Physiological vagility and its relationship to dispersal and neutral genetic heterogeneity in vertebrates

Vagility is the inherent power of movement by individuals. Vagility and the available duration of movement determine the dispersal distance individuals can move to interbreed, which affects the fine-scale genetic structure of vertebrate populations. Vagility and variation in population genetic structure are normally explained by geographic variation and not by the inherent power of movement by individuals. We present a new, quantitative definition for physiological vagility that incorporates aerobic capacity, body size, body temperature and the metabolic cost of transport, variables that are independent of the physical environment. Physiological vagility is the speed at which an animal can move sustainably based on these parameters. This meta-analysis tests whether this definition of physiological vagility correlates with empirical data for maximal dispersal distances and measured microsatellite genetic differentiation with distance {[F(ST)/[1-F(ST))]/ln distance} for amphibians, reptiles, birds and mammals utilizing three locomotor modes (running, flying, swimming). Maximal dispersal distance and physiological vagility increased with body mass for amphibians, reptiles and mammals utilizing terrestrial movement. The relative slopes of these relationships indicate that larger individuals require longer movement durations to achieve maximal dispersal distances. Both physiological vagility and maximal dispersal distance were independent of body mass for flying vertebrates. Genetic differentiation with distance was greatest for terrestrial locomotion, with amphibians showing the greatest mean and variance in differentiation. Flying birds, flying mammals and swimming marine mammals showed the least differentiation. Mean physiological vagility of different groups (class and locomotor mode) accounted for 98% of the mean variation in genetic differentiation with distance in each group. Genetic differentiation with distance was not related to body mass. The physiological capacity for movement (physiological vagility) quantitatively predicts genetic isolation by distance in the vertebrates examined.

Cardiorespiratory adjustments of homing pigeons to steady wind tunnel flight

We made detailed cardiorespiratory measurements from homing pigeons during quiet rest and steady wind tunnel flight. Our pigeons satisfied their 17.4-fold increase in oxygen consumption during flight with a 7.4-fold increase in cardiac output (Q) and a 2.4-fold increase in blood oxygen extraction. Q was increased primarily by increasing heart rate sixfold. Comparisons between our study and those from the only other detailed cardiorespiratory study on flying birds reveal a number of similarities and important differences. Although the avian allometric equations from this earlier study accurately predicted the flight Q of our pigeons, this was primarily due to due to compensating discrepancies in their heart rate and stroke volume predictions. Additionally, the measured heart mass (MH)-specific Q (Q/MH) of our pigeons during wind tunnel flight was about 22% lower than the estimated value. Compared to running mammals in previous studies, the 1.65-fold Q of our pigeons is consistent with their larger heart mass.

Functional and structural optimization of the respiratory system of the batTadarida brasiliensis(Chiroptera, Molossidae): does airway geometry matter?

We studied structure and function of the respiratory system in the bat Tadarida brasiliensis and compared it with those of two species of rodents, Abrothrix andinus and A. olivaceus. Tadarida brasiliensis had lower resting oxygen consumption, but higher maximum oxygen consumption and aerobic scope, than the rodents. The blood–gas barrier of the bat was thinner and its relative lung size was larger; however,alveolar surface density was similar among the three species. In consequence, T. brasiliensis has an oxygen diffusion capacity two or three times higher than that of the rodents. In Tadarida brasiliensis the characteristics of the lung were accompanied by geometrical changes in the proximal airway, such as high physical optimization as a consequence of small variations in the symmetry and the scaling ratio of the bronchial diameters. These may constitute an efficient way to save energy in respiratory mechanics and are the first report of airway adjustments to decrease entropy generation in bats.

Flight respiration and energetics

We use a comparative approach to examine some of the physiological traits that make flight possible. Comparisons of related fliers and runners suggest that fliers generally have higher aerobic metabolic capacities than runners but that the difference is highly dependent on the taxa studied. The high metabolic rates of fliers relative to runners, especially in insects, are correlated with high locomotory muscle cycle frequencies and low efficiencies of conversion of metabolic power to mechanical power. We examine some factors that produce variation in flight respiration and energetics. Air temperature strongly affects the flight metabolic rate of some insects and birds. Flight speed interacts with flier mass, so that small fliers tend to exhibit a J-shaped power curve and larger fliers a U-shaped power curve. As body size increases, mass-specific aerobic flight metabolism decreases in most studies, but mass-specific power output is constant or increases, leading to an increase in efficiency with size. Intraspecific studies have revealed specific genetically based effects on flight metabolism and power output and multiple ecological correlates of flight capabilities.

The development of the ventilatory response to cold in very young rats

To assess the range of functional responses of the ventilatory apparatus of developing rats and the degree to which ventilatory function is developed in advance of other functional characteristics, rat pups at five ages (between 4 and 20 days old) were exposed to temperatures of 28, 32 and 36 degrees C while in a flow through metabolic chamber modified to serve as a whole body plethysmograph. Ventilatory frequency, tidal volume and oxygen extraction 'efficiency' (EO2 = VO2/FEO2 x VI) were measured at each age and temperature. Mean breathing frequency at 4 days old was 2.56 breaths per second, decreasing to 1.99 at 20 days old. There was insignificant modification of breathing frequency with temperature. Four day old rat pups at 28 degrees C had mass specific tidal volumes of 0.017 ml/g, 142% of the value at 36 degrees C (0.012 ml/g). Twenty day old pups at 28 degrees C had mass specific tidal volumes of 0.027 ml/g, also 142% of the thermoneutral value (0.019 ml/g at 32 degrees C). At all ages, increases in tidal volumes were similar and increases in tidal volume were the only response to increased metabolic demand. Oxygen extraction 'efficiency' was about half that previously observed in adult rodents. These observations of ventilation during a cold challenge suggest that although structural development is not complete until much later, functional development is sufficient, either at birth or shortly thereafter.

Ventilatory and metabolic responses of a bat, Phyllostomus discolor, to hypoxia and CO2: implications for the allometry of respiratory control

The ventilatory and metabolic responses of lesser spear-nosed bats to hypoxia and hypercapnia were measured to determine whether these corresponded to preliminary allometries and a positive relationship between hypoxic ventilatory threshold and P50. Ventilatory responses of lesser spear-nosed bats to 3, 5 and 7% CO2 differed significantly from ventilation on air and each other. The magnitude of their ventilatory response to CO2 is consistent with the prediction of a smaller ventilatory response to hypercapnia in small compared to large mammals [% delta V varies MB0.130; Williams et al. (1994)]. Among 12, 10 and 8% O2 treatments only the ventilatory response to 8% O2 differed significantly from ventilation on air or the other treatments. Metabolic rate was significantly reduced at both 10 and 8% O2. The hypoxic ventilatory response of these bats does not support the prediction of a greater response in small compared to large mammals [% delta V veries MB0.273; Boggs and Tenney (1984)]. Their metabolic response is consistent with the hypoxic hypometabolism typical of small mammals, though not of comparable magnitude. The response, expressed as percent change in convection requirement (V/VO2), is also less than that observed in other small mammals. This relative insensitivity to hypoxia may be associated with this bat's unusually high affinity hemoglobin (P50 = 27.5 torr).

Ventilatory accommodation of changing oxygen demand in sciurid rodents

Ventilation was measured across a range of O2 consumption rates in four sciurid rodents: Tamias minimus (47 g), Spermophilus lateralis (189 g), S. beecheyi (531 g), and Marmota flaviventris juveniles (1054 g) and adults (2989 g). Maximum thermogenic oxygen consumption was measured for all but adult M. flaviventris. Aerobic scopes (maximum/minimum O2 consumption rates) were 4.6, 3.8, 5.4, and 4.8 in T. minimus, S. lateralis, S. beecheyi, and juvenile M. flaviventris, respectively. Aerobic scope was at least 4.1 in adult M. flaviventris. Ventilatory accommodation of changing O2 consumption rate was qualitatively similar in the four species, with the bulk of accommodation resulting from changes in minute volume. Nevertheless, there were significant differences in the relative importance of frequency, tidal volume, and O2 extraction in accommodation. In all species, frequency and minute volume were strongly correlated to O2 consumption rate. Tidal volume was significantly correlated to O2 consumption rate in T. minimus and S. beecheyi, but not in the other species. Oxygen extraction was not significantly correlated to O2 consumption rate in any species. Analysis of factorial ventilation changes across a standardized 3.8-fold change in O2 consumption rate revealed significant differences among species in frequency and O2 extraction, but not in tidal or minute volume. When compared to a generalized allometry for mammalian resting ventilation, the four sciurid species had consistently lower respiration frequency and higher O2 extraction than predicted, perhaps because the sciurid measurements were made on unrestrained animals. There was no indication that ventilation constrained maximum O2 consumption rate.

The functional significance of ventilation frequency, and its relationship to oxygen demand in the resting brown long-eared bat, Plecotus auritus

Mean oxygen consumption and simultaneous ventilation frequency of nine non-reproductive brown long-eared bats (body mass 8.53-13.33 g) were measured on 159 occasions. Ambient (chamber) temperature at which the measurements were made ranged from 10.8 to 41.1 degrees C. Apneic ventilation occurred in 22 of the 59 measurements made when mean oxygen consumption was less than 0.5 ml.min-1. No records of apneic ventilation were obtained when it was over 0.5 ml.min-1. The relationship between ventilation frequency and mean oxygen consumption depended on whether ventilation was apneic or non-apneic. When ventilation was non-apneic the relationship was positive and log-linear. When ventilation was apneic the relationship was log-log. Within the thermoneutral zone ventilation frequency was not significantly different from that predicted from allometric equations for a terrestrial mammal of equivalent body mass, but was significantly greater than that predicted for a bird. A reduction in the amount of oxygen consumed per breath occurred at ambient temperatures above the upper critical temperature (39 degrees C).

Temperature effects on metabolism, ventilation, and oxygen extraction in a Neotropical bat

We examined the relationship between ambient temperature (Ta), body temperature (Tb), oxygen consumption (VO2), carbon dioxide production (VCO2), evaporative water loss (mH2O), respiratory frequency (f), tidal volume (VT), minute volume (VI), and oxygen extraction (EO2) in the Neotropical bat Noctilio albiventris (mean mass 40 g). The factorial aerobic scope was 7.2 between Ta of 1-35 degrees C (VO2 = 0.119 and 0.0165 ml/(g.min), respectively). The respiratory exchange ratio (VCO2/VO2) did not change with Ta and mH2O was constant between Ta of 10-35 degrees C. Thermal conductance was minimal at 30 degrees C and constant and low at Ta less than 30 degrees C. Between 10 and 35 degrees C, Noctilio accommodated changing VO2 with parallel and roughly equivalent changes in f, VT, and EO2. The change in VO2 between 10 and 1 degrees C was accommodated mainly through changing f. Ventilation parameters in resting thermoneutral Noctilio are intermediate between allometric values for birds and mammals. Maximal EO2 in Noctilio (35-40%) is higher than for other mammals but considerably less than maximal EO2 in some birds.

Pulmonary design in a microchiropteran bat (Pipistrellus subflavus) during hibernation

The Eastern pipistrelle (Pipistrellus subflavus) is typical of exceptionally small bats capable of a 30-fold range in aerobic metabolism as they arouse from hypothermia and sustain foraging flight. This report describes their basic lung structure and the extent to which this organ is protected from protein depletion during hibernation. Bats were collected at the beginning (Fall), middle (Winter), and end (Spring) of hibernation from a permanent overwintering cave, and analyzed within several days of capture. Regardless of whether bats were examined in the Fall (average body weight = 6.22 g) or in the Spring (4.58 g) no significant differences existed for total lung volume (237 microliter), alveolar surface area (338 cm2), harmonic mean septal thickness, tau ht (0.221 micron), or membrane diffusing capacity (4.13 microliter O2/sec/mbar). These parameters exceed predictions based on body weights for either season, and resemble published data for another highly active mammalian group, the insectivorous shrews. Both tau ht and the minimal septal thickness of 0.083 micron approach the anatomical limits for thinning of alveolar septa without loss of epithelial continuity. Although both the heart and lungs lost 13% of their fresh weights during hibernation, compared to 25% for the liver, the lung contents of DNA (0.14 mg) and blood-free protein (7.38 mg) were not altered significantly. These small bats possess lungs which are well suited for the high aerobic cost of flight. Those lungs are resistant to hibernation-induced proteolysis, and also resistant to the deterioration of alveolar membranes which occurs in nonhibernators subjected to starvation-induced weight losses of similar magnitude.