Patterns of oxygen consumption during simultaneously occurring elevated metabolic states in the viviparous snake Thamnophis marcianus

Ecological and life-history correlates of erythrocyte size and shape in Lepidosauria

Blood oxygen-carrying capacity is shaped both by the ambient oxygen availability as well as species-specific oxygen demand. Erythrocytes are a critical part of oxygen transport and both their size and shape can change in relation to species-specific life-history, behavioural or ecological conditions. Here, we test whether components of the environment (altitude), life history (reproductive mode, body temperature) and behaviour (diving, foraging mode) drive erythrocyte size variation in the Lepidosauria (lizards, snakes and rhynchocephalians). We collected data on erythrocyte size (area) and shape (L/W: elongation ratio) from Lepidosauria across the globe (N = 235 species). Our analyses show the importance of oxygen requirements as a driver of erythrocyte size. Smaller erythrocytes were associated with the need for faster delivery (active foragers, high-altitude species, warmer body temperatures), whereas species with greater oxygen demands (diving species, viviparous species) had larger erythrocytes. Erythrocyte size shows considerable cross-species variation, with a range of factors linked to the oxygen delivery requirements being major drivers of these differences. A key future aspect for study would include within-individual plasticity and how changing states, for example, pregnancy, perhaps alter the size and shape of erythrocytes in Lepidosaurs.

Lizards at the Peak: Physiological Plasticity Does Not Maintain Performance in Lizards Transplanted to High Altitude

Warming climates are facilitating the range expansion of many taxa to habitats that were formerly thermally inhospitable, including to higher latitudes and elevations. The potential for such colonization, however, varies widely among taxa. Because environmental factors may interact to affect colonization potential, an understanding of underlying physiological and behavioral mechanisms is necessary to predict how species will respond to potentially suitable habitats. For example, temperature and oxygen availability will interact to shape physiological and performance traits. Our model species, the wall lizard, Podarcis muralis, is a widely distributed ectotherm that continues to expand its range in Europe despite being limited by cold temperatures at high elevations and latitudes. To test the potential for organisms to expand to warming high-altitude environments, we conducted a transplant experiment to quantify the within-individual effects of high-altitude hypoxia on physiological and performance traits. Transplanted lizards maintained individual differences in physiological traits related to oxygen capacity and metabolism (hemoglobin concentration, hematocrit, and peak postexhaustion metabolic rate), as well as performance traits tied to fitness (sprint speed and running endurance). Although lizards altered blood biochemistry to increase oxygen-carrying capacity, their performance was reduced at high altitude. Furthermore, lizards at high altitude suffered a rapid loss of body condition over the 6-wk experiment, suggesting an energetic cost to hypoxia. Taken together, this demonstrates a limited potential for within-individual plasticity to facilitate colonization of novel high-altitude environments.

Resting vs. active: a meta-analysis of the intra- and inter-specific associations between minimum, sustained, and maximum metabolic rates in vertebrates

Variation in aerobic capacity has far reaching consequences for the physiology, ecology, and evolution of vertebrates. Whether at rest or active, animals are constrained to operate within the energetic bounds determined by their minimum (minMR) and sustained or maximum metabolic rates (upperMR). MinMR and upperMR can differ considerably among individuals and species but are often presumed to be mechanistically linked to one another. Specifically, minMR is thought to reflect the idling cost of the machinery needed to support upperMR. However, previous analyses based on limited datasets have come to conflicting conclusions regarding the generality and strength of their association.

Here we conduct the first comprehensive assessment of their relationship, based on a large number of published estimates of both the intra‐specific (n = 176) and inter‐specific (n = 41) phenotypic correlations between minMR and upperMR, estimated as either exercise‐induced maximum metabolic rate (VO₂max), cold‐induced summit metabolic rate (Msum), or daily energy expenditure (DEE).

Our meta‐analysis shows that there is a general positive association between minMR and upperMR that is shared among vertebrate taxonomic classes. However, there was stronger evidence for intra‐specific correlations between minMR and Msum and between minMR and DEE than there was for a correlation between minMR and VO₂max across different taxa. As expected, inter‐specific correlation estimates were consistently higher than intra‐specific estimates across all traits and vertebrate classes.

An interesting exception to this general trend was observed in mammals, which contrast with birds and exhibit no correlation between minMR and Msum. We speculate that this is due to the evolution and recruitment of brown fat as a thermogenic tissue, which illustrates how some species and lineages might circumvent this seemingly general association.

We conclude that, in spite of some variability across taxa and traits, the contention that minMR and upperMR are positively correlated generally holds true both within and across vertebrate species. Ecological and comparative studies should therefore take into consideration the possibility that variation in any one of these traits might partly reflect correlated responses to selection on other metabolic parameters.

A lay summary is available for this article.

Simultaneously Occurring Elevated Metabolic States Expose Constraints in Maximal Levels of Oxygen Consumption in the Oviparous Snake Lamprophis fuliginosus

African house snakes (Lamprophis fuliginosus) were used to compare the metabolic increments associated with reproduction, digestion, and activity both individually and when combined simultaneously. Rates of oxygen consumption ([Formula: see text]) and carbon dioxide production ([Formula: see text]) were measured in adult female (nonreproductive and reproductive) and adult male snakes during rest, digestion, activity while fasting, and postprandial activity. We also compared the endurance time (i.e., time to exhaustion) during activity while fasting and postprandial activity in males and females. For nonreproductive females and males, our results indicate that the metabolic increments of digestion (∼3-6-fold) and activity while fasting (∼6-10-fold) did not interact in an additive fashion; instead, the aerobic scope associated with postprandial activity was 40%-50% lower, and animals reached exhaustion up to 11 min sooner. During reproduction, there was no change in digestive [Formula: see text], but aerobic scope for activity while fasting was 30% lower than nonreproductive values. The prioritization pattern of oxygen delivery exhibited by L. fuliginosus during postprandial activity (in both males and females) and for activity while fasting (in reproductive females) was more constrained than predicted (i.e., instead of unchanged [Formula: see text], peak values were 30%-40% lower). Overall, our results indicate that L. fuliginosus's cardiopulmonary system's capacity for oxygen delivery was not sufficient to maintain the metabolic increments associated with reproduction, digestion, and activity simultaneously without limiting aerobic scope and/or activity performance.