likely effects of standing and lying on the radiating heat load experienced by a resting kangaroo on a summer day

A study was carried out to examine the effect of posture (standing or lying) on the radiation heat load which would be experienced by a kangaroo resting under a small desert tree during a summer day. Measurements were made to assess the temperature and radiation characteristics in three situations judged to be equivalent to the following: (1) a kangaroo standing in the sun in open country; (2) an animal standing in the shade of a small tree; (3) an animal lying in the same shade. The overall pattern of results was shown by the effective radiation temperature measurements made at midday: 78.3, 57.6, and 52.5C for positions (I), (2), and (3) respectively. The relative contributions of solar and long-wave infrared radiation to the pattern of results are discussed.

Seasonal variation in the plasma and the urine electrolyte concentration of the arid zone kangaroos Megaleia rufa and the Macropus robustus

The seasonal variation in the plasma and urine electrolyte concentration and osmolality of the arid zone kangaroos Megaleia rufa (red kangaroo) and Macropus robustus (euro) has been examined. The urine osmolality of both species appeared to be influenced by environmental temperatures and also the status of feed. The osmolality of urine from red kangaroos was generally higher than that from euros. However, the relative medullary thickness of euro kidneys was greater than that of red kangaroo kidneys, indicating that euros may have the greater urine-concentrating capability. The pattern of sodium and potassium excretion indicated that species of Atriplex (saltbush) and Kochia (bluebush), which are high in sodium relative to potassium, may comprise a major part of the diet of the red kangaroo and euro in western New South Wales.