Centralized red muscle in Odontaspis ferox and the prevalence of regional endothermy in sharks

The order Lamniformes contains charismatic species such as the white shark Carcharodon carcharias and extinct megatooth shark Otodus megalodon, and is of particular interest given their influence on marine ecosystems, and because some members exhibit regional endothermy. However, there remains significant debate surrounding the prevalence and evolutionary origin of regional endothermy in the order, and therefore the development of phenomena such as gigantism and filter-feeding in sharks generally. Here we show a basal lamniform shark, the smalltooth sand tiger shark Odontaspis ferox, has centralized skeletal red muscle and a thick compact-walled ventricle; anatomical features generally consistent with regionally endothermy. This result, together with the recent discovery of probable red muscle endothermy in filter feeding basking sharks Cetorhinus maximus, suggests that this thermophysiology is more prevalent in the Lamniformes than previously thought, which in turn has implications for understanding the evolution of regional endothermy, gigantism, and extinction risk of warm-bodied shark species both past and present.

Reliability of the Enterprise Point-of-Care (EPOC) blood analyzer's calculated arterial oxygen-hemoglobin saturation in immobilized white rhinoceroses (Ceratotherium simum)

BACKGROUND

Enterprise Point-of-Care (EPOC) blood analysis is used routinely in wildlife veterinary practice to monitor blood oxygenation, but the reliability of the EPOC calculated arterial oxygen-hemoglobin saturation (cSaO2 ) has never been validated in the white rhinoceros (Ceratotherium simum), despite their susceptibility to hypoxemia during chemical immobilization.

OBJECTIVES

We aimed to evaluate the reliability of the EPOC cSaO2 by comparing it against arterial oxygen-hemoglobin saturation (SaO2 ) measured by a co-oximeter reference method in immobilized white rhinoceroses.

METHODS

Male white rhinoceroses in two studies (both n = 8) were immobilized by darting with different etorphine-based drug combinations, followed by butorphanol or saline (administered intravenously). Animals in both studies received oxygen via intranasal insufflation after 60 min. Blood samples were drawn, at predetermined time points, from a catheter inserted into the auricular artery and analyzed using the EPOC and a co-oximeter. Bland-Altman (to estimate bias and precision) and area root mean squares (ARMS) plots were used to determine the reliability of the EPOC cSaO2 compared with simultaneous co-oximeter SaO2 readings.

RESULTS

The rhinoceros were acidotic (pH of 7.3 ± 0.1 [mean ± standard deviation]), hypercapnic (PaCO2 of 73.7 ± 10.5 mmHg), and normothermic (body temperature of 37.4 ± 1.8°C). In total, 389 paired cSaO2 -SaO2 measurements were recorded (the cSaO2 ranged between 13.2% and 99.0%, and the SaO2 ranged between 11.8% and 99.9%). The EPOC cSaO2 readings were unreliable (inaccurate, imprecise, and poor ARMS) across the entire saturation range (bias -6%, precision 5%, and ARMS 8%).

CONCLUSIONS

The EPOC cSaO2 is unreliable and should not be used to monitor blood oxygenation in immobilized white rhinoceroses.

Scaling of ear morphology across 127 bird species and its implications for hearing performance

The dimensions of auditory structures among animals of varying body size can have implications for hearing performance. Larger animals often have a hearing range focused on lower frequencies than smaller animals, which may be explained by several anatomical mechanisms in the ear and their scaling relationships. While the effect of size on ear morphology and hearing performance has been explored in some mammals, anurans and lizards, much less is known about the scaling relationships for the single-ossicle, internally-coupled ears of birds. Using micro- and nano-CT scans of the tympanic middle and inner ears of 127 ecologically and phylogenetically diverse bird species, spanning more than 400-fold in head mass (2.3 to 950 g), we undertook phylogenetically-informed scaling analyses to test whether 12 morphological traits, of functional importance to hearing, maintain their relative proportions with increasing head mass. We then extended our analysis by regressing these morphological traits with measures of hearing sensitivity and range to better understand morphological underpinnings of hearing performance. We find that most auditory structures scale together in equal proportions, whereas columella length increases disproportionately. We also find that the size of several auditory structures is associated with increased hearing sensitivity and frequency hearing limits, while head mass did not explain these measures. Although both birds and mammals demonstrate proportional scaling between auditory structures, the consequences for hearing in each group may diverge due to unique morphological predictors of auditory performance.

Reliability of methods to determine cutaneous evaporative water loss rate in furred and fleeced mammals

We used a high-precision weighing system and flow-through respirometry to quantify cutaneous evaporative water loss rates in woolly sheep (wool thickness, ca. 6.5 cm) and haired goats (coat thickness, ca. 2.5 cm), while simultaneously recording parallel data obtained from (1) a flow-through ventilated capsule, (2) a closed hand-held electronic evaporimeter chamber, and (3) a closed colorimetric paper disc chamber. In comparison to the weighing system and respirometry, used here as a "gold standard" measure of cutaneous evaporative water loss rate, we found relatively good agreement with data obtained from the flow-through ventilated capsules. However, we found poor agreement with data obtained from the closed electronic evaporimeter chambers (underestimated by 60%, on average) and the closed colorimetric paper disc chambers (overestimated by 52%, on average). This deviation was likely associated with a requirement for shaved skin in the closed chamber methods. Our results therefore cast doubt on the validity of the closed chamber methods for measurement of cutaneous evaporative water loss rates in furred and fleeced mammals, and instead show that more accurate values can be obtained using flow-through ventilated capsules.

Relationship between capillaries, mitochondria and maximum power of the heart: a meta-study from shrew to elephant

This meta-study uses phylogenetic scaling models across more than 30 species, spanning five orders of magnitude in body mass, to show that cardiac capillary numerical density and mitochondrial volume density decrease with body mass raised to the -0.07 ± 0.03 and -0.04 ± 0.01 exponents, respectively. Thus, while an average 10 g mammal has a cardiac capillary density of approximately 4150 mm^-2 and a mitochondrial density of 33%, a 1 t mammal has considerably lower corresponding values of 1850 mm^-2 and 21%. These similar scaling trajectories suggest quantitative matching for the primary oxygen supply and oxygen consuming structures of the heart, supporting economic design at the cellular level of the oxygen cascade in this aerobic organ. These scaling trajectories are nonetheless somewhat shallower than the exponent of -0.11 calculated for the maximum external mechanical power of the cardiac tissue, under conditions of heavy exercise, when oxygen flow between capillaries and mitochondria is probably fully exploited. This mismatch, if substantiated, implies a declining external mechanical efficiency of the heart with increasing body mass, whereby larger individuals put more energy in but get less energy out, a scenario with implications for cardiovascular design, aerobic capacity and limits of body size.

Scaling at different ontogenetic stages: Gastrointestinal tract contents of a marsupial foregut fermenter, the western grey kangaroo Macropus fuliginosus melanops

Prominent ontogenetic changes of the gastrointestinal tract (GIT) should occur in mammals whose neonatal diet of milk differs from that of adults, and especially in herbivores (as vegetation is particularly distinct from milk), and even more so in foregut fermenters, whose forestomach only becomes functionally relevant with vegetation intake. Due to the protracted lactation in marsupials, ontogenetic differences can be particularly well investigated in this group. Here, we report body mass (BM) scaling relationships of wet GIT content mass in 28 in-pouch young (50 g to 3 kg) and 15 adult (16-70 kg) western grey kangaroos Macropus fuliginosus melanops. Apart from the small intestinal contents, in-pouch young and adults did not differ in the scaling exponents ('slope' in log-log plots) but did differ in the scaling factor ('intercept'), with an implied substantial increase in wet GIT content mass during the out-of-pouch juvenile period. In contrast to forestomach contents, caecum contents were elevated in juveniles still in the pouch, suggestive of fermentative digestion of milk and intestinal secretion residues, particularly in the caecum. The substantial increase in GIT contents (from less than 1 to 10-20% of BM) was associated mainly with the increase in forestomach contents (from 25 to 80% of total GIT contents) and a concomitant decrease in small intestine contents (from 50 to 8%), emphasizing the shifting relevance of auto-enzymatic and allo-enzymatic (microbial) digestion. There was a concomitant increase in the contents-to-tissue ratio of the fermentation chambers (forestomach and caecum), but this ratio generally did not change for the small intestine. Our study not only documents significant ontogenetic changes in digestive morpho-physiology, but also exemplifies the usefulness of intraspecific allometric analyses for quantifying these changes.

How dryland mammals will respond to climate change: the effects of body size, heat load and a lack of food and water

Mammals in drylands are facing not only increasing heat loads but also reduced water and food availability as a result of climate change. Insufficient water results in suppression of evaporative cooling and therefore increases in body core temperature on hot days, while lack of food reduces the capacity to maintain body core temperature on cold nights. Both food and water shortage will narrow the prescriptive zone, the ambient temperature range over which body core temperature is held relatively constant, which will lead to increased risk of physiological malfunction and death. Behavioural modifications, such as shifting activity between night and day or seeking thermally buffered microclimates, may allow individuals to remain within the prescriptive zone, but can incur costs, such as reduced foraging or increased competition or predation, with consequences for fitness. Body size will play a major role in predicting response patterns, but identifying all the factors that will contribute to how well dryland mammals facing water and food shortage will cope with increasing heat loads requires a better understanding of the sensitivities and responses of mammals exposed to the direct and indirect effects of climate change.

Functional morphology of the ankle extensor muscle-tendon units in the springhare Pedetes capensis shows convergent evolution with macropods for bipedal hopping locomotion

This study assesses the functional morphology of the ankle extensor muscle-tendon units of the springhare Pedetes capensis, an African bipedal hopping rodent, to test for convergent evolution with the Australian bipedal hopping macropods. We dissect and measure the gastrocnemius, soleus, plantaris, and flexor digitorum longus in 10 adult springhares and compare them against similar-sized macropods using phylogenetically informed scaling analyses. We show that springhares align reasonably well with macropod predictions, being statistically indistinguishable with respect to the ankle extensor mean weighted muscle moment arm (1.63 vs. 1.65 cm, respectively), total muscle mass (41.1 vs. 29.2 g), total muscle physiological cross-sectional area (22.9 vs. 19.3 cm² ), mean peak tendon stress (26.2 vs. 35.2 MPa), mean tendon safety factor (4.7 vs. 3.6), and total tendon strain energy return capacity (1.81 vs. 1.82 J). However, total tendon cross-sectional area is significantly larger in springhares than predicted for a similar-sized macropod (0.26 vs. 0.17 cm² , respectively), primarily due to a greater plantaris tendon thickness (0.084 vs. 0.048 cm² ), and secondarily because the soleus muscle-tendon unit is present in springhares but is vestigial in macropods. The overall similarities between springhares and macropods indicate that evolution has favored comparable lower hindlimb body plans for bipedal hopping locomotion in the two groups of mammals that last shared a common ancestor ~160 million years ago. The springhare's relatively thick plantaris tendon may facilitate rapid transfer of force from muscle to skeleton, enabling fast and accelerative hopping, which could help to outpace and outmaneuver predators.

Cerebral blood flow rates in recent great apes are greater than in Australopithecus species that had equal or larger brains

Brain metabolic rate (MR) is linked mainly to the cost of synaptic activity, so may be a better correlate of cognitive ability than brain size alone. Among primates, the sizes of arterial foramina in recent and fossil skulls can be used to evaluate brain blood flow rate, which is proportional to brain MR. We use this approach to calculate flow rate in the internal carotid arteries (Q˙ICA), which supply most of the primate cerebrum. Q˙ICA is up to two times higher in recent gorillas, chimpanzees and orangutans compared with 3-million-year-old australopithecine human relatives, which had equal or larger brains. The scaling relationships between Q˙ICA and brain volume (V_br) show exponents of 1.03 across 44 species of living haplorhine primates and 1.41 across 12 species of fossil hominins. Thus, the evolutionary trajectory for brain perfusion is much steeper among ancestral hominins than would be predicted from living primates. Between 4.4-million-year-old Ardipithecus and Homo sapiens, V_br increased 4.7-fold, but Q˙ICA increased 9.3-fold, indicating an approximate doubling of metabolic intensity of brain tissue. By contrast, Q˙ICA is proportional to V_br among haplorhine primates, suggesting a constant volume-specific brain MR.

Blood flow rate and wall shear stress in seven major cephalic arteries of humans

Blood flow rate ( Q ˙ ) in relation to arterial lumen radius (r_i ) is commonly modelled according to theoretical equations and paradigms, including Murray's Law ( Q ˙ ∝ r i 3 ) and da Vinci's Rule ( Q ˙ ∝ r i 2 ). Wall shear stress (τ) is independent of r_i with Murray's Law (τ ∝  r i 0 ) and decreases with da Vinci's Rule (τ ∝  r i - 1 ). These paradigms are tested empirically with a meta-analysis of the relationships between Q ˙ and r_i in seven major arteries of the human cephalic circulation from 19 imaging studies in which both variables were presented. The analysis shows that Q ˙ ∝ r i 2.16 and τ ∝  r i - 1.02 , more consistent with da Vinci's Rule than Murray's Law. This meta-analysis provides standard values for Q ˙ , r_i and τ in the human cephalic arteries that may be a useful baseline in future investigations. On average, the paired internal carotid arteries supply 75%, and the vertebral arteries supply 25%, of total brain blood flow. The internal carotid arteries contribute blood entirely to the anterior and middle cerebral arteries and also partly to the posterior cerebral arteries via the posterior communicating arteries of the circle of Willis. On average, the internal carotid arteries provide 88% of the blood flow to the cerebrum and the vertebral arteries only 12%.

Scaling of cardiac morphology is interrupted by birth in the developing sheep Ovis aries

Scaling of the heart across development can reveal the degree to which variation in cardiac morphology depends on body mass. In this study, we assessed the scaling of heart mass, left and right ventricular masses, and ventricular mass ratio, as a function of eviscerated body mass across fetal and postnatal development in Horro sheep Ovis aries (~50-fold body mass range; N = 21). Whole hearts were extracted from carcasses, cleaned, dissected into chambers and weighed. We found a biphasic relationship when heart mass was scaled against body mass, with a conspicuous 'breakpoint' around the time of birth, manifest not by a change in the scaling exponent (slope), but rather a jump in the elevation. Fetal heart mass (g) increased with eviscerated body mass (M_b , kg) according to the power equation 4.90 M_b ^{0.88 ± 0.26 (± 95% CI )} , whereas postnatal heart mass increased according to 10.0 M_b ^{0.88 ± 0.10} . While the fetal and postnatal scaling exponents are identical (0.88) and reveal a clear dependence of heart mass on body mass, only the postnatal exponent is significantly less than 1.0, indicating the postnatal heart becomes a smaller component of body mass as the body grows, which is a pattern found frequently with postnatal cardiac development among mammals. The rapid doubling in heart mass around the time of birth is independent of any increase in body mass and is consistent with the normalization of wall stress in response to abrupt changes in volume loading and pressure loading at parturition. We discuss variation in scaling patterns of heart mass across development among mammals, and suggest that the variation results from a complex interplay between hard-wired genetics and epigenetic influences.

Interspecific scaling of blood flow rates and arterial sizes in mammals

This meta-study investigated the relationships between blood flow rate (Q̇; cm³ s^-1), wall shear stress (τ_w; dyn cm^-2) and lumen radius (r _i; cm) in 20 named systemic arteries of nine species of mammals, ranging in mass from 23 g mice to 652 kg cows, at rest. In the dataset, derived from 50 studies, lumen radius varied between 3.7 µm in a cremaster artery of a rat and 11.2 mm in the aorta of a human. The 92 logged data points of [Formula: see text] and r _i are described by a single second-order polynomial curve with the equation: [Formula: see text] The slope of the curve increased from approximately 2 in the largest arteries to approximately 3 in the smallest ones. Thus, da Vinci's rule ([Formula: see text]) applies to the main arteries and Murray's law ([Formula: see text]) applies to the microcirculation. A subset of the data, comprising only cephalic arteries in which [Formula: see text] is fairly constant, yielded the allometric power equation: [Formula: see text] These empirical equations allow calculation of resting perfusion rates from arterial lumen size alone, without reliance on theoretical models or assumptions on the scaling of wall shear stress in relation to body mass. As expected, [Formula: see text] of individual named arteries is strongly affected by body mass; however, [Formula: see text] of the common carotid artery from six species (mouse to horse) is also sensitive to differences in whole-body basal metabolic rate, independent of the effect of body mass.

Ameliorating the adverse cardiorespiratory effects of chemical immobilization by inducing general anaesthesia in sheep and goats: implications for physiological studies of large wild mammals

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.

Flight metabolic rate of Locusta migratoria in relation to oxygen partial pressure in atmospheres of varying diffusivity and density

Flying insects have the highest mass-specific metabolic rate of all animals. Oxygen is supplied to the flight muscles by a combination of diffusion and convection along the internal air-filled tubes of the tracheal system. This study measured maximum flight metabolic rate (FMR) during tethered flight in the migratory locust Locusta migratoria under varying oxygen partial pressure (P_O2 ) in background gas mixtures of nitrogen (N₂), sulfur hexafluoride (SF₆) and helium (He), to vary O₂ diffusivity and gas mixture density independently. With N₂ as the sole background gas (normodiffusive-normodense), mass-independent FMR averaged 132±19 mW g^-0.75 at normoxia (P_O2 =21 kPa), and was not limited by tracheal system conductance, because FMR did not increase in hyperoxia. However, FMR declined immediately with hypoxia, oxy-conforming nearly completely. Thus, the locust respiratory system is matched to maximum functional requirements, with little reserve capacity. With SF₆ as the sole background gas (hypodiffusive-hyperdense), the shape of the relationship between FMR and P_O2  was similar to that in N₂, except that FMR was generally lower (e.g. 24% lower at normoxia). This appeared to be due to increased density of the gas mixture rather than decreased O₂ diffusivity, because hyperoxia did not reverse it. Normoxic FMR was not significantly different in He-SF₆ (hyperdiffusive-normodense) compared with the N₂ background gas, and likewise there was no significant difference between FMR in SF₆-He (normodiffusive-hyperdense) compared with the SF₆ background gas. The results indicate that convection, not diffusion, is the main mechanism of O₂ delivery to the flight muscle of the locust when demand is high.

Femoral bone perfusion through the nutrient foramen during growth and locomotor development of western grey kangaroos (Macropus fuliginosus)

The nutrient artery passes through the nutrient foramen on the shaft of the femur and supplies more than half of the total blood flow to the bone. Assuming that the size of the nutrient foramen correlates with the size of the nutrient artery, an index of blood flow rate (Q_i) can be calculated from nutrient foramen dimensions. Interspecific Q_i is proportional to locomotor activity levels in adult mammals, birds and reptiles. However, no studies have yet estimated intraspecific Q_i to test for the effects of growth and locomotor development on bone blood flow requirements. In this study, we used micro-CT and medical CT scanning to measure femoral dimensions and foramen radius to calculate femoral Q_i during the in-pouch and post-pouch life stages of western grey kangaroos (Macropus fuliginosus) weighing 5.7 g to 70.5 kg and representing a 12,350-fold range in body mass. A biphasic scaling relationship between Q_i and body mass was observed (breakpoint at ca. 1-5 kg body mass right before permanent pouch exit), with a steep exponent of 0.96±0.09 (95% CI) during the in-pouch life stage and a statistically independent exponent of -0.59±0.90 during the post-pouch life stage. In-pouch joeys showed Q_i values that were 50-100 times higher than those of adult diprotodont marsupials of the same body mass, but gradually converged with them as post-pouch adults. Bone modelling during growth appears to be the main determinant of femoral bone blood flow during in-pouch development, whereas bone remodelling for micro-fracture repair due to locomotion gradually becomes the main determinant when kangaroos leave the pouch and become more active.

Scaling of morphology and ultrastructure of hearts among wild African antelope

The hearts of smaller mammals tend to operate at higher mass-specific mechanical work rates than those of larger mammals. The ultrastructural characteristics of the heart that allow for such variation in work rate still is largely unknown. We have used perfusion-fixation, transmission electron microscopy and stereology to assess the morphology and anatomical aerobic power density of the heart as a function of body mass across six species of wild African antelope differing by approximately 20-fold in body mass. The survival of wild antelope, as prey animals, depends on competent cardiovascular performance. We found that relative heart mass (g kg−1 body mass) decreases with body mass according to a power equation with an exponent of –0.12±0.07 (± 95% CI) (P=0.0027). Likewise, capillary length density (km cm−3 of cardiomyocyte), mitochondrial volume density (fraction of cardiomyocyte), and mitochondrial inner membrane surface density (m2 cm−3 of mitochondria) also decrease with body mass with exponents of –0.17±0.16 (P=0.039), –0.06±0.05 (P=0.018), and –0.07±0.05 (P=0.015), respectively, trends likely to be associated with the greater mass-specific mechanical work rates of the hearts in smaller antelope. Finally, we found proportionality between quantitative characteristics of a structure responsible for the delivery of oxygen (total capillary length) and those of a structure that ultimately uses that oxygen (total mitochondrial inner membrane surface area), which provides support for the economic principle of symmorphosis at the cellular level of the oxygen cascade in an aerobic organ.

The evolution of scale sensilla in the transition from land to sea in elapid snakes

Scale sensilla are small tactile mechanosensory organs located on the head scales of many squamate reptiles (lizards and snakes). In sea snakes and sea kraits (Elapidae: Hydrophiinae), these scale organs are presumptive scale sensilla that purportedly function as both tactile mechanoreceptors and potentially as hydrodynamic receptors capable of sensing the displacement of water. We combined scanning electron microscopy, silicone casting of the skin and quadrate sampling with a phylogenetic analysis to assess morphological variation in sensilla on the postocular head scale(s) across four terrestrial, 13 fully aquatic and two semi-aquatic species of elapids. Substantial variation exists in the overall coverage of sensilla (0.8-6.5%) among the species sampled and is broadly overlapping in aquatic and terrestrial lineages. However, two observations suggest a divergent, possibly hydrodynamic sensory role of sensilla in sea snake and sea krait species. First, scale sensilla are more protruding (dome-shaped) in aquatic species than in their terrestrial counterparts. Second, exceptionally high overall coverage of sensilla is found only in the fully aquatic sea snakes, and this attribute appears to have evolved multiple times within this group. Our quantification of coverage as a proxy for relative 'sensitivity' represents the first analysis of the evolution of sensilla in the transition from terrestrial to marine habitats. However, evidence from physiological and behavioural studies is needed to confirm the functional role of scale sensilla in sea snakes and sea kraits.

Scaling of the ankle extensor muscle-tendon units and the biomechanical implications for bipedal hopping locomotion in the post-pouch kangaroo Macropus fuliginosus

Bipedal hopping is used by macropods, including rat-kangaroos, wallabies and kangaroos (superfamily Macropodoidea). Interspecific scaling of the ankle extensor muscle-tendon units in the lower hindlimbs of these hopping bipeds shows that peak tendon stress increases disproportionately with body size. Consequently, large kangaroos store and recover more strain energy in their tendons, making hopping more efficient, but their tendons are at greater risk of rupture. This is the first intraspecific scaling analysis on the functional morphology of the ankle extensor muscle-tendon units (gastrocnemius, plantaris and flexor digitorum longus) in one of the largest extant species of hopping mammal, the western grey kangaroo Macropus fuliginosus (5.8-70.5 kg post-pouch body mass). The effective mechanical advantage of the ankle extensors does not vary with post-pouch body mass, scaling with an exponent not significantly different from 0.0. Therefore, larger kangaroos balance rotational moments around the ankle by generating muscle forces proportional to weight-related gravitational forces. Maximum force is dependent upon the physiological cross-sectional area of the muscle, which we found scales geometrically with a mean exponent of only 0.67, rather than 1.0. Therefore, larger kangaroos are limited in their capacity to oppose large external forces around the ankle, potentially compromising fast or accelerative hopping. The strain energy return capacity of the ankle extensor tendons increases with a mean exponent of ~1.0, which is much shallower than the exponent derived from interspecific analyses of hopping mammals (~1.4-1.9). Tendon safety factor (ratio of rupture stress to estimated peak hopping stress) is lowest in the gastrocnemius (< 2), and it decreases with body mass with an exponent of -0.15, extrapolating to a predicted rupture at 160 kg. Extinct giant kangaroos weighing 250 kg could therefore not have engaged in fast hopping using 'scaled-up' lower hindlimb morphology of extant western grey kangaroos.

Correction to 'Fossil skulls reveal that blood flow rate to the brain increased faster than brain volume during human evolution'

[This corrects the article DOI: 10.1098/rsos.160305.].

A structure-function analysis of the left ventricle

This study presents a structure-function analysis of the mammalian left ventricle and examines the performance of the cardiac capillary network, mitochondria, and myofibrils at rest and during simulated heavy exercise. Left ventricular external mechanical work rate was calculated from cardiac output and systemic mean arterial blood pressure in resting sheep (Ovis aries; n = 4) and goats (Capra hircus; n = 4) under mild sedation, followed by perfusion-fixation of the left ventricle and quantification of the cardiac capillary-tissue geometry and cardiomyocyte ultrastructure. The investigation was then extended to heavy exercise by increasing cardiac work according to published hemodynamics of sheep and goats performing sustained treadmill exercise. Left ventricular work rate averaged 0.017 W/cm³ of tissue at rest and was estimated to increase to ∼0.060 W/cm³ during heavy exercise. According to an oxygen transport model we applied to the left ventricular tissue, we predicted that oxygen consumption increases from 195 nmol O₂·s^-1·cm^-3 of tissue at rest to ∼600 nmol O₂·s^-1·cm^-3 during heavy exercise, which is within 90% of the oxygen demand rate and consistent with work remaining predominantly aerobic. Mitochondria represent 21-22% of cardiomyocyte volume and consume oxygen at a rate of 1,150 nmol O₂·s^-1·cm^-3 of mitochondria at rest and ∼3,600 nmol O₂·s^-1·cm^-3 during heavy exercise, which is within 80% of maximum in vitro rates and consistent with mitochondria operating near their functional limits. Myofibrils represent 65-66% of cardiomyocyte volume, and according to a Laplacian model of the left ventricular chamber, generate peak fiber tensions in the range of 50 to 70 kPa at rest and during heavy exercise, which is less than maximum tension of isolated cardiac tissue (120-140 kPa) and is explained by an apparent reserve capacity for tension development built into the left ventricle.

Fossil skulls reveal that blood flow rate to the brain increased faster than brain volume during human evolution

The evolution of human cognition has been inferred from anthropological discoveries and estimates of brain size from fossil skulls. A more direct measure of cognition would be cerebral metabolic rate, which is proportional to cerebral blood flow rate (perfusion). The hominin cerebrum is supplied almost exclusively by the internal carotid arteries. The sizes of the foramina that transmitted these vessels in life can be measured in hominin fossil skulls and used to calculate cerebral perfusion rate. Perfusion in 11 species of hominin ancestors, from Australopithecus to archaic Homo sapiens, increases disproportionately when scaled against brain volume (the allometric exponent is 1.41). The high exponent indicates an increase in the metabolic intensity of cerebral tissue in later Homo species, rather than remaining constant (1.0) as expected by a linear increase in neuron number, or decreasing according to Kleiber's Law (0.75). During 3 Myr of hominin evolution, cerebral tissue perfusion increased 1.7-fold, which, when multiplied by a 3.5-fold increase in brain size, indicates a 6.0-fold increase in total cerebral blood flow rate. This is probably associated with increased interneuron connectivity, synaptic activity and cognitive function, which all ultimately depend on cerebral metabolic rate.

Gas exchange and dive characteristics of the free-swimming backswimmer Anisops deanei

Many aquatic insects utilise air bubbles on the surface of their bodies to supply O2 while they dive. The bubbles can simply store O2, as in the case of an ‘air store’, or they can act as a physical ‘gas gill’, extracting O2 from the water. Backswimmers of the genus Anisops augment their air store with O2 from haemoglobin cells located in the abdomen. The O2 release from the haemoglobin helps stabilise bubble volume, enabling backswimmers to remain near neutrally buoyant for a period of the dive. It is generally assumed that the backswimmer air store does not act as a gas gill and that gas exchange with the water is negligible. This study combines measurements of dive characteristics under different exotic gases (N2, He, SF6, CO) with mathematical modelling, to show that the air store of the backswimmer Anisops deanei does exchange gases with the water. Our results indicate that approximately 20% of O2 consumed during a dive is obtained directly from the water. Oxygen from the water complements that released from the haemoglobin, extending the period of near-neutral buoyancy and increasing dive duration.

Scaling of cerebral blood perfusion in primates and marsupials

The evolution of primates involved increasing body size, brain size and presumably cognitive ability. Cognition is related to neural activity, metabolic rate and rate of blood flow to the cerebral cortex. These parameters are difficult to quantify in living animals. This study shows that it is possible to determine the rate of cortical brain perfusion from the size of the internal carotid artery foramina in skulls of certain mammals, including haplorrhine primates and diprotodont marsupials. We quantify combined blood flow rate in both internal carotid arteries as a proxy of brain metabolism in 34 species of haplorrhine primates (0.116-145 kg body mass) and compare it to the same analysis for 19 species of diprotodont marsupials (0.014-46 kg). Brain volume is related to body mass by essentially the same exponent of 0.70 in both groups. Flow rate increases with haplorrhine brain volume to the 0.95 power, which is significantly higher than the exponent (0.75) expected for most organs according to 'Kleiber's Law'. By comparison, the exponent is 0.73 in marsupials. Thus, the brain perfusion rate increases with body size and brain size much faster in primates than in marsupials. The trajectory of cerebral perfusion in primates is set by the phylogenetically older groups (New and Old World monkeys, lesser apes) and the phylogenetically younger groups (great apes, including humans) fall near the line, with the highest perfusion. This may be associated with disproportionate increases in cortical surface area and mental capacity in the highly social, larger primates.

Scaling of left ventricle cardiomyocyte ultrastructure across development in the kangaroo Macropus fuliginosus

The heart and left ventricle of the marsupial western grey kangaroo Macropus fuliginosus exhibit biphasic allometric growth, whereby a negative shift in the trajectory of cardiac growth occurs at pouch exit. In this study, we used transmission electron microscopy to examine the scaling of left ventricle cardiomyocyte ultrastructure across development in the western grey kangaroo over a 190-fold body mass range (0.355-67.5 kg). The volume-density (%) of myofibrils, mitochondria, sarcoplasmic reticuli and T-tubules increase significantly during in-pouch growth, such that the absolute volume (ml) of these organelles scales with body mass (Mb; kg) with steep hyperallometry: 1.41Mb (1.38), 0.64Mb (1.29), 0.066Mb (1.45) and 0.035Mb (1.87), respectively. Maturation of the left ventricle ultrastructure coincides with pouch vacation, as organelle volume-densities scale independent of body mass across post-pouch development, such that absolute organelle volumes scale in parallel and with relatively shallow hypoallometry: 4.65Mb (0.79), 1.75Mb (0.77), 0.21Mb (0.79) and 0.35Mb (0.79), respectively. The steep hyperallometry of organelle volumes and volume-densities across in-pouch growth is consistent with the improved contractile performance of isolated cardiac muscle during fetal development in placental mammals, and is probably critical in augmenting cardiac output to levels necessary for endothermy and independent locomotion in the young kangaroo as it prepares for pouch exit. The shallow hypoallometry of organelle volumes during post-pouch growth suggests a decrease in relative cardiac requirements as body mass increases in free-roaming kangaroos, which is possibly because the energy required for hopping is independent of speed, and the capacity for energy storage during hopping could increase as the kangaroo grows.

Burrowing energetics of the Giant Burrowing Cockroach Macropanesthia rhinoceros: an allometric study

Burrowing is an important life strategy for many insects, yet the energetic cost of constructing burrows has never been studied in insects of different sizes. Open flow respirometry was used to determine the allometric scaling of standard metabolic rate (MRS) and burrowing metabolic rate (MRB) in the heaviest extant cockroach species, the Giant Burrowing Cockroach Macropanesthia rhinoceros, at different stages of development. At 10 °C, MRS (mW) scales with body mass (M; g) according to the allometric power equation, MRS=0.158M(0.74), at 20 °C the equation is MRS=0.470M(0.53), and at 30 °C the equation is MRS=1.22M(0.49) (overall Q10=2.23). MRS is much lower in M. rhinoceros compared to other insect species, which is consistent with several aspects of their life history, including flightlessness, extreme longevity (>5 years), burrowing, parental behaviour, and an energy-poor diet (dry eucalypt leaf litter). Energy expenditure during burrowing at 25 °C scales according to MRB=16.9M(0.44), and is approximately 17 times higher than resting rates measured at the same temperature, although the metabolic cost over a lifetime is probably low, because the animal does not burrow to find food. The net cost of transport by burrowing (Jm(-1)) scales according to NCOT=120M(0.49), and reflects the energetically demanding task of burrowing compared to other forms of locomotion. The net cost of excavating the soil (J cm(-3)) is statistically independent of body size.

Blood flow for bone remodelling correlates with locomotion in living and extinct birds

Nutrient arteries enter limb bones through discrete foramina on the shafts. They are required for bone remodelling in response to mechanical loading and dynamic forces imposed by locomotion. The cross-sectional area of the nutrient foramen of the femur represents an index of blood flow rate to the shaft and thus provides insight into the animal's level of activity. Morphometric data on femoral length, mass and foramen size from 100 extant bird species and eight extinct moa species were analysed allometrically and phylogenetically. The nutrient foramen blood flow index (Qi) and femur mass (Mf) increase with body mass (Mb). At 1 kg body mass, cursorial species have approximately 2.1 times higher Qi and 1.9 times heavier Mf than volant species. The scaling of Qi on Mf is independent of the primary mode of locomotion, but the ratio Qi/Mf decreases significantly in larger birds, although absolute Qi increases. The overall avian equation for Qi on Mb is not significantly different from previous data from mammals, but when differences in blood pressure are accounted for, estimated blood flow to the femur is approximately 1.9 times higher in cursorial birds than in mammals, possibly in relation to bipedalism and quadrupedalism, respectively. Femoral bone blood flow in both endothermic groups is estimated to be 50-100 times higher than in ectothermic reptiles.

The effects of temperature and body mass on jump performance of the locust Locusta migratoria

Locusts jump by rapidly releasing energy from cuticular springs built into the hind femur that deform when the femur muscle contracts. This study is the first to examine the effect of temperature on jump energy at each life stage of any orthopteran. Ballistics and high-speed cinematography were used to quantify the energy, distance, and take-off angle of the jump at 15, 25, and 35°C in the locust Locusta migratoria. Allometric analysis across the five juvenile stages at 35°C reveals that jump distance (D; m) scales with body mass (M; g) according to the power equation D = 0.35M^{0.17±0.08 (95% CI)}, jump take-off angle (A; degrees) scales as A = 52.5M^0.00±0.06, and jump energy (E; mJ per jump) scales as E = 1.91M^1.14±0.09. Temperature has no significant effect on the exponent of these relationships, and only a modest effect on the elevation, with an overall Q₁₀ of 1.08 for jump distance and 1.09 for jump energy. On average, adults jump 87% farther and with 74% more energy than predicted based on juvenile scaling data. The positive allometric scaling of jump distance and jump energy across the juvenile life stages is likely facilitated by the concomitant relative increase in the total length (L_f+t; mm) of the femur and tibia of the hind leg, L_f+t = 34.9M^0.37±0.02. The weak temperature-dependence of jump performance can be traced to the maximum tension of the hind femur muscle and the energy storage capacity of the femur's cuticular springs. The disproportionately greater jump energy and jump distance of adults is associated with relatively longer (12%) legs and a relatively larger (11%) femur muscle cross-sectional area, which could allow more strain loading into the femur's cuticular springs. Augmented jump performance in volant adult locusts achieves the take-off velocity required to initiate flight.

Maximum metabolic rate, relative lift, wingbeat frequency and stroke amplitude during tethered flight in the adult locust Locusta migratoria

Flying insects achieve the highest mass-specific aerobic metabolic rates of all animals. However, few studies attempt to maximise the metabolic cost of flight and so many estimates could be sub-maximal, especially where insects have been tethered. To address this issue, oxygen consumption was measured during tethered flight in adult locusts Locusta migratoria, some of which had a weight attached to each wing (totalling 30-45% of body mass). Mass-specific metabolic rate increased from 28±2 μmol O(2) g(-1) h(-1) at rest to 896±101 μmol O(2)g(-1) h(-1) during flight in weighted locusts, and to 1032±69 μmol O(2) g(-1) h(-1) in unweighted locusts. Maximum metabolic rate of locusts during tethered flight (m(O(2)); μmol O(2) h(-1)) increased with body mass (M(b); g) according to the allometric equation m(O(2))=994M(b)(0.75±0.19), whereas published metabolic rates of moths and orchid bees during hovering free flight (h(O(2))) are approximately 2.8-fold higher, h(O(2))=2767M(b)(0.72±0.08). The modest flight metabolic rate of locusts is unlikely to be an artefact of individuals failing to exert themselves, because mean maximum lift was not significantly different from that required to support body mass (95±8%), mean wingbeat frequency was 23.7±0.6 Hz, and mean stroke amplitude was 105±5 deg in the forewing and 96±5 deg in the hindwing - all of which are close to free-flight values. Instead, the low cost of flight could reflect the relatively small size and relatively modest anatomical power density of the locust flight motor, which is a likely evolutionary trade-off between flight muscle maintenance costs and aerial performance.

A test of the oxidative damage hypothesis for discontinuous gas exchange in the locust Locusta migratoria

The discontinuous gas exchange cycle (DGC) is a breathing pattern displayed by many insects, characterized by periodic breath-holding and intermittently low tracheal O(2) levels. It has been hypothesized that the adaptive value of DGCs is to reduce oxidative damage, with low tracheal O(2) partial pressures (PO(2) ≈ 2-5 kPa) occurring to reduce the production of oxygen free radicals. If this is so, insects displaying DGCs should continue to actively defend a low tracheal PO(2) even when breathing higher than atmospheric levels of oxygen (hyperoxia). This behaviour has been observed in moth pupae exposed to ambient PO(2) up to 50 kPa. To test this observation in adult insects, we implanted fibre-optic oxygen optodes within the tracheal systems of adult migratory locusts Locusta migratoria exposed to normoxia, hypoxia and hyperoxia. In normoxic and hypoxic atmospheres, the minimum tracheal PO(2) that occurred during DGCs varied between 3.4 and 1.2 kPa. In hyperoxia up to 40.5 kPa, the minimum tracheal PO(2) achieved during a DGC exceeded 30 kPa, increasing with ambient levels. These results are consistent with a respiratory control mechanism that functions to satisfy O(2) requirements by maintaining PO(2) above a critical level, not defend against high levels of O(2).

Scaling of resting and maximum hopping metabolic rate throughout the life cycle of the locust Locusta migratoria

The hemimetabolous migratory locust Locusta migratoria progresses through five instars to the adult, increasing in size from 0.02 to 0.95 g, a 45-fold change. Hopping locomotion occurs at all life stages and is supported by aerobic metabolism and provision of oxygen through the tracheal system. This allometric study investigates the effect of body mass (Mb) on oxygen consumption rate (MO2, μmol h(-1)) to establish resting metabolic rate (MRO2), maximum metabolic rate during hopping (MMO2) and maximum metabolic rate of the hopping muscles (MMO2,hop) in first instar, third instar, fifth instar and adult locusts. Oxygen consumption rates increased throughout development according to the allometric equations MRO2=30.1Mb(0.83±0.02), MMO2=155Mb(1.01±0.02), MMO2,hop=120Mb(1.07±0.02) and, if adults are excluded, MMO2,juv=136Mb(0.97±0.02) and MMO2,juv,hop=103Mb(1.02±0.02). Increasing body mass by 20-45% with attached weights did not increase mass-specific MMO2 significantly at any life stage, although mean mass-specific hopping MO2 was slightly higher (ca. 8%) when juvenile data were pooled. The allometric exponents for all measures of metabolic rate are much greater than 0.75, and therefore do not support West, Brown and Enquist’s optimised fractal network model, which predicts that metabolism scales with a 3⁄4-power exponent owing to limitations in the rate at which resources can be transported within the body.

Symmorphosis and the insect respiratory system: a comparison between flight and hopping muscle

Weibel and Taylor's theory of symmorphosis predicts that the structural components of the respiratory system are quantitatively adjusted to satisfy, but not exceed, an animal's maximum requirement for oxygen. We test this in the respiratory system of the adult migratory locust Locusta migratoria by comparing the aerobic capacity of hopping and flight muscle with the morphology of the oxygen cascade. Maximum oxygen uptake by flight muscle during tethered-flight is 967 ± 76 μmol h-1 g-1 (body mass-specific, ± 95% CI), whereas the hopping muscles consume a maximum of 158 ± 8 during jumping. The 6.1-fold difference in aerobic capacity between the two muscles is matched by a 6.4-fold difference in tracheole lumen volume, which is 3.5×108 ± 1.2×108 μm3 g-1 in flight muscle and 5.5×107 ± 1.8×107 in the hopping muscles, a 6.4-fold difference in tracheole inner cuticle surface area, which is 3.2×109 ± 1.1×109 μm2 g-1 in flight muscle and 5.0×108 ± 1.7×108 in the hopping muscles, and a 6.8-fold difference in tracheole radial diffusing capacity, which is 113 ± 47 μmol kPa-1 h-1 g-1 in flight muscle and 16.7 ± 6.5 in the hopping muscles. However, there is little congruence between the 6.1-fold difference in aerobic capacity and the 19.8-fold difference in mitochondrial volume, which is 3.2×1010 ± 3.9×109 μm3 g-1 in flight muscle and only 1.6×109 ± 1.4×108 in the hopping muscles. Therefore, symmorphosis is upheld in the design of the tracheal system, but not in relation to the amount of mitochondria, which might be due to other factors operating on the molecular level.

Allometric scaling of discontinuous gas exchange patterns in the locust Locusta migratoria throughout ontogeny

The discontinuous gas exchange cycle (DGC) is a three-phase breathing pattern displayed by many insects at rest. The pattern consists of an extended breath-hold period (closed phase), followed by a sequence of rapid gas exchange pulses (flutter phase), and then by a period in which respiratory gases move freely between insect and environment (open phase). This study measured CO2 emission in resting locusts Locusta migratoria throughout ontogeny, in normoxia (21 kPa PO2), hypoxia (7 kPa PO2) and hyperoxia (40 kPa PO2), to determine whether body mass and ambient O2 affects DGC phase duration. In normoxia, mean CO2 production rate (MCO2; μmol h-1) scales with body mass (Mb; g) according to the allometric power equation, MCO2 = 9.9Mb0.95±0.09, closed phase duration (C; min) scales with body mass according to the equation, C = 18.0Mb0.38±0.29, closed+flutter period (C+F; min) scales with body mass according to the equation, C+F = 26.6Mb0.20±0.25, and open phase duration (O; min) scales with body mass according to the equation, O = 13.3Mb0.23±0.18. Hypoxia results in a shorter closed phase and longer open phase across all life stages, whereas hyperoxia elicits a shorter closed, closed+flutter, and open phase across all life stages. The tendency for larger locusts to exhibit both a longer closed, and closed+flutter period, might arise if the positive allometric scaling of locust tracheal volume prolongs the time taken to reach the minimum O2 and maximum CO2 set-points that determine the duration of these respective periods, whereas an increasingly protracted open phase could reflect the additional time required for larger locusts to expel CO2 through a relatively longer tracheal pathway. Observed changes in phase duration under hypoxia possibly serve to maximise O2 uptake from the environment, while the response of the DGC to hyperoxia is difficult to explain, but could be affected by elevated levels of reactive oxygen species.

Symmorphosis and the insect respiratory system: allometric variation

Taylor and Weibel's theory of symmorphosis predicts that structures of the respiratory system are matched to maximum functional requirements with minimal excess capacity. We tested this hypothesis in the respiratory system of the migratory locust, Locusta migratoria, by comparing the aerobic capacity of the jumping muscles with the morphology of the oxygen cascade in the hopping legs using an intraspecific allometric analysis of different body mass (Mb) at selected juvenile life stages. The maximum oxygen consumption rate of the hopping muscle during jumping exercise scales as Mb1.02±0.02, which parallels the scaling of mitochondrial volume in the hopping muscle, Mb1.02±0.08, and the total surface area of inner mitochondrial membrane, Mb0.99±0.10. Likewise, at the oxygen supply end of the insect respiratory system, there is congruence between the aerobic capacity of the hopping muscle and the total volume of tracheoles in the hopping muscle, Mb0.99±0.16, the total inner surface area of the tracheoles, Mb0.99±0.16, and the anatomical radial diffusing capacity of the tracheoles, Mb0.99±0.18. Therefore, the principles of symmorphosis are upheld at each step of the oxygen cascade in the respiratory system of the migratory locust.

Moulting of insect tracheae captured by light and electron-microscopy in the metathoracic femur of a third instar locust Locusta migratoria

The insect tracheal system is an air-filled branching network of internal tubing that functions to exchange respiratory gases between the tissues and the environment. The light and electron-micrographs presented in this study show tracheae in the process of moulting, captured from the metathoracic hopping femur of a juvenile third instar locust (Locusta migratoria). The images provide evidence for the detachment of the cuticular intima from the tracheal epithelial cells, the presence of moulting fluid between the new and old cuticle layers, and the withdrawal of the shed cuticular lining through larger upstream regions of the tracheal system during moulting. The micrographs also reveal that the cuticular intima of the fine terminal branches of the tracheal system is cast at ecdysis. Therefore, the hypothesis that tracheoles retain their cuticle lining at each moult may not apply to all insect species or developmental stages.