Discovery of the world's highest-dwelling mammal

Extreme High-Elevation Mammal Surveys Reveal Unexpectedly High Upper Range Limits of Andean Mice

AbstractIn the world's highest mountain ranges, uncertainty about the upper elevational range limits of alpine animals represents a critical knowledge gap regarding the environmental limits of life and presents a problem for detecting range shifts in response to climate change. Here we report results of mountaineering mammal surveys in the Central Andes, which led to the discovery of multiple species of mice living at extreme elevations that far surpass previously assumed range limits for mammals. We livetrapped small mammals from ecologically diverse sites spanning >6,700 m of vertical relief, from the desert coast of northern Chile to the summits of the highest volcanoes in the Andes. We used molecular sequence data and whole-genome sequence data to confirm the identities of species that represent new elevational records and to test hypotheses regarding species limits. These discoveries contribute to a new appreciation of the environmental limits of vertebrate life.

Convergent High O2 Affinity but Distinct ATP-Mediated Allosteric Regulation of Hemoglobins in Oviparous and Viviparous Eremias Lizards from the Qinghai-Tibet Plateau

The functional adaptation and underlying molecular mechanisms of hemoglobins (Hbs) have primarily concentrated on mammals and birds, with few reports on reptiles. This study aimed to investigate the convergent and species-specific high-altitude adaptation mechanisms of Hbs in two Eremias lizards from the Qinghai-Tibet Plateau. The Hbs of high-altitude E. argus and E. multiocellata were characterized by significantly high overall and intrinsic Hb-O2 affinity compared to their low-altitude populations. Despite the similarly low Cl- sensitivities, the Hbs of high-altitude E. argus exhibited higher ATP sensitivity and ATP-dependent Bohr effects than that of E. multiocellata, which could facilitate O2 unloading in respiring tissues. Eremias lizards Hbs exhibited similarly low temperature sensitivities and relatively high Bohr effects at lower temperatures, which could help to stably deliver and release O2 to cold extremities at low temperatures. The oxygenation properties of Hbs in high-altitude populations might be attributed to varying ratios of β2/β1 globin and substitutions on the β2-type globin. Notably, the Asn12Ala in lowland E. argus could cause localized destabilization of the E-helix in the tetrameric Hb by elimination of hydrogen bonds, thereby resulting in its lowest O2 affinity. This study provides a valuable reference for the high-altitude adaptation mechanisms of hemoglobins in reptiles.

To what extent do physiological tolerances determine elevational range limits of mammals?

A key question in biology concerns the extent to which distributional range limits of species are determined by intrinsic limits of physiological tolerance. Here, we use common-garden data for wild rodents to assess whether species with higher elevational range limits typically have higher thermogenic capacities in comparison to closely related lowland species. Among South American leaf-eared mice (genus Phyllotis), mean thermogenic performance is higher in species with higher elevational range limits, but there is little among-species variation in the magnitude of plasticity in this trait. In the North American rodent genus Peromyscus, highland deer mice (Peromyscus maniculatus) have greater thermogenic maximal oxygen uptake (V ̇ O 2 max ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}{\mathrm{max}}}}$ ) than lowland white-footed mice (Peromyscus leucopus) at a level of hypoxia that matches the upper elevational range limit of the former species. In highland deer mice, the enhanced thermogenicV ̇ O 2 max ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}{\mathrm{max}}}}$ in hypoxia is attributable to a combination of evolved and plastic changes in physiological pathways that govern the transport and utilization of O2 and metabolic substrates. Experiments with Peromyscus mice also demonstrate that exposure to hypoxia during different stages of development elicits plastic changes in cardiorespiratory traits that improve thermogenicV ̇ O 2 max ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}{\mathrm{max}}}}$ via distinct physiological mechanisms. Evolved differences in thermogenic capacity provide clues about why some species are able to persist in higher-elevation habitats that lie slightly beyond the tolerable limits of other species. Such differences in environmental tolerance also suggest why some species might be more vulnerable to climate change than others.

Genomic insights into the mystery of mouse mummies on the summits of Atacama volcanoes

Our understanding of the limits of animal life is continually revised by scientific exploration of extreme environments. Here we report the discovery of mummified cadavers of leaf-eared mice, Phyllotis vaccarum, from the summits of three different Andean volcanoes at elevations 6,029-6,233 m above sea level in the Puna de Atacama in Chile and Argentina. Such extreme elevations were previously assumed to be completely uninhabitable by mammals. In combination with a live-captured specimen of the same species from the nearby summit of Volcán Llullaillaco (6,739 m)1, the summit mummies represent the highest altitude physical records of mammals in the world. We also report a chromosome-level genome assembly for P. vaccarum that, in combination with a whole-genome re-sequencing analysis and radiocarbon dating analysis, provides insights into the provenance and antiquity of the summit mice. Radiocarbon data indicate that the most ancient of the mummies are, at most, a few centuries old. Genomic polymorphism data revealed a high degree of continuity between the summit mice and conspecifics from lower elevations in the surrounding Altiplano. Genomic data also revealed equal numbers of males and females among the summit mice and evidence of close kinship between some individuals from the same summits. These findings bolster evidence for resident populations of Phyllotis at elevations >6,000 m and challenge assumptions about the environmental limits of vertebrate life and the physiological tolerances of small mammals.

Elevational range extension of the Puna Mouse, Punomys (Cricetidae), with the first record of the genus from Chile

We report an elevational record for the Andean sigmodontine Puna Mouse Punomys, which is also the first record of the genus in Chile. The record is based on a mummified specimen that we discovered at an elevation of 5,461 m (17,917 feet) in the caldera of Volcán Acamarachi, Región de Antofagasta, Chile. Results of a morphological assessment suggest that the specimen can be provisionally referred to the species P. lemminus. This new record also extends the known geographic distribution of the genus by 700 km to the south and brings the known Chilean mammal richness to a total of 170 living species and 88 genera. This finding highlights the need for increased survey efforts in more remote, high-elevation regions and demonstrates that there is still much to be learned about the mammal fauna of the Andean Altiplano.

Extreme high-elevation mammal surveys reveal unexpectedly high upper range limits of Andean mice

In the world's highest mountain ranges, uncertainty about the upper elevational range limits of alpine animals represents a critical knowledge gap regarding the environmental limits of life and presents a problem for detecting range shifts in response to climate change. Here we report results of mountaineering mammal surveys in the Central Andes, which led to the discovery of multiple species of mice living at extreme elevations that far surpass previously assumed range limits for mammals. We live-trapped small mammals from ecologically diverse sites spanning >6700 m of vertical relief, from the desert coast of northern Chile to the summits of the highest volcanoes in the Andes. We used molecular sequence data and whole-genome sequence data to confirm the identities of species that represent new elevational records and to test hypotheses regarding species limits. These discoveries contribute to a new appreciation of the environmental limits of vertebrate life.

Evidence of a population of leaf-eared mice Phyllotis vaccarum above 6,000 m in the Andes and a survey of high-elevation mammals

Biologists have long pondered the extreme limits of life on Earth, including the maximum elevation at which species can live and reproduce. Here we review evidence of a self-sustaining population of mice at an elevation that exceeds that of all previously reported for mammals. Five expeditions over 10 years to Volcán Llullaillaco on the Argentina/Chile border observed and collected mice at elevations ranging from 5,070 m at the mountain's base to the summit at 6,739 m (22,110 feet). Previously unreported evidence includes observations and photographs of live animals and mummified remains, environmental DNA, and a soil microbial community reflecting animal activity that are evaluated in combination with previously reported video recordings and capture of live mice. All of the evidence identifies the mouse as the leaf-eared mouse Phyllotis vaccarum, and it robustly places the population within a haplotype group containing individuals from the Chilean Atacama Desert and nearby regions of Argentina. A critical review of the literature affirms that this population is not only an elevational record for mammals but for all terrestrial vertebrates to date, and we further find that many extreme elevations previously reported for mammals are based on scant or dubious evidence.

Uncovering cryptic diversity does not end: a new species of leaf-eared mouse, genus Phyllotis (Rodentia, Cricetidae), from Central Sierras of Argentina

Based on previously published molecular (mitochondrial) and herein provided morphological (qualitative and quantitative data) evidence, we describe a new species of leaf-eared mouse of the genus Phyllotis. The new species is morphometrically distinct when compared with other phylogenetically or geographically close species of Phyllotis, showing several quantitative differences in their external and craniodental characters (e.g., proportionally broader nasals and interorbital region, and proportionally smaller tympanic bullae). The new species is endemic to central Argentina, occurring on rocky grasslands at elevations of 650–2,800 m a.s.l. This is the only species of Phyllotis inhabiting the Central Sierras, a mountain system of medium elevation, isolated from the Andes by low elevation arid and semiarid environments.

DNA barcodes highlight genetic diversity patterns in rodents from lowland desert and andean areas in argentina

Rodents are an important component of South America fauna. Their high diversity has motivated researchers to continually review their taxonomy, genetic diversity, species limits, and phylogenetic relationships. We apply DNA-barcodes for assessing the taxonomic and genetic diversity in the two major lineages of South American rodents: caviomorphs and sigmodontines. We analyzed 335 COI barcodes in 34 morphologically determined species from 39 localities along central Andes and arid lands of Argentina. Neighbor Joining and Maximum Likelihood reconstruction provided clear separation between species. The Barcode Index Number and Bayesian Poisson Tree Processes were used to confirm concordance between sequence clusters and species designations by taxonomy. We found deep divergence within the Phyllotis xanthopygus species complex, with distances up to 13.0% between geographically separated lineages. Minor divergences (3.30% and 2.52%) were found within Abrothrix hirta, and Tympanoctomys barrerae, respectively, with differentiation in their genetic lineages. Also, we documented geographically separated clusters for Akodon spegazzinii and A.oenos with up to 2.3% divergence, but clustering methods failed to distinguish them as different species. Sequence results show a clear barcode gap with a mean intraspecific divergence (0.56%) vs. a minimum nearest-neighbor distance averaging (10.1%). Distances between congeneric species varied from 4.1 to 14%, with the exception of two related forms within Euneomys and the sister species Akodon spegazzinii and A. oenos. This study constitutes a substantial contribution to the global barcode reference library. It provides insights into the complex phylogeographic patterns and speciation scenarios in rodents, while highlighting areas that require in-depth taxonomic and integrative research.

Increased Reliance on Carbohydrates for Aerobic Exercise in Highland Andean Leaf-Eared Mice, but Not in Highland Lima Leaf-Eared Mice

Exercise is an important performance trait in mammals and variation in aerobic capacity and/or substrate allocation during submaximal exercise may be important for survival at high altitude. Comparisons between lowland and highland populations is a fruitful approach to understanding the mechanisms for altitude differences in exercise performance. However, it has only been applied in very few highland species. The leaf-eared mice (LEM, genus Phyllotis) of South America are a promising taxon to uncover the pervasiveness of hypoxia tolerance mechanisms. Here we use lowland and highland populations of Andean and Lima LEM (P. andium and P. limatus), acclimated to common laboratory conditions, to determine exercise-induced maximal oxygen consumption (V˙O₂max), and submaximal exercise metabolism. Lowland and highland populations of both species showed no difference in V˙O₂max running in either normoxia or hypoxia. When run at 75% of V˙O₂max, highland Andean LEM had a greater reliance on carbohydrate oxidation to power exercise. In contrast, highland Lima LEM showed no difference in exercise fuel use compared to their lowland counterparts. The higher carbohydrate oxidation seen in highland Andean LEM was not explained by maximal activities of glycolytic enzymes in the gastrocnemius muscle, which were equivalent to lowlanders. This result is consistent with data on highland deer mouse populations and suggests changes in metabolic regulation may explain altitude differences in exercise performance.

Fetal growth, high altitude, and evolutionary adaptation: a new perspective

Residence at high altitude is consistently associated with low birthweight among placental mammals. This reduction in birthweight influences long-term health trajectories for both the offspring and mother. However, the physiological processes that contribute to fetal growth restriction at altitude are still poorly understood, and thus our ability to safely intervene remains limited. One approach to identify the factors that mitigate altitude-dependent fetal growth restriction is to study populations that are protected from fetal growth restriction through evolutionary adaptations (e.g., high altitude-adapted populations). Here, we examine human gestational physiology at high altitude from a novel evolutionary perspective that focuses on patterns of physiological plasticity, allowing us to identify 1) the contribution of specific physiological systems to fetal growth restriction and 2) the mechanisms that confer protection in highland-adapted populations. Using this perspective, our review highlights two general findings: first, that the beneficial value of plasticity in maternal physiology is often dependent on factors more proximate to the fetus; and second, that our ability to understand the contributions of these proximate factors is currently limited by thin data from altitude-adapted populations. Expanding the comparative scope of studies on gestational physiology at high altitude and integrating studies of both maternal and fetal physiology are needed to clarify the mechanisms by which physiological responses to altitude contribute to fetal growth outcomes. The relevance of these questions to clinical, agricultural, and basic research combined with the breadth of the unknown highlight gestational physiology at high altitude as an exciting niche for continued work.

High Altitude Limits of Living Things

West, John B. High altitude limits of living things. High Alt Med Biol. 22:342-345, 2021.-The tolerance of animals to high altitude is generally limited by the low partial pressure of oxygen (PO₂) in the air. Plant growth at high altitude is also limited but by different mechanisms. This article is a brief survey of the limiting factors of all living things. By a curious coincidence, the highest point on earth, that is Mt. Everest at 8,848 m, appears to be right at the limit of human tolerance to hypoxia. The altitude of the highest permanent human habitation, that is a town, is 5,100 m. This altitude is partly determined by the hypoxia, but also by economic factors. For other terrestrial mammals, birds, and insects, the highest altitudes for permanent habitation apparently belong to field mice (Phyllotis xanthopygus rupestris) and jumping spiders (Euophrys omnisuperstes) at about 6,700 m. Birds have been known to fly as high as 11,000 m although how much they are elevated by atmospheric updrafts is not clear. The record for animals for survival in extreme hypoxia is arguably held by the primitive invertebrate, the tardigrade (Hypsibius dujardini). This has been shown to tolerate the hard vacuum of space where the PO₂ is essentially zero for many days. Less is known about the tolerance of plants to extreme altitude. However, vascular plants have been collected at >6,000 m in the Himalayas, and moss grows even higher. Lichens are very tolerant of severe hypoxia. There is evidence that global warming is increasing the highest altitudes at which plants can survive.

Evolved changes in maternal care in high-altitude native deer mice

At high altitude (HA), unremitting low oxygen and persistent cold push small mammals close to their metabolic ceilings, leaving limited scope for aerobically demanding activities. However, HA breeding seasons are relatively short and endemic rodents compensate with larger litters than low altitude (LA) conspecifics. Rodent mothers are the sole source of heat and nutrition for altricial offspring and lactation is energetically costly. Thus, it is unclear how HA females balance energy allocation during the nursing period. We hypothesized that HA female rodents invest heavily in each litter to ensure postnatal survival. We measured maternal energetic output and behaviour in nursing deer mice (Peromyscus maniculatus) native to LA (400 m a.s.l.) and HA (4350 m a.s.l.) under control (24°C, 760 mmHg) and cold hypoxia conditions, simulating HA (5°C, 430 mmHg). Strikingly, resting metabolic rates of lactating HA and LA females under cold hypoxia were 70-85% of their maximum aerobic capacity. In cold hypoxia, LA mothers increased both nursing time and milk fat content, however their pups were leaner and severely growth restricted at weaning. HA mothers also increased nursing in cold hypoxia but for far less time than LA mothers. Despite receiving less care, HA pups in cold hypoxia only experienced small growth restrictions at weaning and maintained body composition. As adults, HA mice raised in cold hypoxia had increased aerobic capacity compared to controls. These data suggest that HA mothers prioritize their own maintenance costs over investing heavily in their offspring. Pups compensate for this lack of care, likely by reducing their own metabolic costs during development.

Alexander von Humboldt (1769-1859): early high-altitude explorer and renowned plant naturalist

Alexander von Humboldt (1769-1859) was one of the most distinguished German scientists of the late 18th and early 19th centuries. His fame came chiefly from his extensive explorations in South America and his eminence as a plant naturalist. He attempted to climb the inactive volcano Chimborazo in Ecuador, which was thought to be the highest mountain in the world at the time, and he reached an altitude of about 5,543 m, which was a record height for humans. During the climb, he had typical symptoms of acute mountain sickness, which he correctly attributed to the low level of oxygen, and he was apparently the first person to make this connection. His ability as a naturalist enabled him to recognize the effect of high altitude on the distribution of plants, and by comparing his observations on Chimborazo with those in the European Alps and elsewhere, he inferred that the deleterious effects of high altitude were universal. During his return trip to Europe, he called on President Thomas Jefferson in Washington, where he was given a warm reception, and discussed conservation issues. He then returned to Paris, where he produced 29 volumes over a period of 31 years describing his travels. Here the effects of high altitude on the distribution of plants compared with animals are briefly reviewed. Following Humboldt's death in 1859, there was extensive coverage of his contributions, but curiously, his fame has diminished over the years, and inexplicably, he now has a lower profile in North America.

Physiological Genomics of Adaptation to High-Altitude Hypoxia

Population genomic studies of humans and other animals at high altitude have generated many hypotheses about the genes and pathways that may have contributed to hypoxia adaptation. Future advances require experimental tests of such hypotheses to identify causal mechanisms. Studies to date illustrate the challenge of moving from lists of candidate genes to the identification of phenotypic targets of selection, as it can be difficult to determine whether observed genotype-phenotype associations reflect causal effects or secondary consequences of changes in other traits that are linked via homeostatic regulation. Recent work on high-altitude models such as deer mice has revealed both plastic and evolved changes in respiratory, cardiovascular, and metabolic traits that contribute to aerobic performance capacity in hypoxia, and analyses of tissue-specific transcriptomes have identified changes in regulatory networks that mediate adaptive changes in physiological phenotype. Here we synthesize recent results and discuss lessons learned from studies of high-altitude adaptation that lie at the intersection of genomics and physiology.

Developmental and reproductive physiology of small mammals at high altitude: challenges and evolutionary innovations

High-altitude environments, characterized by low oxygen levels and low ambient temperatures, have been repeatedly colonized by small altricial mammals. These species inhabit mountainous regions year-round, enduring chronic cold and hypoxia. The adaptations that allow small mammals to thrive at altitude have been well studied in non-reproducing adults; however, our knowledge of adaptations specific to earlier life stages and reproductive females is extremely limited. In lowland natives, chronic hypoxia during gestation affects maternal physiology and placental function, ultimately limiting fetal growth. During post-natal development, hypoxia and cold further limit growth both directly by acting on neonatal physiology and indirectly via impacts on maternal milk production and care. Although lowland natives can survive brief sojourns to even extreme high altitude as adults, reproductive success in these environments is very low, and lowland young rarely survive to sexual maturity in chronic cold and hypoxia. Here, we review the limits to maternal and offspring physiology - both pre-natal and post-natal - that highland-adapted species have overcome, with a focus on recent studies on high-altitude populations of the North American deer mouse (Peromyscus maniculatus). We conclude that a combination of maternal and developmental adaptations were likely to have been critical steps in the evolutionary history of high-altitude native mammals.

Elevation patterns and critical environmental drivers of the taxonomic, functional, and phylogenetic diversity of small mammals in a karst mountain area

Understanding how biodiversity components are related under different environmental factors is a fundamental challenge for ecology studies, yet there is little knowledge of this interplay among the biotas, especially small mammals, in karst mountain areas. Here, we examine the elevation patterns of the taxonomic diversity (TD), phylogenetic diversity (PD), and functional diversity (FD) of small mammals in a karst mountain area, the Wuling Mountains, Southwest China, and compare these patterns between taxa (Rodentia and Eulipotyphla) and scales (broad‐ and narrow‐range species). We also disentangle the impacts of the human influence index, net primary productivity (NPP), normalized difference vegetation index (NDVI), annual precipitation (AP), and annual mean temperature (AMT) on these three facets of biodiversity by using structural equation modeling. We recorded a total of 39 small mammal species, including 26 rodents and 13 species of the order Eulipotyphla. Our study shows that the facets of biodiversity are spatially incongruent. Net primary productivity has a positive effect on the three facets for most groups, while the effect of the NDVI is negative for TD and PD in most groups. AMT temperature and AP have negative effects on FD and PD, whereas TD is dependent on the species range scale. The human influence index effect on TD and PD also depends on the species range scale. These findings provide robust evidence that the ecological drivers of biodiversity differ among different biotas and different range scales, and future research should use multifacet approach to determine biodiversity conservation strategies.