Ecogeographical patterns describe predictable variation in phenotypic traits between ecological communities. For example, high-altitude animals are expected to show elevated hematological values as an adaptation to the lower oxygen pressure. Mountains act like ecological islands and therefore are considered natural laboratories. However, the majority of ecophysiological studies on blood traits lack replication that would allow us to infer if the pattern reported is a local event or whether it is a widespread pattern resulting from larger-scale ecological processes. In lizards, in fact, the increase of hematological values at high altitudes has received mixed support. Here, for the first time, we compare blood traits in lizards along elevational gradients with replication. We tested the repeatability of blood traits in mesquite lizards between different elevations in three different mountains from the Trans-Mexican Volcanic Belt. We measured hematocrit, hemoglobin concentration, mean corpuscular hemoglobin concentration, and erythrocyte size in blood samples of low, medium, and high-elevation lizards. We obtained similar elevational patterns between mountains, but the blood traits differed among mountains. Middle-altitude populations had greater oxygen-carrying capacity than lizards from low and high altitudes. The differences found between mountain systems could be the result of phenotypic plasticity or genetic differentiation as a consequence of abiotic factors not considered.

The ectothermic vertebrates are a diverse group that includes the Fishes (Agnatha, Chondrichthyes, and Osteichthyes), and the stem Tetrapods (Amphibians and Reptiles). From an evolutionary perspective, it is within this group that we see the origin of air-breathing and the transition from the use of water to air as a respiratory medium. This is accompanied by a switch from gills to lungs as the major respiratory organ and from oxygen to carbon dioxide as the primary respiratory stimulant. This transition first required the evolution of bimodal breathing (gas exchange with both water and air), the differential regulation of O2 and CO2 at multiple sites, periodic or intermittent ventilation, and unsteady states with wide oscillations in arterial blood gases. It also required changes in respiratory pump muscles (from buccopharyngeal muscles innervated by cranial nerves to axial muscles innervated by spinal nerves). The question of the extent to which common mechanisms of respiratory control accompany this progression is an intriguing one. While the ventilatory control systems seen in all extant vertebrates have been derived from common ancestors, the trends seen in respiratory control in the living members of each vertebrate class reflect both shared-derived features (ancestral traits) as well as unique specializations. In this overview article, we provide a comprehensive survey of the diversity that is seen in the afferent inputs (chemo and mechanoreceptor), the central respiratory rhythm generators, and the efferent outputs (drive to the respiratory pumps and valves) in this group. © 2022 American Physiological Society. Compr Physiol 12: 1-120, 2022.

High CO2 (hypercapnia) can impose significant physiological challenges associated with acid-base regulation in fishes, impairing whole animal performance and survival. Unlike other environmental conditions such as temperature and O2, the acute CO2 tolerance thresholds of fishes are not understood. While some fish species are highly tolerant, the extent of acute CO2 tolerance and the associated physiological and ecological traits remain largely unknown. To investigate this, we used a recently developed ramping assay, termed the Carbon Dioxide maximum (CDmax), that increases CO2 exposure until loss of equilibrium (LOE) is observed. We investigated if there was a relationship between CO2 tolerance and the Root effect, β-adrenergic sodium proton exchanger (βNHE), air-breathing, and fish habitat in 17 species. We hypothesized that CO2 tolerance would be higher in fishes that lack both a Root effect and βNHE, breathe air, and reside in tropical habitats. Our results showed that CDmax ranged from 2.7 to 26.7 kPa, while LOE was never reached in four species at the maximum PCO2 we could measure (26.7 kPa); CO2 tolerance was only associated with air-breathing, but not the presence of a Root effect or a red blood cell (RBC) βNHE, or fish habitat. This study demonstrates that the diverse group of fishes investigated here are incredibly tolerant of CO2 and that although this tolerance is associated with air-breathing, further investigations are required to understand the basis for CO2 tolerance.

Tracing the evolution of the central rhythm generators associated with ventilation in vertebrates is hindered by a lack of information surrounding key transitions. To begin with, central rhythm generation has been studied in detail in only a few species from four vertebrate groups, lamprey, anuran amphibians, turtles, and mammals (primarily rodents). Secondly, there is a lack of information regarding the transition from water breathing fish to air breathing amniotes (reptiles, birds, and mammals). Specifically, the respiratory rhythm generators of fish appear to be single oscillators capable of generating both phases of the respiratory cycle (expansion and compression) and projecting to motoneurons in cranial nerves innervating bucco-pharyngeal muscles. In the amniotes we find oscillators capable of independently generating separate phases of the respiratory cycle (expiration and inspiration) and projecting to pre-motoneurons in the ventrolateral medulla that in turn project to spinal motoneurons innervating thoracic and abdominal muscles (reptiles, birds, and mammals). Studies of the one group of amphibians that lie at this transition (the anurans), raise intriguing possibilities but, for a variety of reasons that we explore, also raise unanswered questions. In this review we summarize what is known about the rhythm generating circuits associated with breathing that arise from the different rhombomeric segments in each of the different vertebrate classes. Assuming oscillating circuits form in every pair of rhombomeres in every vertebrate during development, we trace what appears to be the evolutionary fate of each and highlight the questions that remain to be answered to properly understand the evolutionary transitions in vertebrate central respiratory rhythm generation.

August Krogh's 1929 principle is referenced as the cornerstone of comparative physiology (CP). However, there are diverse views as to what type of research falls under the CP approach. This study had three aims: 1) determine how CP is defined through an online survey (OS) of physiologists and a systematic review (SR), 2) put forth an updated definition of CP by summarizing OS and SR results, and 3) outline the numerous CP research approaches. Professional physiology societies (n = 54) were invited to share the OS with their members, and a SR was conducted, which yielded 197 and 70 definitions, respectively. The three most common words in descending order in the OS definitions were "different," "animals," and "species" and in the SR definitions, "animals," "species," and "organisms." The three most prevalent themes from the OS and SR definitions were comparing/differences/diversity across species (78% and 51%, respectively), response to the environment/ecology (28% and 43%, respectively), and included evolution or adaptation (24% and 60%, respectively). Ten research approaches were identified, which include broad comparison (i.e., many species generalization), specific comparison (e.g., 2 species; for traits that are different, exaggerated, extreme, missing, or not induced), or comparison while considering evolution (i.e., evolutionary physiology), ecology (i.e., ecophysiology), or human physiology/medicine. Only 5% and 33% of OS and SR definitions described or mentioned Krogh's principle. In conclusion, CP can best be defined as a compilation of research approaches that utilize different types of comparisons to elucidate physiological mechanisms and not simply comparing physiologies as the name implies.

Anurans have an exceptional capacity for maintaining vascular volume compared with other groups of vertebrates. They can mobilize interstitial fluids via lymphatic return at rates that are ten-fold higher than mammals. This extraordinary capacity is the result of coordination of specialized skeletal muscles and pulmonary ventilation that vary volume and pressure of subcutaneous lymph sacs, thus moving lymph to dorsally located lymph hearts that return lymph to the vascular space. Variation in the capacity to mobilize lymph within anurans varies with the degree of terrestriality, development of skeletal muscles, lung volume and lung compliance, and lymph heart pressure development. This ability enable anurans, which have the highest rates of evaporative water loss among terrestrial vertebrates, to withstand levels of dehydration far exceeding that of other vertebrates, and to successfully occupy virtually all terrestrial environments during their evolution. Maintenance of vascular fluid volume for all vertebrates can be achieved primarily by moving fluid from the interstitial space to the vascular space by transcapillary uptake and mobilization of interstitial (lymphatic) fluid. Transcapillary fluid uptake at the capillary level has been analyzed historically by Krogh and others from a Starling perspective and involves a balance of hydrostatic and oncotic forces. A complete evaluation of blood volume homeostasis also incorporates pressures and compliances of the vascular and interstitial spaces, but has been applied to only a few species. In this review we outline the current understanding of how anurans and other vertebrates maintain blood volume during hypovolemic challenges such as dehydration and hemorrhage which is crucial for maintaining cardiac output.

August Krogh (1874-1949) was amongst the most influential physiologists in the first part of the 20th century. This was an era when physiology emerged as a quantitative research field and when many of the current physiological disciplines were defined; Krogh can rightfully be viewed as having introduced comparative physiology, epithelial transport and - together with Johannes Lindhard - exercise physiology as independent disciplines. With a unique ability to design and construct equipment, Krogh could address novel questions in both human and animal physiology with unprecedented precision. Krogh would characteristically focus on a given physiological problem over a couple of years, delineate the focal mechanisms, provide a solution to the major problems, and then move onto new academic ground. For each of his major research areas (respiratory gas exchange, capillary function, osmoregulation), he wrote comprehensive books or monographs that remain important resources for scholars today, and he engaged in the writing of physiology textbooks for the Danish high school. Krogh's research appears to have been driven by curiosity to understand how animals (including humans) work, but he did not hesitate to apply his insight to societal and clinical problems throughout his long academic career.

This review serves as an introduction to a Special Issue of Comparative Biochemistry and Physiology, focused on using non-human models to study biomedical physiology. The concept of a model differs across disciplines. For example, several models are used primarily to gain an understanding of specific human pathologies and disease states, whereas other models may be focused on gaining insight into developmental or evolutionary mechanisms. It is often the case that animals initially used to gain knowledge of some unique biochemical or physiological process finds foothold in the biomedical community and becomes an established model. The choice of a particular model for biomedical research is an ongoing process and model validation must keep pace with existing and emerging technologies. While the importance of non-mammalian models, such as Caenorhabditis elegans, Drosophila melanogaster, Danio rerio and Xenopus laevis, is well known, we also seek to bring attention to emerging alternative models of both invertebrates and vertebrates, which are less established but of interest to the comparative biochemistry and physiology community.

August Krogh’s principle, “For such a large number of problems there will be some animal of choice, or a few such animals, on which it can be most conveniently studied” is credited as the cornerstone of comparative physiology. Today, there appears to be some confusion among physiologists as to what type of research falls under the comparative physiology approach. To identify how comparative physiologists and general physiologists define comparative physiology, 54 physiology organizations were contacted and requested to share a cross‐sectional online survey with their members. In total, 245 participants consented to the study with 87% identifying as comparative physiologists (22% as general physiologists). Participants came from 18 countries and were members of 45 physiology or science societies/organizations. When asked to define comparative physiology, the three most common words in the definitions (n=197) were (in descending order): ‘different’, ‘physiology’, ‘animals’. The three most prevalent themes among responses were: comparing/differences across species (68% of responses), only animal research (42%), and environmental/ecological variable (26%). While Krogh’s principle is central to comparative physiology, only 5% of responses either mentioned or described Krogh’s principle. To further determine the participants’ understanding of comparative physiology, a multiple‐choice question was created which asked “Please identify the Comparative Physiology perspective below. Select all that apply.” The most common selected answer among participants (91%; no significant difference between comparative and general physiologists; Chi‐square test: p=0.088) was the ‘comparative physiology ‐ human focused’ perspective which states “Studying how digestion functions in snakes compared to humans.” Further, 49% of all participants selected all five choices for their answer; however, more comparative physiologists selected all five choices compared with general physiologists (50% versus 8%; Chi‐square test: p=<0.001). In conclusion, it appears that comparative physiology has become an umbrella term for a collection of research approaches including Krogh’s Principle and other research themes, such as, but not limited to: 1) simply comparing across species with or without considering evolution, 2) comparing animal to human physiology, or 3) studying extreme or unique traits. While Krogh emphasized comparing physiological systems across animals to determine generalizations, among surveyed physiologists, Krogh’s Principle per se is not the guiding focus in the field of comparative physiology.

This excel file shows the results of the Defining Comparative Physiology Survey.

A review of Lovegrove, Barry G. Fires of Life – Endothermy in Birds and Mammals. Yale University Press, New Haven, Connecticut, USA, 2019. Birds and mammals are unique among the vertebrates for their ability to maintain high, stable body temperatures through endogenous heat production; this “endothermic” lifestyle contrasts with the vast vertebrate and invertebrate fauna that, with a few notable exceptions, are incapable of this metabolic feat and rely, instead, on the external environment, or ectothermy, to determine body temperature. Birds and mammals have evolved this ability, independently, from separate reptilian clades; how and why these groups have evolved endothermy from their ectothermic ancestors is the subject of a new book by Professor Barry G. Lovegrove. The title of Lovegrove's book is a nod to Max Kleiber's similarly titled classic treatise on animal energetics (Kleiber 1961). Although there is an extensive literature on the evolution of endothermy, Lovegrove has written the first book specifically dedicated to this topic in a holistic manner. Lovegrove is an expert on the metabolism of African mammals, and his synthesis results from a decade-long career that has examined the thermal and metabolic characteristics of some unusual animals of the Southern Hemisphere, most notably the odd tenrec hedgehog of Madagascar. As Lovegrove points out, his ideas on the evolution of endothermy are the direct result of his work in the “hot, humid and swampy” conditions of Madagascar where he and his students investigated the metabolic and thermal characteristics of the elusive tenrec. Many of the animals that Lovegrove and his students studied in the tropics exhibit a great deal of variation in body temperature, and some, such as the tenrec, are known to hibernate, and these surprising data helped to inform Lovegrove's views on the evolution of endothermy. The book is divided into two parts. The first nine chapters are a paleontological survey on some of the key fossil forms of amphibians and reptiles that are related to the extant endothermic birds and mammals. Lovegrove also provides some background on various ideas on the evolution of endothermy and why many of these extinct animals in particular support his arguments on the evolution of endothermy. Along the way, Lovegrove touches on a number of subjects relevant to the evolution of endothermy: the physiological problems of a water-to-land transition, energetics, flight, nocturnal vision, insulation, effects of body mass on physiology, and locomotion, to name a few. I am not a paleontologist, and I found this section of the book difficult to follow given its length and the number of fossil forms to keep clear. Fortunately, Lovegrove provides a useful appendix of family trees that show the phylogenetic relationships of all the relevant players. In addition to the various fossil forms Lovegrove deems key to his thesis, he also adds some interesting history of the regions in which many of these fossils are located and the scientists who discovered them. For example, the Karoo region of South Africa holds a great number of fossils from the Middle Permian to the Middle Triassic that are relevant to this story. And among the many paleontologists who have worked in the Karoo, none was perhaps more important or interesting than the South African fossil hunter, Robert Broom, who discovered hundreds of important fossils. Lovegrove's descriptions of Broom's methods and eccentricities are highly entertaining. A weakness of the first section of the book is the number of definitive statements about the supposed physiology of particular fossil species; I found many of these statements unnecessarily dogmatic. For example, the first terrestrial vertebrates that were precursors of modern amphibians were large salamander-like animals that the author asserts were large to limit evaporative water loss due to their low surface area-to-volume ratio. This ignores the fact that nearly all modern amphibians are small with high surface area-to-volume ratios yet also have high rates of evaporative water loss, characteristics that are in direct opposition to his argument. He does not consider potential alternative hypotheses, such as that these stem-amphibians may have been large to reduce energy when moving from water to land, a step that incurs an approximate 10-fold increase in the cost of transport; the amount of energy expended per unit distance traveled. There is a universal inverse relationship between body mass and cost of transport; thus, being large helps to ameliorate the energetic costs of terrestrial locomotion. I could cite other examples where Lovegrove neglects to consider or dismisses data that are not consistent with his ideas on the evolution of endothermy. In his defense, however, exploring all alternative hypotheses and all sides of these arguments would greatly lengthen the book, and the author is trying to present his own story on the evolution of endothermy, including some novel ideas that he admits have not undergone peer review. But an acknowledgment of contrary evidence would provide a more complete and balanced story. Nevertheless, Lovegrove does provide a thorough, although not exhaustive, literature review of each chapter, and the curious reader might investigate the relevant literature and come to their own conclusions about the data. The second half of the book, an additional nine chapters, focuses primarily on the physiology of endothermy in extant animals, and much of the data presented are from his own work on African mammals. In my view, the most interesting and well-written sections of the book are Lovegrove's descriptions of attempts to collect physiological data on tenrecs and other animals in Madagascar despite the numerous barriers imposed upon him and his students. Anyone who has spent time working in the field can certainly relate to the joy and frustration inherent in collecting data under adverse conditions such as those Lovegrove vividly describes. It is in this section that Lovegrove's passion for his research shines through and I felt that the book could have been improved by more stories such as these. Although Lovegrove depicts his overall model of endothermy as a figure in the first chapter of the book, he finally describes the model in detail in the final chapter titled “Why We Are Hot.” Based on the paleontological and physiological data covered in the preceding chapters, he presents a three-phase model that explains how endothermy in birds and mammals evolved from ectothermic ancestors. His triphasic model, as he explains, is a “multiple-causes model” that is superior to previous “single-cause” models, such as the Aerobic Capacity and Parental Care models described in the book. In Lovegrove's view, endothermy evolved in an “iterative, pulse-like way” in birds and mammals over several hundred million years. The first two phases of the model are built on previous single-cause models, and the third phase, which he calls the “Locomotory and Climate Adaptation Phase,” contains novel ideas that he explains are not currently supported by any current single-cause models. Although Lovegrove's model may be plausible, the entire scenario struck me as a kind of “Theory of Everything” for endothermy and it is unclear to me that many of the ideas incorporated in this three-phase model are, in fact, directly testable hypotheses. I found myself in disagreement with many of the physiological arguments that Lovegrove uses to support his point of view; however, Lovegrove is well aware that his ideas are open to argument and interpretation. The difficulty, of course, is testing hypotheses and making inferences about the physiology of extinct animals; that is, explaining how and why endothermy evolved. Regardless, there are a number of interesting ideas in the book that could keep scientists busy for years to come. A theme that permeates the book is Lovegrove's antithesis to what he calls the “Holarctic Hibernation Paradigm,” the ideas promulgated by the North American and European scientists working in the field of hibernation. He mentions this throughout the book and gives the impression that one impetus for writing this book was to promote ideas he feels have been ignored by the scientists he refers to as the “Holarctic Hibernation Paradigmists.” As Lovegrove explains, his research has shown that many tropical animals undergo daily torpor or hibernate, and because many of these animals belong to more primitive mammalian clades, he argues that hibernation and torpor are ancient physiological adaptations that evolved in the tropics, not in cold, upper latitudes that one normally associates with hibernation. While Lovegrove raises important physiological distinctions between Holarctic and African hibernators, it is unclear whether this argument will convince the hibernation research community, which seems somewhat uninvolved with the evolution of endothermy; thus, his repeated invocations of this “paradigm” seemed somewhat odd. Overall, Lovegrove has written a highly entertaining story about the parallel evolution of endothermy in birds and mammals from distinct ectothermic reptilian clades. A major strength of the book is that Lovegrove pulls together information from a large number of topics and ideas relevant to endothermy that have been previously relegated to disparate field-specific journals. The evolution of endothermy is a fascinating subplot to the story of vertebrate evolution, and anyone with an interest in this story would do well to start with this book.

Acute (< 96 h) exposure to elevated environmental CO2 (hypercarbia) induces a pH disturbance in fishes that is often compensated by concurrent recovery of intra- and extracellular pH (pHi and pHe, respectively; coupled pH regulation). However, coupled pH regulation may be limited at PCO2 tensions far below levels that some fishes naturally encounter. Previously, four hypercarbia tolerant fishes had been shown to completely and rapidly regulate heart, brain, liver, and white muscle pHi during acute exposure to>4 kPa PCO2 (preferential pHi regulation) before pHe compensation was observed. Here we test the hypothesis that preferential pHi regulation is a wide spread strategy of acid-base regulation among fish by measuring pHi regulation in 10 different fish species that are broadly phylogenetically separated, spanning 6 orders, 8 families and 10 genera. Contrary to previous views, we show that preferential pHi regulation is the most common strategy for acid-base regulation within these fishes during exposure to severe acute hypercarbia and that this strategy is associated with increased hypercarbia tolerance. This suggests preferential pHi regulation may confer tolerance to the respiratory acidosis associated with hypercarbia and we propose that it is an exaptation that facilitated key evolutionary transitions in vertebrate evolution, such as the evolution of air breathing.

We would like to thank Hedrick and colleagues for their thoughtful comments and for the opportunity to further elaborate on the splenic blood-boosting strategy of Pagothenia borchgrevinki ([Brijs et al., 2020][1]). [Hedrick et al. (2020)][2] argue that the reduction in spleen mass observed in our

The African cheetah (Acinonyx jubatus) is the world's fastest terrestrial mammal, capable of reaching speeds of about 100 km h−1 over short distances. Central to its exceptional athleticism is the ability to deliver oxygen to muscles, where the cardiovascular system plays a critical role in supplying oxygen and preventing the accumulation of metabolic waste products even during intensive sprints. However, little is known about the blood transport properties of cheetahs. Therefore, we measured the effects of temperature on blood viscosity (ηbl) and hemoglobin (Hb) O2 affinity (P50) at resting body temperature (38 °C) and post-sprint body temperature (41 °C) in 5 adult animals undergoing routine health checks. Mean hematocrit (Hct) was 41.5 ± 2.5 (SD) %. Whole blood ηbl was exponentially-related to Hct (P<0.001; r2 >0.99) at both temperatures; the slopes (0.024 ± 0.0003) of this relationship were not different at the two temperatures, but the y-intercept was lower at 41 °C (P<0.001). Increased temperature from 38 °C to 41 °C significantly (P<0.001) reduced η of whole blood (3.39 ± 0.23 vs. 3.08 ± 0.23 mPa s) and plasma (1.27 ± 0.04 vs. 1.15 ± 0.04 mPa s). Purified Hb had a P50 of 22.2 mm Hg at 38° C and 25.8 mm Hg at 41°C. O2-affinity of Hb decreased only slightly (~3–4 mm Hg) in the presence of 2-fold molar excess DPG, similar to other felid Hbs, and decreased by ~13–14 mm Hg with the addition 100 mM Clat both temperatures. Neither temperature nor allosteric modifier significantly changed the Hill coefficient of Hb. These results indicate that cheetah blood has rheological properties similar to that of other mammals and Hb properties similar to other felids. The reduced viscosity at higher temperatures, typical of exercising muscles, would facilitate blood flow and O2 transport during sprints. Support or Funding Information Supported by Natur og Univers, Det Frie Forskningsråd (FNU), Denmark. Blood viscosity as a function of Hct at two temperatures for cheetah blood Purified cheetah Hb P50, Hb P50with DPG and Hb P50with chloride at 38 °C. Different letters reflect significant differences (P<0.05). This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

This article serves as an introduction to a Virtual Special Issue of Comparative Biochemistry and Physiology (CBP) focused on aquaculture. CBP has not traditionally had a focus on aquaculture, and the Editors sought to use this Special Issue to identify opportunities for synergy between traditional comparative physiology and applied physiology, such as aquaculture. Each of the four CBP journals has a dedicated special issue, with manuscripts that span the breadth of vertebrate and invertebrate species cultured around the globe. This overview is intended to identify the major themes of the submissions, as well as articulate a vision for the types of aquaculture-focused research that are well suited for CBP publications.