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Respiratory gas exchange in avian embryos progresses through three stages inside the egg. During the first 3-5 days of incubation, the chicken embryo has no specialised respiratory organs and is not reliant on blood circulation. At this stage, it obtains oxygen mainly by diffusion through the eggshell, albumen, amniotic fluid and embryonic tissues. In the second stage, gas exchange relies on diffusion through the shell in the gas phase and convection by blood circulation through the chorioallantoic membrane and body. Day 19 starts the third stage, the transition from chorioallantoic to pulmonary gas exchange, which is complete when the chick hatches on day 20. Metabolism is thought to be aerobic throughout incubation, although the early embryo is covered by fluids (albumen and amniotic fluid) which would greatly resist oxygen diffusion. This study uses fibre-optic sensors to measure oxygen partial pressure (PO 2 ) near, and inside of, the embryo during days 3-5, and relates the data to total body lactate levels. The study shows that fluids surrounding the embryo greatly impede oxygen diffusion, with PO 2 becoming severely hypoxic near the embryo, occasionally almost anoxic inside it. Meanwhile, lactate rises to high levels, and the stored lactate can be later oxidised by the embryo when the chorioallantois takes over and metabolism becomes entirely aerobic., (© 2021. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

The relationship between body mass and the respiratory microenvironment of burrowing animals is examined using artificial burrows containing surrogate animals that simulate O 2 consumption by removal of air and simultaneous replacement with N 2 . Allometric relationships between body mass and burrow radius, nest chamber radius, and O 2 consumption rate show that published mathematical predictions of diffusion-mediated gas exchange are adequate to describe the respiratory environments of animals in small blind-ending burrows through porous substrata. Diffusion is sufficient to ventilate burrows containing small mammals weighing less than 340 g, or subterranean nest chambers connected to the surface by one or more tunnels containing mammals weighing less than 30 kg. Outside of these limits, convection prevails and prevents the development of hypoxic conditions, particularly in burrows of mammals weighing more than 1300 g., (© 2021. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

As an animal grows, the relative sizes of their organs may grow proportionately or disproportionately, depending on ontogenetic changes in function. If organ growth is proportional (isometric), then the exponent of the scaling equation is 1.0. Relative decreases or increases in size result in exponents less than 1 (hypoallometric) or greater than 1 (hyperallometric). Thus, the empirical exponent can indicate potential changes in function. The entire gastrointestinal tract (GIT) of the foregut-fermenting western grey kangaroo Macropus fuliginosus melanops exhibited biphasic allometry across five orders of magnitude body mass (M b ; 52.0 g-70.5 kg). Prior to weaning at around 12 kg M b , the entire empty GIT mass scaled with hyperallometry (M b 1.13 ), shifting to hypoallometry (M b 0.80 ) post-weaning. In addition, there were varying patterns of hyper-, hypo-, and isometric scaling for select GIT organs, with several displaying phase shifts associated with major life-history events, specifically around exit from the maternal pouch and around weaning. Mass of the kangaroo forestomach, the main fermentation site, scaled with hyperallometry (M b 1.16 ) before the stage of increased vegetation intake, and possibly after this stage (M b 1.12 ; P = 0.07), accompanied by a higher scaling factor (elevation of the curve) probably associated with more muscle for processing fibrous vegetation. The acid hindstomach mass showed hyperallometry (M b 1.15 ) before weaning, but hypoallometry (M b 0.58 ) post-weaning, presumably associated with decreasing intake of milk. Small intestine mass and length each scaled isometrically throughout ontogeny, with no discernible breakpoints at any life-history stage. The caecum and colon mass were steeply hyperallometric early in-pouch life (M b 1.59-1.66 ), when the young were ectothermic, hairless, and supported solely by milk. After around 295 g M b , caecum mass remained hyperallometric (M b 1.14 ), possibly supporting its early development as a nidus for microbial populations to provide for secondary fermentation in this organ after the young transition from milk to vegetation.

Chemical immobilization is necessary for the physiological study of large wild animals. However, the immobilizing drugs can adversely affect the cardiovascular and respiratory systems, yielding data that do not accurately represent the normal, resting state. We hypothesize that these adverse effects can be ameliorated by reversing the immobilizing agent while holding the animal under general anaesthesia. We used habituated sheep Ovis aries (N = 5, 46.9 ± 5.3 kg body mass, mean ± SEM) and goats Capra hircus (N = 4, 27.7 ± 2.8 kg) as ungulate models for large wild animals, and measured their cardiorespiratory function under three conditions: (1) mild sedation (midazolam), as a proxy for the normal resting state, (2) immobilization (etorphine and azaperone), and (3) general anaesthesia (propofol) followed by etorphine antagonism (naltrexone). Cardiac output for both sheep and goats remained unchanged across the three conditions (overall means of 6.2 ± 0.9 and 3.3 ± 0.3 L min -1 , respectively). For both sheep and goats, systemic and pulmonary mean arterial pressures were significantly altered from initial midazolam levels when administered etorphine + azaperone, but those arterial pressures were restored upon transition to propofol anaesthesia and antagonism of the etorphine. Under etorphine + azaperone, minute ventilation decreased in the sheep, though this decrease was corrected under propofol, while the minute ventilation in the goats remained unchanged throughout. Under etorphine + azaperone, both sheep and goats displayed arterial blood hypoxia and hypercapnia (relative to midazolam levels), which failed to completely recover under propofol, indicating that more time might be needed for the blood gases to be adequately restored. Nonetheless, many of the confounding cardiorespiratory effects of etorphine were ameliorated when it was antagonized with naltrexone while the animal was held under propofol, indicating that this procedure can largely restore the cardiovascular and respiratory systems closer to a normal, resting state.

Pollination biology is often associated with mutualistic interactions between plants and their animal pollen vectors, with energy rewards as the foundation for co-evolution. Energy is supplied as food (often nectar from flowers) or as heat (in sun-tracking or thermogenic plants). The requirements of pollinators for these resources depend on many factors, including the costs of living, locomotion, thermoregulation and behaviour, all of which are influenced by body size. These requirements are modified by the availability of energy offered by plants and environmental conditions. Endothermic insects, birds and bats are very effective, because they move faster and are more independent of environmental temperatures, than are ectothermic insects, but they are energetically costly for the plant. The body size of endothermic pollinators appears to be influenced by opposing requirements of the animals and plants. Large body size is advantageous for endotherms to retain heat. However, plants select for small body size of endotherms, as energy costs of larger size are not matched by increases in flight speed. If high energy costs of endothermy cannot be met, birds and mammals employ daily torpor, and large insects reduce the frequency of facultative endothermy. Energy uptake can be limited by the time required to absorb the energy or eliminate the excess water that comes with it. It can also be influenced by variations in climate that determine temperature and flowering season.

Standard metabolic rate (SMR, ml O2 min(-1)) of captive Crocodylus porosus at 30 °C scales with body mass (kg) according to the equation, SMR = 1.01 M(0.829), in animals ranging in body mass of 3.3 orders of magnitude (0.19-389 kg). The exponent is significantly higher than 0.75, so does not conform to quarter-power scaling theory, but rather is likely an emergent property with no single explanation. SMR at 1 kg body mass is similar to the literature for C. porosus and for alligators. The high exponent is not related to feeding, growth, or obesity of captive animals. The log-transformed data appear slightly curved, mainly because SMR is somewhat low in many of the largest animals (291-389 kg). A 3-parameter model is scarcely different from the linear one, but reveals a declining exponent between 0.862 and 0.798. A non-linear model on arithmetic axes overestimates SMR in 70% of the smallest animals and does not satisfactorily represent the data.

This study uses stereological techniques to examine body, internal gill and cardiovascular morphology of two larval amphibians, Pseudophryne bibronii and Crinia georgiana, to evaluate the roles of diffusive and convective gas exchange. Gosner stage 27 specimens were prepared for light microscopy and six parallel sections of equal distance taken through the body as well as a further six through the heart and internal gills. Body, internal gill and heart volume as well as body and internal gill surface areas were determined. The harmonic mean distance across the internal gills was also measured and used to estimate oxygen diffusive conductance, DO₂. The species were of similar body size and surface area, but the heart and internal gills were larger in P. bibronii, which may represent precursors for greater growth of the species beyond stage 27. The much larger surface area of the skin compared to the internal gills in both species suggests it is the main site for gas exchange, with the gills supplementing oxygen uptake. The sparse cutaneous capillary network suggests diffusion is the main oxygen transport mechanism across the skin and directly into deeper tissues. A numerical model that simplifies larval shape, and has an internal (axial vessels) and external oxygen source, confirms that diffusion is able to maintain tissue oxygen with limited convective input.

The effects of oxygen partial pressure ([Formula: see text]) on development and respiration were investigated in the eggs of the Australian lungfish, Neoceratodus forsteri. At 20°C, embryonic survival and development was optimal at 15 and 20.9 kPa. Development was slowed at 5 and 10 kPa and embryos did not survive 2 kPa. At lower [Formula: see text], the rate of oxygen consumption also decreased. Embryos responded to hypoxia by hatching at an earlier age and stage of development, and hatching wet and dry gut-free masses were reduced. The role of oxygen conductance ([Formula: see text]) in gas exchange was also examined under selected environmental [Formula: see text] and temperatures. The breakdown of the vitelline membrane changed capsule geometry, allowed water to be absorbed into the perivitelline space and increased capsule [Formula: see text]. This occurred at embryonic stage 32 under all treatments and was largely independent of both [Formula: see text] and temperature (15, 20 and 25°C), demonstrating that capsule [Formula: see text] cannot adaptively respond to altered environmental conditions. The membrane breakdown increased capsule diffusive [Formula: see text] and stabilised perivitelline [Formula: see text], but reduced the convective [Formula: see text] of the perivitelline fluid, as the large perivitelline volume and inadequate convective current resulted in a [Formula: see text] gradient within the egg prior to hatch.

The rate of oxygen consumption throughout embryonic development is used to indirectly determine the 'cost' of development, which includes both differentiation and growth. This cost is affected by temperature and the duration of incubation in anamniote fish and amphibian embryos. The influences of temperature on embryonic development rate, respiration rate and energetics were investigated in the Australian lungfish, Neoceratodus forsteri, and compared with published data. Developmental stage and oxygen consumption rate were measured until hatching, upon which wet and dry gut-free masses were determined. A measure of the cost of development, the total oxygen required to produce 1 mg of embryonic dry tissue, increased as temperature decreased. The relationship between the oxygen cost of development (C, ml mg(-1)) and dry hatchling mass (M, mg) in fishes and amphibians is described by C = 0.30 M(0.22 0.13 (95% CI)), r (2) = 0.52. The scaling exponent indicates that the cost of embryonic development increases disproportionally with increasing hatchling mass. At 15 and 20°C, N. forsteri cost of development is significantly lower than the regression mean for all species, and at 25°C is lower than the allometrically scaled data set. Unexpectedly, incubation of N. forsteri is long, despite natural development under relatively warm conditions, and may be related to a large genome size. The low cost of development may be associated with construction of a rather sluggish fish with a low capacity for aerobic metabolism. The metabolic rate is lower in N. forsteri hatchlings than in any other fishes or amphibians at the same temperature, which matches the extremely low aerobic metabolic scope of the juveniles.

The Pacific tarpon is an elopomorph teleost fish with an air-breathing organ (ABO) derived from a physostomous gas bladder. Oxygen partial pressure (PO(2)) in the ABO was measured on juveniles (238 g) with fiber-optic sensors during exposure to selected aquatic PO(2) and swimming speeds. At slow speed (0.65 BL s(-1)), progressive aquatic hypoxia triggered the first breath at a mean PO(2) of 8.3 kPa. Below this, opercular movements declined sharply and visibly ceased in most fish below 6 kPa. At aquatic PO(2) of 6.1 kPa and swimming slowly, mean air-breathing frequency was 0.73 min(-1), ABO PO(2) was 10.9 kPa, breath volume was 23.8 ml kg(-1), rate of oxygen uptake from the ABO was 1.19 ml kg(-1) min(-1), and oxygen uptake per breath was 2.32 ml kg(-1). At the fastest experimental speed (2.4 BL s(-1)) at 6.1 kPa, ABO oxygen uptake increased to about 1.90 ml kg(-1) min(-1), through a variable combination of breathing frequency and oxygen uptake per breath. In normoxic water, tarpon rarely breathed air and apparently closed down ABO perfusion, indicated by a drop in ABO oxygen uptake rate to about 1% of that in hypoxic water. This occurred at a wide range of ABO PO(2) (1.7-26.4 kPa), suggesting that oxygen level in the ABO was not regulated by intrinsic receptors.

Cold acclimation lowers the selected body temperature (Tb) in many ectothermic vertebrates. This change in behavioural thermoregulation is accompanied by an increase in the proportion of polyunsaturated fatty acids in tissues and cellular membranes. We investigated how diets containing different fatty acids, known to significantly alter the fatty acid composition of animal tissues and membranes, affect the selected Tb of the lizard Tiliqua rugosa. Lizards on a diet containing many polyunsaturated fatty acids (10% sunflower oil) showed a 3-5 degrees C decrease in Tb, whereas Tb in animals on a diet containing mainly saturated fatty acids (10% sheep fat) did not change. Our study suggests that the composition of dietary lipids influences thermoregulation in ectothermic vertebrates and may thus play a role in the seasonal adjustment of their physiology.