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

   https://doi.org/10.1113/JP284586  

   Storz, Jay F. ; Scott, Graham R. ( October 2023 , The Journal of Physiology)

   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 (genusPhyllotis), 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 genusPeromyscus, highland deer mice (Peromyscus maniculatus) have greater thermogenic maximal oxygen uptake () 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 thermogenic in hypoxia is attributable to a combination of evolved and plastic changes in physiological pathways that govern the transport and utilization of O₂and metabolic substrates. Experiments withPeromyscusmice also demonstrate that exposure to hypoxia during different stages of development elicits plastic changes in cardiorespiratory traits that improve thermogenic 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.image

    

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2. Survival is negatively related to basal metabolic rate in tropical Andean birds

   https://doi.org/10.1111/1365-2435.13375  

   Scholer, Micah N. ; Arcese, Peter ; Puterman, Martin L. ; Londoño, Gustavo A. ; Jankowski, Jill E. ; Portugal, ed., Steven ( June 2019 , Functional Ecology)

   Life‐history theory postulates that physiological traits, such as energy metabolism, can be understood in terms of allocation trade‐offs between self‐maintenance and reproduction over an organism's life span, and data show that metabolic intensity and survival vary inversely with latitude, with tropical birds exhibiting a “slow” pace of life relative to temperature species. However, tropical regions harbour strong environmental gradients of their own, and it remains to be shown whether similar life‐history trade‐offs between metabolism and longevity are reflected among tropical birds of the same latitude.

   We estimated apparent annual survival in 37 species of tropical passerine birds along an elevational gradient (400–3,000 m) in Peru to test whether variation in survival was influenced by basal metabolic rate (BMR; estimated at the same sites), elevation or both factors. We used path analysis to test our prediction that survival would decline as BMR increased, while accounting for the potential direct effects of elevation on survival due to differences in predation pressure or environmental conditions as well as potential indirect effects of elevation on BMR via temperature and the costs of thermoregulation.

   Higher BMR in tropical passerine birds predicted lower apparent survival, regardless of the elevation at which species occurred. In addition, elevation had a direct negative effect on apparent survival, perhaps due to harsher abiotic conditions, low site fidelity or both at high elevations.

   We provide evidence of a link between metabolic rate and longevity previously undescribed in populations of free‐living birds. Our results illustrate that tropical montane species may be characterized by a unique suite of traits in their pace of life, in which BMR does not differ from lowland birds, but survival does.

   Aplain language summaryis available for this article.

    

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3. Adaptive Shifts in Gene Regulation Underlie a Developmental Delay in Thermogenesis in High-Altitude Deer Mice

   https://doi.org/10.1093/molbev/msaa086  

   Velotta, Jonathan P ; Robertson, Cayleih E ; Schweizer, Rena M ; McClelland, Grant B ; Cheviron, Zachary A ( April 2020 , Molecular Biology and Evolution)

   Ruvinsky, Ilya (Ed.)

   Abstract Aerobic performance is tied to fitness as it influences an animal’s ability to find food, escape predators, or survive extreme conditions. At high altitude, where low O2 availability and persistent cold prevail, maximum metabolic heat production (thermogenesis) is an aerobic performance trait that is closely linked to survival. Understanding how thermogenesis evolves to enhance survival at high altitude will yield insight into the links between physiology, performance, and fitness. Recent work in deer mice (Peromyscus maniculatus) has shown that adult mice native to high altitude have higher thermogenic capacities under hypoxia compared with lowland conspecifics, but that developing high-altitude pups delay the onset of thermogenesis. This finding suggests that natural selection on thermogenic capacity varies across life stages. To determine the mechanistic cause of this ontogenetic delay, we analyzed the transcriptomes of thermoeffector organs—brown adipose tissue and skeletal muscle—in developing deer mice native to low and high altitude. We demonstrate that the developmental delay in thermogenesis is associated with adaptive shifts in the expression of genes involved in nervous system development, fuel/O2 supply, and oxidative metabolism pathways. Our results demonstrate that selection has modified the developmental trajectory of the thermoregulatory system at high altitude and has done so by acting on the regulatory systems that control the maturation of thermoeffector tissues. We suggest that the cold and hypoxic conditions of high altitude force a resource allocation tradeoff, whereby limited energy is allocated to developmental processes such as growth, versus active thermogenesis, during early development. 

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4. The adaptive benefit of evolved increases in hemoglobin-O2 affinity is contingent on tissue O2 diffusing capacity in high-altitude deer mice

   https://doi.org/10.1186/s12915-021-01059-4  

   Wearing, Oliver H. ; Ivy, Catherine M. ; Gutiérrez-Pinto, Natalia ; Velotta, Jonathan P. ; Campbell-Staton, Shane C. ; Natarajan, Chandrasekhar ; Cheviron, Zachary A. ; Storz, Jay F. ; Scott, Graham R. ( June 2021 , BMC Biology)

   Complex organismal traits are often the result of multiple interacting genes and sub-organismal phenotypes, but how these interactions shape the evolutionary trajectories of adaptive traits is poorly understood. We examined how functional interactions between cardiorespiratory traits contribute to adaptive increases in the capacity for aerobic thermogenesis (maximal O₂consumption,V̇O₂max, during acute cold exposure) in high-altitude deer mice (Peromyscus maniculatus). We crossed highland and lowland deer mice to produce F₂inter-population hybrids, which expressed genetically based variation in hemoglobin (Hb) O₂affinity on a mixed genetic background. We then combined physiological experiments and mathematical modeling of the O₂transport pathway to examine the links between cardiorespiratory traits andV̇O₂max.

   Physiological experiments revealed that increases in Hb-O₂affinity of red blood cells improved blood oxygenation in hypoxia but were not associated with an enhancement inV̇O₂max. Sensitivity analyses performed using mathematical modeling showed that the influence of Hb-O₂affinity onV̇O₂max in hypoxia was contingent on the capacity for O₂diffusion in active tissues.

   These results suggest that increases in Hb-O₂affinity would only have adaptive value in hypoxic conditions if concurrent with or preceded by increases in tissue O₂diffusing capacity. In high-altitude deer mice, the adaptive benefit of increasing Hb-O₂affinity is contingent on the capacity to extract O₂from the blood, which helps resolve controversies about the general role of hemoglobin function in hypoxia tolerance.

    

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5. Pervasive Genomic Signatures of Local Adaptation to Altitude Across Highland Specialist Andean Hummingbird Populations

   https://doi.org/10.1093/jhered/esab008  

   Lim, Marisa C ; Bi, Ke ; Witt, Christopher C ; Graham, Catherine H ; Dávalos, Liliana M ( February 2021 , Journal of Heredity)

   Chapman, Mark (Ed.)

   Abstract Populations along steep environmental gradients are subject to differentiating selection that can result in local adaptation, despite countervailing gene flow, and genetic drift. In montane systems, where species are often restricted to narrow ranges of elevation, it is unclear whether the selection is strong enough to influence functional differentiation of subpopulations differing by a few hundred meters in elevation. We used targeted capture of 12 501 exons from across the genome, including 271 genes previously implicated in altitude adaptation, to test for adaptation to local elevations for 2 highland hummingbird species, Coeligena violifer (n = 62) and Colibri coruscans (n = 101). For each species, we described population genetic structure across the complex geography of the Peruvian Andes and, while accounting for this structure, we tested whether elevational allele frequency clines in single nucleotide polymorphisms (SNPs) showed evidence for local adaptation to elevation. Although the 2 species exhibited contrasting population genetic structures, we found signatures of clinal genetic variation with shifts in elevation in both. The genes with SNP-elevation associations included candidate genes previously discovered for high-elevation adaptation as well as others not previously identified, with cellular functions related to hypoxia response, energy metabolism, and immune function, among others. Despite the homogenizing effects of gene flow and genetic drift, natural selection on parts of the genome evidently optimizes elevation-specific cellular function even within elevation range-restricted montane populations. Consequently, our results suggest local adaptation occurring in narrow elevation bands in tropical mountains, such as the Andes, may effectively make them “taller” biogeographic barriers. 

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