The evolution of endothermy in mammals and birds: from physiology to fossils

Molecular genetic foundation of a sex-linked tailless trait in Hongshan chicken by whole genome data analysis

As a Chinese local chicken breed, Hongshan chickens have 2 kinds of tail feather phenotypes, normal and taillessness. Our previous studies showed that taillessness was a sex-linked dominant trait. Abnormal development of the tail vertebrae could be explained this phenomenon in some chicken breeds. However, the number of caudal vertebrae in rumpless Hongshan chickens was normal, so rumplessness in Hongshan chicken was not related to the development of the caudal vertebrae. Afterwards, we found that rumplessness in Hongshan was due to abnormal development of tail feather rather than abnormal development of caudal vertebrae. In order to understand the genetic foundation of the rumplessness of Hongshan chickens, we compared and reanalyzed 2 sets of data in normal and rumpless Hongshan chickens from our previous studies. By joint analysis of genome-wide selection signature analysis and genome-wide association approach, we found that 1 overlapping gene (EDIL3) and 16 peak genes (ENSGALG00000051843, ENSGALG00000053498, ENSGALG00000054800, KIF27, PTPRD, ENSGALG00000047579, ENSGALG00000041052, ARHGEF28, CAMK4, SERINC5, ENSGALG00000050776, ERCC8, MCC, ADAMTS19, ENSGALG00000053322, CHRNA8) located on the Z chromosome was associated with the rumpless trait. The results of this study furtherly revealed the molecular mechanism of the rumpless trait in Hongshan chickens, and identified the candidate genes associated with this trait. Our results will help to improve the shape of chicken tail feathers and to rise individual economic value in some specific market in China.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 4: evolution, thermal adaptation and unsupported theories of thermoregulation

This review is the final contribution to a four-part, historical series on human exercise physiology in thermally stressful conditions. The series opened with reminders of the principles governing heat exchange and an overview of our contemporary understanding of thermoregulation (Part 1). We then reviewed the development of physiological measurements (Part 2) used to reveal the autonomic processes at work during heat and cold stresses. Next, we re-examined thermal-stress tolerance and intolerance, and critiqued the indices of thermal stress and strain (Part 3). Herein, we describe the evolutionary steps that endowed humans with a unique potential to tolerate endurance activity in the heat, and we examine how those attributes can be enhanced during thermal adaptation. The first of our ancestors to qualify as an athlete was Homo erectus, who were hairless, sweating specialists with eccrine sweat glands covering almost their entire body surface. Homo sapiens were skilful behavioural thermoregulators, which preserved their resource-wasteful, autonomic thermoeffectors (shivering and sweating) for more stressful encounters. Following emigration, they regularly experienced heat and cold stress, to which they acclimatised and developed less powerful (habituated) effector responses when those stresses were re-encountered. We critique hypotheses that linked thermoregulatory differences to ancestry. By exploring short-term heat and cold acclimation, we reveal sweat hypersecretion and powerful shivering to be protective, transitional stages en route to more complete thermal adaptation (habituation). To conclude this historical series, we examine some of the concepts and hypotheses of thermoregulation during exercise that did not withstand the tests of time.

Rare and Opportunistic Use of Torpor in Mammals-An Echo from the Past?

Torpor was traditionally seen as a winter survival mechanism employed by animals living in cold and highly seasonal habitats. Although we now know that torpor is also used by tropical and subtropical species, and in response to a variety of triggers, torpor is still largely viewed as a highly controlled, seasonal mechanism shown by Northern hemisphere species. To scrutinize this view, we report data from a macroanalysis in which we characterized the type and seasonality of torpor use from mammal species currently known to use torpor. Our findings suggest that predictable, seasonal torpor patterns reported for Northern temperate and polar species are highly derived forms of torpor expression, whereas the more opportunistic and variable forms of torpor that we see in tropical and subtropical species are likely closer to the patterns expressed by ancestral mammals. Our data emphasize that the torpor patterns observed in the tropics and subtropics should be considered the norm and not the exception.

Nasal turbinates of the dicynodont Kawingasaurus fossilis and the possible impact of the fossorial habitat on the evolution of endothermy

The nasal region of the fossorial anomodont Kawingasaurus fossilis was virtually reconstructed from neutron-computed tomographic data and compared with the terrestrial species Pristerodon mackayi and other nonmammalian synapsids. The tomography of the Kawingasaurus skull reveals a pattern of maxillo-, naso-, fronto- and ethmoturbinal ridges that strongly resemble the mammalian condition. On both sides of the nasal cavity, remains of scrolled maxilloturbinals were preserved that were still partially articulated with maxilloturbinal ridges. Furthermore, possible remains of the lamina semicircularis as well as fronto- or ethmoturbinals were found. In Kawingasaurus, the maxilloturbinal ridges were longer and stronger than in Pristerodon. Except for the nasoturbinal ridges, no other ridges in the olfactory region and no remains of turbinates were recognized. This supports the hypothesis that naso-, fronto-, ethmo- and maxilloturbinals were a plesiomorphic feature of synapsids, but due to their cartilaginous nature in most taxa were, in almost all cases, not preserved. The well-developed maxilloturbinals in Kawingasaurus were probably an adaptation to hypoxia-induced hyperventilation in the fossorial habitat, maintaining the high oxygen demands of Kawingasaurus' large brain. The surface area of the respiratory turbinates in Kawingasaurus falls into the mammalian range, which suggests that they functioned as a countercurrent exchange system for thermoregulation and conditioning of the respiratory airflow. Our results suggest that the environmental conditions of the fossorial habitat led to specific sensory adaptations, accompanied by a pulse in brain evolution and of endothermy in cistecephalids, ~50 million years before the origin of endothermy in the mammalian stem line. This supports the Nocturnal Bottleneck Theory, in that we found evidence for a similar evolutionary scenario in cistecephalids as proposed for early mammals.

A Critical Review on the Dosing and Safety of Antifungals Used in Exotic Avian and Reptile Species

Antifungals are used in exotic avian and reptile species for the treatment of fungal diseases. Dose extrapolations across species are common due to lack of species-specific pharmacological data. This may not be ideal because interspecies physiological differences may result in subtherapeutic dosing or toxicity. This critical review aims to collate existing pharmacological data to identify antifungals with the most evidence to support their safe and effective use. In the process, significant trends and gaps are also identified and discussed. An extensive search was conducted on PubMed and JSTOR, and relevant data were critically appraised. Itraconazole or voriconazole showed promising results in Japanese quails, racing pigeons and inland bearded dragons for the treatment of aspergillosis and CANV-related infections. Voriconazole neurotoxicity manifested as seizures in multiple penguins, but as lethargy or torticollis in cottonmouths. Itraconazole toxicity was predominantly hepatotoxicity, observed as liver abnormalities in inland bearded dragons and a Parson's chameleon. Differences in formulations of itraconazole affected various absorption parameters. Non-linearities in voriconazole due to saturable metabolism and autoinduction showed opposing effects on clearance, especially in multiple-dosing regimens. These differences in pharmacokinetic parameters across species resulted in varying elimination half-lives. Terbinafine has been used in dermatomycoses, especially in reptiles, due to its keratinophilic nature, and no significant adverse events were observed. The use of fluconazole has declined due to resistance or its narrow spectrum of activity.

Feather function and the evolution of birds

The ability of feathers to perform many functions either simultaneously or at different times throughout the year or life of a bird is integral to the evolutionary history of birds. Many studies focus on single functions of feathers, but any given feather performs many functions over its lifetime. These functions necessarily interact with each other throughout the evolution and development of birds, so our knowledge of avian evolution is incomplete without understanding the multifunctionality of feathers, and how different functions may act synergistically or antagonistically during natural selection. Here, we review how feather functions interact with avian evolution, with a focus on recent technological and discovery-based advances. By synthesising research into feather functions over hierarchical scales (pattern, arrangement, macrostructure, microstructure, nanostructure, molecules), we aim to provide a broad context for how the adaptability and multifunctionality of feathers have allowed birds to diversify into an astounding array of environments and life-history strategies. We suggest that future research into avian evolution involving feather function should consider multiple aspects of a feather, including multiple functions, seasonal wear and renewal, and ecological or mechanical interactions. With this more holistic view, processes such as the evolution of avian coloration and flight can be understood in a broader and more nuanced context.

Thermal adaptation of pelage in desert rodents balances cooling and insulation

Phenotypic convergence across distantly related taxa can be driven by similar selective pressures from the environment or intrinsic constraints. The roles of these processes on physiological strategies, such as homeothermy, are poorly understood. We studied the evolution of thermal properties of mammalian pelage in a diverse community of rodents inhabiting the Mojave Desert, USA. We used a heat flux device to measure the thermal insulation of museum specimens and determined whether thermal properties were associated with habitat preferences while assessing phylogenetic dependence. Species that prefer arid habitats exhibited lower conductivity and thinner pelage relative to species with other habitat preferences. Despite being thinner, the pelage of arid species exhibited comparable insulation to the pelage of the other species due to its lower conductivity. Thus, arid species have insulative pelage while simultaneously benefitting from thin pelage that promotes convective cooling. We found no evidence of intrinsic constraints or phylogenetic dependence, indicating pelage readily evolves to environmental pressures. Thermoregulatory simulations demonstrated that arid specialists reduced energetic costs required for homeothermy by 14.5% by evolving lower conductivity, providing support for adaptive evolution of pelage. Our study indicates that selection for lower energetic requirements of homeothermy has shaped evolution of pelage thermal properties.

Fossil bone histology reveals ancient origins for rapid juvenile growth in tetrapods

Patterns of growth throughout the lifetime of an animal reflect critical life history traits such as reproductive timing, physiology, and ecological interactions. The ancestral growth pattern for tetrapods has traditionally been described as slow-to-moderately paced, akin to modern amphibians, with fast growth and high metabolic rates considered a specialized physiological trait of amniotes. Here, we present bone histology from an ontogenetic series of the Early Carboniferous stem tetrapod Whatcheeria deltae, and document evidence of fibrolamellar bone-primary bone tissue associated with fast growth. Our data indicate that Whatcheeria juveniles grew rapidly and reached skeletal maturity quickly, allowing them to occupy a large-bodied predator niche in their paleoenvironment. This life history strategy contrasts with those described for other stem tetrapods and indicates that a diversity of growth patterns existed at the origins of tetrapod diversification. Importantly, Whatcheeria marks an unexpectedly early occurrence of fibrolamellar bone in Tetrapoda, both temporally and phylogenetically. These findings reveal that elevated juvenile growth is not limited to amniotes, but has a deep history in the tetrapod clade and may have played a previously unrecognized role in the tetrapod invasion of land.

Avian cardiomyocyte architecture and what it reveals about the evolution of the vertebrate heart

Bird cardiomyocytes are long, thin and lack transverse (t)-tubules, which is akin to the cardiomyocyte morphology of ectothermic non-avian reptiles, who are typified by low maximum heart rates and low pressure development. However, birds can achieve greater contractile rates and developed pressures than mammals, whose wide cardiomyocytes contain a dense t-tubular network allowing for uniform excitation-contraction coupling and strong contractile force. To address this apparent paradox, this paper functionally links recent electrophysiological studies on bird cardiomyocytes with decades of ultrastructure measurements. It shows that it is the strong transsarcolemmal Ca2+ influx via the L-type Ca2+ current (ICaL) and the high gain of Ca2+-induced Ca2+ release from the sarcoplasmic reticulum (SR), coupled with an internal SR Ca2+ release relay system, that facilitates the strong fast contractions in the long thin bird cardiomyocytes, without the need for t-tubules. The maintenance of an elongated myocyte morphology following the post-hatch transition from ectothermy to endothermy in birds is discussed in relation to cardiac load, myocyte ploidy, and cardiac regeneration potential in adult cardiomyocytes. Overall, the paper shows how little we know about cellular Ca2+ dynamics in the bird heart and suggests how increased research efforts in this area would provide vital information in our quest to understand the role of myocyte architecture in the evolution of the vertebrate heart. This article is part of the theme issue 'The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease'. Please see glossary at the end of the paper for definitions of specialized terms.

The genetic basis of a novel reproductive strategy in Sulawesi ricefishes: How modularity and a low number of loci shape pelvic brooding

The evolution of complex phenotypes like reproductive strategies is challenging to understand, as they often depend on multiple adaptations that only jointly result in a specific functionality. Sulawesi ricefishes (Adrianichthyidae) evolved a reproductive strategy termed as pelvic brooding. In contrast to the more common transfer brooding, female pelvic brooders carry an egg bundle connected to their body for weeks until the fry hatches. To examine the genetic architecture of pelvic brooding, we crossed the pelvic brooding Oryzias eversi and the transfer brooding Oryzias nigrimas (species divergence time: ∼3.6 my). We hypothesize, that a low number of loci and modularity have facilitated the rapid evolution of pelvic brooding. Traits associated to pelvic brooding, like rib length, pelvic fin length, and morphology of the genital papilla, were correlated in the parental species but correlations were reduced or lost in their F1 and F2 hybrids. Using the Castle-Wright estimator, we found that generally few loci underlie the studied traits. Further, both parental species showed modularity in their body plans. In conclusion, morphological traits related to pelvic brooding were based on a few loci and the mid-body region likely could evolve independently from the remaining body parts. Both factors presumably facilitated the evolution of pelvic brooding.

A Culture-Based Study of Micromycetes Isolated from the Urban Nests of Grey Heron (Ardea cinerea) in SW Poland

Simple Summary

Fungi inhabiting bird nests may pose a serious threat to living organisms. Therefore, the main goal of the study was to identify cultivable fungi in the nest of grey heron (Ardea cinerea) located near the city centre of Wrocław (Poland). Overall, 10 different fungal species were obtained which were both cosmopolitan and potentially hazardous to humans, homoiothermous animals and plants. The greatest number of fungal species was obtained from the nest fragments with visible fungal growth, and the least from western conifer seed bugs (Leptoglossus occidentalis) inhabiting the nests. The damp chamber allowed isolation of Aspergillus fumigatus, Penicillium coprophilum, and P. griseofulvum as directly related to the occurrence of visible fungal growth on plant fragments of grey heron nests.

Abstract

There are many positive relationships between micromycetes and birds: They can spread fungal spores, and fungi facilitate cavity woodpecker excavation by preparing and modifying excavation sites. In turn, bird nests are mainly a source of potentially zoopathogenic fungi. The Wrocław city centre hosts the biggest grey heron breeding colony in Poland with at least 240 breeding birds pairs. To assess the possible public health risks associated with bird nests, the goal of the present study was to identify cultivable fungi present in the nests of grey herons (Ardea cinerea) in Wrocław. Additionally, attempts were made to determine whether the obtained species of fungi may pose a potential threat to animal health. Fungi were cultured at 23 and 37 ± 0.5 °C, and identified based on phenotypic and genotypic traits. Moreover, during routine inspection, visible fungal growth in some of the nests was found. Overall, 10 different fungal species were obtained in the study (Alternaria alternata, Aspergillus fumigatus, Botryotrichum piluliferum, Cladosporium cladosporioides, Epicoccum layuense, Mucor circinelloides, M.hiemalis, Penicillium atramentosum, P.coprophilum, and P.griseofulvum). They are both cosmopolitan species and a source of potential threat to humans, homoiothermous animals and plants. The greatest number of fungal species was obtained from the nest fragments with visible fungal growth incubated at 23 °C, and the least from western conifer seed bugs (Leptoglossus occidentalis) inhabiting the nests. The species such as A. fumigatus, P. coprophilum, and P.griseofulvum can be directly related to the occurrence of visible fungal growth on plant fragments of grey heron’s nests.

Effect of Different Water Cooling Treatments on Changes in Rectal and Surface Body Temperature in Leisure Horses after Medium-Intensity Effort

Simple Summary

The study was performed in order to answer the question: should horses be cooled after not very intensive exercise? Thus, the efficiency of four different water cooling methods was studied in horses after medium-intensity effort for leisure horses under moderate air temperature. The water cooling methods used included spraying with cold water: (1) lower body parts, i.e., lower parts of legs; (2) upper body parts, i.e., back of the head and flanks; and (3) both lower and upper body parts. The control group was not treated with water. Water cooling was applied to 19 warmblood geldings immediately after the end of exercise and 10 and 20 min later. The internal and body surface temperatures were registered and analyzed. The water cooling methods used led to a decrease in rectal and body surface temperature. Applying cold water on lower body parts was only effective and can be recommended for practical use under the described conditions.

Abstract

Cooling a horse after intensive exercise under hot conditions is commonly recommended. The study aimed to analyze changes in the rectal and surface temperature of the horses subjected to various water cooling treatments. This followed medium-intensity exercise performed by leisure horses under moderate air temperature. The experiment involved a control group without water application, and three variants of water cooling applied to 19 warmblood geldings after medium-intensity effort. Cooling of lower, upper, and lower and upper body parts was performed. In each variant, the rectal and body surface temperatures were measured five times: before; immediately after; and 10, 20, and 30 min after effort. Using water cooling under the studied conditions did not influence a post-exercise decrease in the rectal temperature. The decrease in body surface temperature depended on the used variant of cooling down the horse. Cooling the limbs by pouring water several times changed the surface body temperature from 34.2 ± 0.37 °C to 32.0 ± 0.32 °C and was more efficient than the repeated application of cool water on both the upper and lower body parts, leading to a temperature change from 34.6 ± 0.26 °C to 33.2 ± 0.36 °C. Thus, the application of cold water on the limbs only is sufficient for cooling the horse after medium-intensity exercise under moderate air temperature (about 24 °C).

Escaping the nocturnal bottleneck, and the evolution of the dorsal and ventral streams of visual processing in primates

Early mammals were small and nocturnal. Their visual systems had regressed and they had poor vision. After the extinction of the dinosaurs 66 mya, some but not all escaped the 'nocturnal bottleneck' by recovering high-acuity vision. By contrast, early primates escaped the bottleneck within the age of dinosaurs by having large forward-facing eyes and acute vision while remaining nocturnal. We propose that these primates differed from other mammals by changing the balance between two sources of visual information to cortex. Thus, cortical processing became less dependent on a relay of information from the superior colliculus (SC) to temporal cortex and more dependent on information distributed from primary visual cortex (V1). In addition, the two major classes of visual information from the retina became highly segregated into magnocellular (M cell) projections from V1 to the primate-specific temporal visual area (MT), and parvocellular-dominated projections to the dorsolateral visual area (DL or V4). The greatly expanded P cell inputs from V1 informed the ventral stream of cortical processing involving temporal and frontal cortex. The M cell pathways from V1 and the SC informed the dorsal stream of cortical processing involving MT, surrounding temporal cortex, and parietal-frontal sensorimotor domains. This article is part of the theme issue 'Systems neuroscience through the lens of evolutionary theory'.

Whole-body endothermy: ancient, homologous and widespread among the ancestors of mammals, birds and crocodylians

The whole‐body (tachymetabolic) endothermy seen in modern birds and mammals is long held to have evolved independently in each group, a reasonable assumption when it was believed that its earliest appearances in birds and mammals arose many millions of years apart. That assumption is consistent with current acceptance that the non‐shivering thermogenesis (NST) component of regulatory body heat originates differently in each group: from skeletal muscle in birds and from brown adipose tissue (BAT) in mammals. However, BAT is absent in monotremes, marsupials, and many eutherians, all whole‐body endotherms. Indeed, recent research implies that BAT‐driven NST originated more recently and that the biochemical processes driving muscle NST in birds, many modern mammals and the ancestors of both may be similar, deriving from controlled ‘slippage’ of Ca²⁺ from the sarcoplasmic reticulum Ca²⁺‐ATPase (SERCA) in skeletal muscle, similar to a process seen in some fishes. This similarity prompted our realisation that the capacity for whole‐body endothermy could even have pre‐dated the divergence of Amniota into Synapsida and Sauropsida, leading us to hypothesise the homology of whole‐body endothermy in birds and mammals, in contrast to the current assumption of their independent (convergent) evolution. To explore the extent of similarity between muscle NST in mammals and birds we undertook a detailed review of these processes and their control in each group. We found considerable but not complete similarity between them: in extant mammals the ‘slippage’ is controlled by the protein sarcolipin (SLN), in birds the SLN is slightly different structurally and its role in NST is not yet proved. However, considering the multi‐millions of years since the separation of synapsids and diapsids, we consider that the similarity between NST production in birds and mammals is consistent with their whole‐body endothermy being homologous. If so, we should expect to find evidence for it much earlier and more widespread among extinct amniotes than is currently recognised. Accordingly, we conducted an extensive survey of the palaeontological literature using established proxies. Fossil bone histology reveals evidence of sustained rapid growth rates indicating tachymetabolism. Large body size and erect stature indicate high systemic arterial blood pressures and four‐chambered hearts, characteristic of tachymetabolism. Large nutrient foramina in long bones are indicative of high bone perfusion for rapid somatic growth and for repair of microfractures caused by intense locomotion. Obligate bipedality appeared early and only in whole‐body endotherms. Isotopic profiles of fossil material indicate endothermic levels of body temperature. These proxies led us to compelling evidence for the widespread occurrence of whole‐body endothermy among numerous extinct synapsids and sauropsids, and very early in each clade's family tree. These results are consistent with and support our hypothesis that tachymetabolic endothermy is plesiomorphic in Amniota. A hypothetical structure for the heart of the earliest endothermic amniotes is proposed. We conclude that there is strong evidence for whole‐body endothermy being ancient and widespread among amniotes and that the similarity of biochemical processes driving muscle NST in extant birds and mammals strengthens the case for its plesiomorphy.

Large-scale evolution of body temperatures in land vertebrates

Body temperature is a crucial variable in animals that affects nearly every aspect of their lives. Here we analyze for the first time largescale patterns in the evolution of body temperatures across terrestrial vertebrates (tetrapods: including amphibians, mammals, birds and other reptiles). Despite the traditional view that endotherms (birds and mammals) have higher body temperatures than ectotherms, we find they are not significantly different. However, rates of body-temperature evolution are significantly different, with lower rates in endotherms than ectotherms, and the highest rates in amphibians. We find that body temperatures show strong phylogenetic signal and conservatism over 350 million years of evolutionary history in tetrapods, and some lineages appear to have retained similar body temperatures over time for hundreds of millions of years. Although body temperatures are often unrelated to climate in tetrapods, we find that body temperatures are significantly related to day-night activity patterns. Specifically, body temperatures are generally higher in diurnal species than nocturnal species, both across ectotherms and, surprisingly, across endotherms also. Overall, our results suggest that body temperatures are significantly linked to phylogeny and diel-activity patterns within and among tetrapod groups, rather than just climate and the endotherm-ectotherm divide.

First case of endothermy in semisessile animals

Endothermy is generally believed to have coevolved with highly active lifestyle in animals, and to be permanent (combined with homeothermy) only in some vertebrates, due in part to size restrictions on endothermic animals. All invertebrates are known to possess endothermy and exhibit it only when engaged in physically intensive behaviors. I report the discovery of permanent endothermy during one part of the life cycle in two species of semisessile lanternflies (Fulgoridae), proving the established assumptions about physiological and morphological prerequisites for permanent endothermy to be wrong: apparently, permanent endothermy can evolve even in very small, semisessile animals as long as they have access to sufficient energy supply.

Two lanternfly species have permanently high, very stable body temperature for weeks during the adult life stage.

This is the first case of full endothermy in invertebrates, in semisessile animals, and in animals smaller than 1 g.

The Genomes of Two Billfishes Provide Insights into the Evolution of Endothermy in Teleosts

Endothermy is a typical convergent phenomenon which has evolved independently at least eight times in vertebrates, and is of significant advantage to organisms in extending their niches. However, how vertebrates other than mammals or birds, especially teleosts, achieve endothermy has not previously been fully understood. In this study, we sequenced the genomes of two billfishes (swordfish and sailfish), members of a representative lineage of endothermic teleosts. Convergent amino acid replacements were observed in proteins related to heat production and the visual system in two endothermic teleost lineages, billfishes and tunas. The billfish-specific genetic innovations were found to be associated with heat exchange, thermoregulation, and the specialized morphology, including elongated bill, enlarged dorsal fin in sailfish and loss of the pelvic fin in swordfish.

Evolution, Emotion, and Episodic Engagement

Although rodent research provides important insights into neural correlates of human psychology, new cortical areas, connections, and cognitive abilities emerged during primate evolution, including human evolution. Comparison of human brains with those of nonhuman primates reveals two aspects of human brain evolution particularly relevant to emotional disorders: expansion of homotypical association areas and expansion of the hippocampus. Two uniquely human cognitive capacities link these phylogenetic developments with emotion: a subjective sense of participating in and reexperiencing remembered events and a limitless capacity to imagine details of future events. These abilities provided evolving humans with selective advantages, but they also created proclivities for emotional problems. The first capacity evokes the "reliving" of past events in the "here-and-now," accompanied by emotional responses that occurred during memory encoding. It contributes to risk for stress-related syndromes, such as posttraumatic stress disorder. The second capacity, an ability to imagine future events without temporal limitations, facilitates flexible, goal-related behavior by drawing on and creating a uniquely rich array of mental representations. It promotes goal achievement and reduces errors, but the mental construction of future events also contributes to developmental aspects of anxiety and mood disorders. With maturation of homotypical association areas, the concrete concerns of childhood expand to encompass the abstract apprehensions of adolescence and adulthood. These cognitive capacities and their dysfunction are amenable to a research agenda that melds experimental therapeutic interventions, cognitive neuropsychology, and developmental psychology in both humans and nonhuman primates.

Individual quality and phenology mediate the effect of radioactive contamination on body temperature in Chernobyl barn swallows

Anthropogenic stressors, such as radioactive contaminants released from the Chernobyl and Fukushima Daiichi accidents, deteriorate ecological and evolutionary processes, as evidence for damaging effects of radioactive contamination on wildlife is accumulating. Yet little is known about physiological traits of animals inhabiting contaminated areas, and how those are affected by individual quality and phenology. We investigated variation in body temperature of wild barn swallows, Hirundo rustica, exposed to radioactive contamination from the Chernobyl accident in Ukraine and Belarus. We tested whether exposure to variable levels of radioactive contamination modified core body temperature of birds, and whether individual and phenological characteristics modulated radiosensitivity of body temperature. We showed that barn swallow body temperature varied with exposure to environmental radioactive contamination and that individual characteristics and phenology affected radioactive exposure. Increased radiosensitivity and up-regulation of body temperature were detected in birds of low body condition, high risk of capture, and in animals captured late during the day but early during the season. These results highlight the complex ways that the body temperature of a wild bird is impacted by exposure to increased radioactive contamination in natural habitats. By impacting body temperature, increased radioactive contamination may compromise energetic balance, jeopardize responsiveness to global warming, and increase risk of overheating.

Pitfalls in Monitoring Mitochondrial Temperature Using Charged Thermosensitive Fluorophores

Mitochondria are the source of internal heat which influences all cellular processes. Hence, monitoring mitochondrial temperature provides a unique insight into cell physiology. Using a thermosensitive fluorescent probe MitoThermo Yellow (MTY), we have shown recently that mitochondria within human cells are maintained at close to 50 °C when active, increasing their temperature locally by about 10 °C. Initially reported in the HEK293 cell line, we confirmed this finding in the HeLa cell line. Delving deeper, using MTY and MTX (MitoThermo X), a modified version of MTY, we unraveled some caveats related to the nature of these charged fluorophores. While enabling the assessment of mitochondrial temperature in HEK and HeLa cell lines, the reactivity of MTY to membrane potential variations in human primary skin fibroblasts precluded local temperature monitoring in these cells. Chemical modification of MTY into MTX did not result in a temperature probe unresponsive to membrane potential variations that could be universally used in any cell type to determine mitochondrial temperature. Thus, the cell-type dependence of MTY in measuring mitochondrial temperature, which is likely due to the variable binding of this dye to specific internal mitochondrial components, should imply cautiousness while using these nanothermometers for mitochondrial temperature analysis.

Warmer, faster, stronger: Ca2+ cycling in avian myocardium

Birds occupy a unique position in the evolution of cardiac design. Their hearts are capable of cardiac performance on par with, or exceeding that of mammals, and yet the structure of their cardiomyocytes resembles those of reptiles. It has been suggested that birds use intracellular Ca2+ stored within the sarcoplasmic reticulum (SR) to power contractile function, but neither SR Ca2+ content nor the cross-talk between channels underlying Ca2+-induced Ca2+ release (CICR) have been studied in adult birds. Here we used voltage clamp to investigate the Ca2+ storage and refilling capacities of the SR and the degree of trans-sarcolemmal and intracellular Ca2+ channel interplay in freshly isolated atrial and ventricular myocytes from the heart of the Japanese quail (Coturnix japonica). A trans-sarcolemmal Ca2+ current (ICa) was detectable in both quail atrial and ventricular myocytes, and was mediated only by L-type Ca2+ channels. The peak density of ICa was larger in ventricular cells than in atrial cells, and exceeded that reported for mammalian myocardium recorded under similar conditions. Steady-state SR Ca2+ content of quail myocardium was also larger than that reported for mammals, and reached 750.6±128.2 μmol l-1 in atrial cells and 423.3±47.2 μmol l-1 in ventricular cells at 24°C. We observed SR Ca2+-dependent inactivation of ICa in ventricular myocytes, indicating cross-talk between sarcolemmal Ca2+ channels and ryanodine receptors in the SR. However, this phenomenon was not observed in atrial myocytes. Taken together, these findings help to explain the high-efficiency avian myocyte excitation-contraction coupling with regard to their reptilian-like cellular ultrastructure.

Does control of insensible evaporative water loss by two species of mesic parrot have a thermoregulatory role?

Insensible evaporative water loss (EWL) at or below thermoneutrality is generally assumed to be a passive physical process. However, some arid zone mammals and a single arid zone bird can control their insensible water loss, so we tested the hypothesis that the same is the case for two parrot species from a mesic habitat. We investigated red-rumped parrots (Psephotus haematonotus) and eastern rosellas (Platycercus eximius), measuring their EWL, and other physiological variables, at a range of relative humidities at ambient temperatures of 20 and 30°C (below and at thermoneutrality). We found that, despite a decrease in EWL with increasing relative humidity, rates of EWL were not fully accounted for by the water vapour deficit between the animal and its environment, indicating that the insensible EWL of both parrots was controlled. It is unlikely that this deviation from physical expectations was regulation with a primary role for water conservation because our mesic-habitat parrots had equivalent regulatory ability as the arid habitat budgerigar (Melopsittacus undulatus). This, together with our observations of body temperature and metabolic rate, instead support the hypothesis that acute physiological control of insensible water loss serves a thermoregulatory purpose for endotherms. Modification of both cutaneous and respiratory avenues of evaporation may be involved, possibly via modification of expired air temperature and humidity, and surface resistance.

A Culture-Based ID of Micromycetes on the Wing Membranes of Greater Mouse-Eared Bats (Myotis myotis) from the "Nietoperek" Site (Poland)

Bats play important functions in ecosystems and many of them are threatened with extinction. Thus, the monitoring of the health status and prevention of diseases seem to be important aspects of welfare and conservation of these mammals. The main goal of the study was the identification of culturable fungal species colonizing the wing membranes of female greater mouse-eared bat (Myotis myotis) during spring emergence from the "Nietoperek" underground hibernation site by the use of genetic and phenotypic analyses. The study site is situated in Western Poland (52°25' N, 15°32' E) and is ranked within the top 10 largest hibernation sites in the European Union. The number of hibernating bats in the winter exceeds 39,000 individuals of 12 species, with M. myotis being the most common one. The wing membranes of M. myotis were sampled using sterile swabs wetted in physiological saline (0.85% NaCl). Potato dextrose agar (PDA) plates were incubated in the dark at 8, 24 and 36 ± 1 °C for 3 up to 42 days. All fungi isolated from the surface of wing membranes were assigned to 17 distinct fungal isolates belonging to 17 fungal species. Penicillium chrysogenum was the most frequently isolated species. Some of these fungal species might have a pathogenic potential for bats and other mammals. However, taking into account habitat preferences and the life cycle of bats, it can be assumed that some fungi were accidentally obtained from the surface of vegetation during early spring activity. Moreover, Pseudogymnoascus destructans (Pd)-the causative agent of the White Nose Syndrome (WNS)-was not found during testing, despite it was found very often in M. myotis during previous studies in this same location.

Stable mitochondrial CICIII2 supercomplex interactions in reptiles versus homeothermic vertebrates

The association of complex I (CI), complex III (CIII) and complex IV (CIV) of the mitochondrial electron transport chain into stable high molecular weight supercomplexes (SCs) has been observed in several prokaryotes and eukaryotes, but among vertebrates it has only been examined in mammals. The biological role of these SCs is unclear but suggestions so far include enhanced electron transfer between complexes, decreased production of the reactive oxygen species (ROS) O2- and H2O2, or enhanced structural stability. Here, we provide the first overview on the stability, composition and activity of mitochondrial SCs in representative species of several vertebrate classes to determine patterns of SC variation across endotherms and ectotherms. We found that the stability of the CICIII2 SC and the inclusion of CIV within the SC varied considerably. Specifically, when solubilized by the detergent DDM, mitochondrial CICIII2 SCs were unstable in endotherms (birds and mammals) and highly stable in reptiles. Using mass-spectrometric complexomics, we confirmed that the CICIII2 is the major SC in the turtle, and that 90% of CI is found in this highly stable SC. Interestingly, the presence of stable SCs did not prevent mitochondrial H2O2 production and was not associated with elevated respiration rates of mitochondria isolated from the examined species. Together, these data show that SC stability varies among vertebrates and is greatest in poikilothermic reptiles and weakest in endotherms. This pattern suggests an adaptive role of SCs to varying body temperature, but not necessarily a direct effect on electron transfer or in the prevention of ROS production.

The evolution of zebrafish RAG2 protein is required for adapting to the elevated body temperature of the higher endothermic vertebrates

The recombination activating gene (RAG or RAG1/RAG2 complex)-mediated adaptive immune system is a hallmark of jawed vertebrates. It has been reported that RAG originated in invertebrates. However, whether RAG further evolved once it arose in jawed vertebrates remains largely unknown. Here, we found that zebrafish RAG (zRAG) had a lower activity than mouse RAG (mRAG). Intriguingly, the attenuated stability of zebrafish RAG2 (zRAG2), but not zebrafish RAG1, caused the reduced V(D)J recombination efficiency compared to mRAG at 37 °C which are the body temperature of most endotherms except birds. Importantly, the lower temperature 28 °C, which is the best temperature for zebrafish growth, made the recombination efficiency of zRAG similar to that of mRAG by improving the stability of zRAG2. Consistent with the prementioned observation, the V(D)J recombination of Rag2^KI/KI mice, which zRAG2 was substituted for mRAG2, was also severely impaired. Unexpectedly, Rag2^KI/KI mice developed cachexia syndromes accompanied by premature death. Taken together, our findings illustrate that the evolution of zebrafish RAG2 protein is required for adapting to the elevated body temperature of the higher endothermic vertebrates.

Is Endothermy an Evolutionary By-Product?

Endothermy alters the energetic relationships between organisms and their environment and thereby influences fundamental niches. Endothermy is closely tied to energy metabolism. Regulation of energy balance is indispensable for all life and regulatory pathways increase in complexity from bacteria to vertebrates. Increasing complexity of metabolic networks also increase the probability for endothermic phenotypes to appear. Adaptive arguments are problematic epistemologically because the regulatory mechanisms enabling endothermy have not evolved for the 'purpose' of endothermy and the utility of current traits is likely to have changed over evolutionary time. It is most parsimonious to view endothermy as the evolutionary by-product of energy balance regulation rather than as an adaptation and interpret its evolution in the context of metabolic networks.

Convergent evolution of bird-mammal shared characteristics for adapting to nocturnality

The diapsid lineage (birds) and synapsid lineage (mammals), share a suite of functionally similar characteristics (e.g. endothermy) that are considered to be a result of their convergent evolution, but the candidate selections leading to this convergent evolution are still under debate. Here, we used a newly developed molecular phyloecological approach to reconstruct the diel activity pattern of the common ancestors of living birds. Our results strongly suggest that they had adaptations to nocturnality during their early evolution, which is remarkably similar to that of ancestral mammals. Given their similar adaptation to nocturnality, we propose that the shared traits in birds and mammals may have partly evolved as a result of the convergent evolution of their early ancestors adapting to ecological factors (e.g. low ambient temperature) associated with nocturnality. Finally, a conceptually unifying ecological model on the evolution of endothermy in diverse organisms with an emphasis on low ambient temperature is proposed. We reason that endothermy may evolve as an adaptive strategy to enable organisms to effectively implement various life-cycle activities under relatively low-temperature environments. In particular, a habitat shift from high-temperature to relatively low-temperature environments is identified as a common factor underlying the evolution of endothermy.

Were the synapsids primitively endotherms? A palaeohistological approach using phylogenetic eigenvector maps

The acquisition of mammalian endothermy is poorly constrained both phylogenetically and temporally. Here, we inferred the resting metabolic rates (RMRs) and the thermometabolic regimes (endothermy or ectothermy) of a sample of eight extinct synapsids using palaeohistology, phylogenetic eigenvector maps (PEMs), and a sample of 17 extant tetrapods of known RMR (quantified using respirometry). We inferred high RMR values and an endothermic metabolism for the anomodonts (Lystrosaurus sp., Oudenodon bainii) and low RMR values and an ectothermic metabolism for Clepsydrops collettii, Dimetrodon sp., Edaphosaurus boanerges, Mycterosaurus sp., Ophiacodon uniformis and Sphenacodon sp. A maximum-likelihood ancestral states reconstruction of RMRs performed using the values inferred for extinct synapsids, and the values measured using respirometry in extant tetrapods, shows that the nodes Anomodontia and Mammalia were primitively endotherms. Finally, we performed a parsimony optimization of the presence of endothermy using the results obtained in the present study and those obtained in previous studies that used PEMs. For this, we assigned to each extinct taxon a thermometabolic regime (ectothermy or endothermy) depending on whether the inferred values were significantly higher, lower or not significantly different from the RMR value separating ectotherms from endotherms (1.5 ml O₂ h^-1 g^-0.67). According to this optimization, endothermy arose independently in Archosauromorpha, Sauropterygia and Therapsida. This article is part of the theme issue 'Vertebrate palaeophysiology'.

The evolution of mechanisms involved in vertebrate endothermy

Endothermy, i.e. the endogenous production of metabolic heat, has evolved multiple times among vertebrates, and several strategies of heat production have been studied extensively by physiologists over the course of the twentieth century. The independent acquisition of endothermy by mammals and birds has been the subject of many hypotheses regarding their origin and associated evolutionary constraints. Many groups of vertebrates, however, are thought to possess other mechanisms of heat production, and alternative ways to regulate thermogenesis that are not always considered in the palaeontological literature. Here, we perform a review of the mechanisms involved in heat production, with a focus on cellular and molecular mechanisms, in a phylogenetic context encompassing the entire vertebrate diversity. We show that endothermy in mammals and birds is not as well defined as commonly assumed by evolutionary biologists and consists of a vast array of physiological strategies, many of which are currently unknown. We also describe strategies found in other vertebrates, which may not always be considered endothermy, but nonetheless correspond to a process of active thermogenesis. We conclude that endothermy is a highly plastic character in vertebrates and provides a guideline on terminology and occurrences of the different types of heat production in vertebrate evolution. This article is part of the theme issue 'Vertebrate palaeophysiology'.

Importance of adipocyte browning in the evolution of endothermy

Endothermy changes the relationship between organisms and their environment fundamentally, and it is therefore of major ecological and evolutionary significance. Endothermy is characterized by non-shivering thermogenesis, that is metabolic heat production in the absence of muscular activity. In many eutherian mammals, brown adipose tissue (BAT) is an evolutionary innovation that facilitates non-shivering heat production in mitochondria by uncoupling food-derived substrate oxidation from chemical energy (ATP) production. Consequently, energy turnover is accelerated resulting in increased heat release. The defining characteristics of BAT are high contents of mitochondria and vascularization, and the presence of uncoupling protein 1. Recent insights, however, reveal that a range of stimuli such as exercise, diet and the immune system can cause the browning of white adipocytes, thereby increasing energy expenditure and heat production even in the absence of BAT. Here, we review the molecular mechanisms that cause browning of white adipose tissue, and their potential contribution to thermoregulation. The significance for palaeophysiology lies in the presence of adipose tissue and the mechanisms that cause its browning and uncoupling in all amniotes. Hence, adipocytes may have played a role in the evolution of endothermy beyond the more specific evolution of BAT in eutherians. This article is part of the theme issue 'Vertebrate palaeophysiology'.

Whistling is metabolically cheap for communicating bottlenose dolphins (Tursiops truncatus)

Toothed whales depend on sound for communication and foraging, making them potentially vulnerable to acoustic masking from increasing anthropogenic noise. Masking effects may be ameliorated by higher amplitudes or rates of calling, but such acoustic compensation mechanisms may incur energetic costs if sound production is expensive. The costs of whistling in bottlenose dolphins (Tursiops truncatus) have been reported to be much higher (20% of resting metabolic rate, RMR) than theoretical predictions (0.5-1% of RMR). Here, we address this dichotomy by measuring the change in the resting O₂ consumption rate (V̇ _O2 ), a proxy for RMR, in three post-absorptive bottlenose dolphins during whistling and silent trials, concurrent with simultaneous measurement of acoustic output using a calibrated hydrophone array. The experimental protocol consisted of a 2-min baseline period to establish RMR, followed by a 2-min voluntary resting surface apnea, with or without whistling as cued by the trainers, and then a 5-min resting period to measure recovery costs. Daily fluctuations in V̇ _O2  were accounted for by subtracting the baseline RMR from the recovery costs to estimate the cost of apnea with and without whistles relative to RMR. Analysis of 52 sessions containing 1162 whistles showed that whistling did not increase metabolic cost (P>0.1, +4.2±6.9%) as compared with control trials (-0.5±5.9%; means±s.e.m.). Thus, we reject the hypothesis that whistling is costly for bottlenose dolphins, and conclude that vocal adjustments such as the Lombard response to noise do not represent large direct energetic costs for communicating toothed whales.

Revisiting the dermatomal recruitment of, and pressure-dependent influences on, human eccrine sweating

Herein we describe two experiments in which the recruitment and pressure-induced modifications of human eccrine sweating were investigated. In one experiment, the longstanding belief that glandular recruitment follows a gradual, caudal-to-rostral (dermatomal) recruitment pattern was re-evaluated. The onset of sweating was simultaneously determined (ventilated capsules) from four spinal (dermatomal) segments (forehead, dorsal hand, lower chest and dorsal foot) during the passive heating of supine participants (N = 8). No evidence was found to support either dermatomal or simultaneous glandular recruitment patterns. Instead, the results were more consistent with individualised (random) patterns of regional activation (P > 0.05), with significant time delays among sites. Such delays in the appearance of discharged sweat may reflect differences in neurotransmitter sensitivity, precursor sweat production or ductal reabsorption. In the second experiment, the pressure-induced hemihidrotic reflex (contralateral sudomotor enhancement) was revisited, using pressures applied over 10 cm² areas of the chest (left side: 6 N cm^-2) and left heel (3 N cm^-2) during both supine and seated postures (N = 12). Participants were passively heated and thermally clamped before pressure application. Hemihidrosis was not observed from the contralateral surfaces within the same (chest) or lower spinal segments (abdomen; both P > 0.05) during chest pressure, but a generalised enhancement followed heel pressure when supine. We suggest that previous observations of hemihidrosis possibly resulted from elevated heat storage, rather than a neural reflex. Chest pressure significantly inhibited ipsilateral sweating (forehead, hand, chest; all P < 0.05), and that influence is hypothesised to result from interactions between ascending mechanoreceptor afferents and the descending sudomotor pathways.

Does Basal Metabolism Set the Limit for Metabolic Downregulation during Torpor?

The evolution of endothermic thermoregulation is rooted in the processes involving high metabolism, which allows the maintenance of high and stable body temperatures (T_b). In turn, selection for high endothermic metabolism correlates with increased size of metabolically active organs and thus with high basal metabolic rate (BMR). Endothermic animals are characterized by an MR several times that of similar-sized ectotherms. However, many small mammals are temporally heterothermic and are able to temporally decrease T_b and MR by entering daily torpor or hibernation. Both BMR and minimum MR during torpor (TMR_min) likely result from oxidative respiration in mitochondria of the same tissues. It should be expected that these two MRs are positively correlated, suggesting that the evolution of endothermy and higher BMR set the limit for the ability to reduce MR while entering torpor. Using published data for 96 mammal species, we tested the hypothesis that, among heterothermic mammals, the processes leading to the evolution of higher BMR limit the ability to downregulate metabolism during torpor. We found that body mass (m_b)-adjusted BMR was positively correlated with m_b- and T_b-adjusted TMR_min, in a phylogenetically corrected analysis. Phylogenetic path modeling indicated that the mechanisms underlying the evolutionary increase of BMR in endotherms most likely constrain their ability to reduce MR during torpor. Given that heterothermy is considered an ancestral state in mammals, these results suggest an increase in BMR during the evolution of endothermy in homeothermic animals, which leads to the loss of their ability to enter torpor.

Can birds do it too? Evidence for convergence in evaporative water loss regulation for birds and mammals

Birds have many physiological characteristics that are convergent with mammals. In the light of recent evidence that mammals can maintain a constant insensible evaporative water loss (EWL) over a range of perturbing environmental conditions, we hypothesized that birds might also regulate insensible EWL, reflecting this convergence. We found that budgerigars (Melopsittacus undulatus) maintain EWL constant over a range of relative humidities at three ambient temperatures. EWL, expressed as a function of water vapour pressure deficit, differed from a physical model where the water vapour pressure deficit between the animal and the ambient air is the driver of evaporation, indicating physiological control of EWL. Regulating EWL avoids thermoregulatory impacts of varied evaporative heat loss; changes in relative humidity had no effect on body temperature, metabolic rate or thermal conductance. Our findings that a small bird can regulate EWL are evidence that this is a common feature of convergently endothermic birds and mammals, and may therefore be a fundamental characteristic of endothermy.

Quantitative Genetic Modeling of the Parental Care Hypothesis for the Evolution of Endothermy

There are two heuristic explanations proposed for the evolution of endothermy in vertebrates: a correlated response to selection for stable body temperatures, or as a correlated response to increased activity. Parental care has been suggested as a major driving force in this context given its impact on the parents' activity levels and energy budgets, and in the offspring's growth rates due to food provisioning and controlled incubation temperature. This results in a complex scenario involving multiple traits and transgenerational fitness benefits that can be hard to disentangle, quantify and ultimately test. Here we demonstrate how standard quantitative genetic models of maternal effects can be applied to study the evolution of endothermy, focusing on the interplay between daily energy expenditure (DEE) of the mother and growth rates of the offspring. Our model shows that maternal effects can dramatically exacerbate evolutionary responses to selection in comparison to regular univariate models (breeder's equation). This effect would emerge from indirect selection mediated by maternal effects concomitantly with a positive genetic covariance between DEE and growth rates. The multivariate nature of selection, which could favor a higher DEE, higher growth rates or both, might partly explain how high turnover rates were continuously favored in a self-reinforcing process. Overall, our quantitative genetic analysis provides support for the parental care hypothesis for the evolution of endothermy. We contend that much has to be gained from quantifying maternal and developmental effects on metabolic and thermoregulatory variation during adulthood.

Muscle Non-shivering Thermogenesis and Its Role in the Evolution of Endothermy

The development of sustained, long-term endothermy was one of the major transitions in the evolution of vertebrates. Thermogenesis in endotherms does not only occur via shivering or activity, but also via non-shivering thermogenesis (NST). Mammalian NST is mediated by the uncoupling protein 1 in the brown adipose tissue (BAT) and possibly involves an additional mechanism of NST in skeletal muscle. This alternative mechanism is based on Ca²⁺-slippage by a sarcoplasmatic reticulum Ca²⁺-ATPase (SERCA) and is controlled by the protein sarcolipin. The existence of muscle based NST has been discussed for a long time and is likely present in all mammals. However, its importance for thermoregulation was demonstrated only recently in mice. Interestingly, birds, which have evolved from a different reptilian lineage than mammals and lack UCP1-mediated NST, also exhibit muscle based NST under the involvement of SERCA, though likely without the participation of sarcolipin. In this review we summarize the current knowledge on muscle NST and discuss the efficiency of muscle NST and BAT in the context of the hypothesis that muscle NST could have been the earliest mechanism of heat generation during cold exposure in vertebrates that ultimately enabled the evolution of endothermy. We suggest that the evolution of BAT in addition to muscle NST was related to heterothermy being predominant among early endothermic mammals. Furthermore, we argue that, in contrast to small mammals, muscle NST is sufficient to maintain high body temperature in birds, which have enhanced capacities to fuel muscle NST by high rates of fatty acid import.

Was endothermy in amniotes induced by an early stop in growth during ontogeny?

Endothermy and its evolution are still an unresolved issue. Here, we present a model which transforms an ectothermic amniote (ancestor) into a derived amniote (descendant) showing many characteristics seen in extant endothermic birds and mammals. Consistent with the fossil record within the ancestral lineages of birds and mammals, the model assumes that mutations in genes which get active during ontogeny and affect body growth resulted in a reduced asymptotic body size and an early growth stop of the descendant. We show that such a postulated early growth stop in the descendant simultaneously increases the growth rate and metabolic rate, and also changes six life history traits (offspring mass, annual clutch/litter mass, number of offspring per year, age and mass at which sexual maturity is reached, age at which the individual is fully grown) of the descendant compared to a similar-sized ancestor. All these changes coincide with known differences between recent ectothermic and endothermic amniotes. We also elaborate many other differences and similarities in biological characteristics supporting the early growth stop. An early stop in growth during ontogeny thus could have played a key role in the evolution of endothermy within the reptilia and therapsids. It generated variability in characteristics of ancestral ectotherms, which was subject to natural selection in the past and resulted in many adaptations linked to endothermy in today's birds and mammals.

Mosaic evolution and the pattern of transitions in the hominin lineage

Humans are uniquely unique, in terms of the extreme differences between them and other living organisms, and the impact they are having on the biosphere. The evolution of humans can be seen, as has been proposed, as one of the major transitions in evolution, on a par with the origins of multicellular organisms or the eukaryotic cell (Maynard Smith & Szathmáry 1997 Major transitions in evolution). Major transitions require the evolution of greater complexity and the emergence of new evolutionary levels or processes. Does human evolution meet these conditions? I explore the diversity of evidence on the nature of transitions in human evolution. Four levels of transition are proposed-baseline, novel taxa, novel adaptive zones and major transitions-and the pattern of human evolution considered in the light of these. The primary conclusions are that changes in human evolution occur continuously and cumulatively; that novel taxa and the appearance of new adaptations are not clustered very tightly in particular periods, although there are three broad transitional phases (Pliocene, Plio-Pleistocene and later Quaternary). Each phase is distinctive, with the first based on ranging and energetics, the second on technology and niche expansion, and the third on cognition and cultural processes. I discuss whether this constitutes a 'major transition' in the context of the evolutionary processes more broadly; the role of behaviour in evolution; and the opportunity provided by the rich genetic, phenotypic (fossil morphology) and behavioural (archaeological) record to examine in detail major transitions and the microevolutionary patterns underlying macroevolutionary change. It is suggested that the evolution of the hominin lineage is consistent with a mosaic pattern of change.This article is part of the themed issue 'Major transitions in human evolution'.

Warming the mouse to model human diseases

Humans prefer to live within their thermal comfort or neutral zone, which they create by making shelters, wearing clothing and, more recently, by regulating their ambient temperature. These strategies enable humans to maintain a constant core temperature (a trait that is conserved across all endotherms, including mammals and birds) with minimal energy expenditure. Although this primordial drive leads us to seek thermal comfort, we house our experimental animals, laboratory mice (Mus musculus), under conditions of thermal stress. In this Review, we discuss how housing mice below their thermoneutral zone limits our ability to model and study human diseases. Using examples from cardiovascular physiology, metabolic disorders, infections and tumour immunology, we show that certain phenotypes observed under conditions of thermal stress disappear when mice are housed at thermoneutrality, whereas others emerge that are more consistent with human biology. Thus, we propose that warming the mouse might enable more predictive modelling of human diseases and therapies.

Modeling the costs and benefits associated with the evolution of endothermy using a robotic python

Endothermy provides considerable benefits to an organism but requires large energy investment. To understand potential driving forces that would lead to the evolution of endothermy, it is important to understand the energy costs and potential benefits of intermediate steps between ectothermy and homeothermic endothermy as well as the influences of environmental conditions on energetic costs. However, efforts to examine intermediate conditions are greatly limited by the predominant natural dichotomy between ectothermy and endothermy. Facultative endothermy by brooding pythons provides a fortunate study system where endothermy is beneficial but not essential. As one cannot control the extent of energy investment in heat production by a female python, we created an artificial snake with controllable heating capability. This enabled us to determine the energetic costs of maintaining a clutch at a preferred temperature, and to determine the relative thermal benefit of limited energy-producing capability (i.e. 50% of the required energy to maintain the preferred developmental temperature). We manipulated the pseudoserpent's clutch size (5, 10, 15 eggs), diel ambient temperature cycle (2, 4, 6°C) and insulation (with and without) at each of these power levels: unlimited power, half required power and no power. We found no significant effect of clutch size on either power requirements or developmental temperature. Energy requirements increased with the amplitude of the diel cycle and decreased with the addition of insulation, while the quality of the thermal environment decreased with the amplitude of the diel cycle. Interestingly, the quality of the thermal environment also decreased with the addition of insulation. We discuss these results within the context of the reproductive model of the evolution of endothermy.

NATURAL SELECTION ON THERMOGENIC CAPACITY OF HIGH-ALTITUDE DEER MICE

Adaptive explanations that rely on physiological arguments are common, but tests of hypotheses about the significance of whole-animal physiological performance (e.g., aerobic capacities) are rare. We studied phenotypic selection on the thermogenic capacity (i.e., maximal rate of oxygen consumption [VO₂ max] elicited via cold exposure) of high-altitude (~3800 m) deer mice (Peromyscus maniculatus). A high VO₂ max equates to a high capacity for heat production and should favor survival in the cold environments prevalent at high altitude. Strong directional selection favored high VO₂ max, at least in one year. The selection for increased VO₂ max is consistent with predictions derived from incorporating our physiological data into a biophysical model. During another year, we found weak evidence of selection for decreased body mass. Nonlinear selection was not significant for any of the selection episodes we studied. The strong directional selection for VO₂ max that we observed suggests that-given ample genetic variation-aerobic metabolism and perhaps endothermy may have evolved rapidly on the geological time scale.