The glass transition is marked by a rapid increase in the barriers for molecular rearrangement, and this leads to the vitrification of supercooled liquids. Recent theories suggested that at low temperature, molecular rearrangements generate elastic stresses that dissipate through the surrounding material and add a non-local contribution to relaxation barriers. However, it is difficult to experimentally measure this elastic contribution. Here we measure the elastic barriers by investigating the transformation of vapour-deposited stable glasses, influenced by distant boundaries of varying elasticity. Rigid boundaries preserve bulk super-Arrhenius dynamics in which both relaxation times and barriers increase on supercooling. By contrast, distant soft boundaries facilitate fast Arrhenius relaxations—even below the glass transition temperature—by allowing faraway stress dissipation. As such, the constant soft-substrate barrier is attributed to local interactions, whereas the differences between barriers on soft and rigid boundaries arise from non-local elastic barriers. Our results show that the rapid dynamical slowdown and vitrification of supercooled liquids are governed by the emergence and growth of elastic barriers, and provide a direct experimental basis for elasticity-based descriptions of the glass transition. Elastic stresses are believed to play a role in the vitrification process in glasses, but measuring them is challenging. Imposing elastic boundary conditions is now shown to provide a way to probe elastic effects within glassy samples.

IntroductionGlobal temperatures are rising, and scientists are mobilizing to uncover which birds are most affected by the problem of heat. Heat shock proteins (HSPs), for example, can shed light on this issue because they prevent damage and promote recovery from heat. However, few studies have investigated the relationship between HSPs and heat outside of experimental contexts. Here, we ask whether natural variation in HSP gene expression can serve as a biomarker of recent ambient conditions in wild nestling tree swallows (Tachycineta bicolor).MethodsWe focused on HSP90AA1 because this HSP increases mRNA abundance in avian blood, after acute heat. Using blood samples collected across ten degrees of latitude, we tested for population differences in constitutive HSP90AA1 gene expression in 12-day-old nestlings. To quantify the specific time period over which ambient conditions best predicted variation in HSP gene expression, we used a climate window analysis, evaluating the predictive value of maximum temperatures and maximum heat index in the hours and days from hatching until sampling.ResultsWe observed a significant difference in constitutive HSP gene expression between populations, with South Carolina nestlings showing nearly double the HSP90AA1 mRNA abundance compared to those in Massachusetts. There was no relationship between HSP90AA1 and heat index at any time (hours or days), meaning that baseline HSP gene expression is not a reliable biomarker for the combined effects of heat and humidity, at least not when applying existing metrics that were developed for poultry. We found some evidence linking HSP90AA1 gene expression with maximum temperatures three to four days before sampling; however, a permutation test could not rule out the possibility of a false positive.DiscussionHSP90AA1 mRNA abundance is not necessarily an effective biomarker of recent heat, and it may instead reflect other inherent population differences. As heat waves intensify, this conclusion could change, and other species could be more reactive to heat. We urge the avian biology community to continue biomarker testing for estimating heat impacts on wild birds, as we seek to better understand and predict avian resilience to environmental challenges.

The ovary is key to linking environmental factors with the timing and quality of offspring development. Focused on free-living female tree swallows (Tachycineta bicolor), we measured temporal variation in ovarian expression of genes involved in the regulation of telomere length. Using qPCR, we quantified mRNA abundance of shelterin proteins (TERF1, TERF2, TERF2IP, TPP1, POT1), telomerase (TERT), antioxidants (SOD1, PRDX-1, GPX), and glucocorticoid receptors (MR, GR). We asked how they differ across breeding stages and social environments, and then we assessed effects on gene co-expression, which reflects coordinated changes across this network of interacting genes. We hypothesized that maintenance of telomeres is upregulated and more strongly coregulated in the lead up to reproduction, i.e., before egg-laying and following a social challenge. We did not find a main effect of environmental context on mRNA abundance, but we did detect subtle differences in gene co-expression networks. Females exhibited stronger coregulation among shelterin proteins and stronger crosstalk with glucocorticoid receptors during incubation. In response to a conspecific challenger, coregulation of antioxidants with shelterin and glucocorticoid receptors was weaker or more negatively correlated, suggesting semi-independent social modulation of these telomere regulatory networks. While the consequences of these transcriptional differences require more research, our results suggest that the environment could contribute to protection of the ovary, including its telomeres.

Although short-lived, easily manipulated wild systems could be useful for studying ageing, developing epigenetic clocks for them is challenging because their chronological age is often unknown. Here, we present a multi-tissue epigenetic clock for the wood mouse ( Apodemus sylvaticus ) that was developed in a laboratory colony and then applied to wild individuals. We used the mammalian methylation array to profile CpG sites across highly conserved stretches of DNA in blood, ear, spleen, and liver of colony-reared mice. We trained an elastic net model with Leave-One-Out-Cross Validation (LOOCV), which identified 77 key age-related CpG sites as being highly predictive of chronological age (r = 0.99; MAE = 3.29 days). Upon validation in an independent dataset, the LOOCV clock predicted age with an MAE of 54.68 days. Epigenome-wide association study and ‘Genomic Regions Enrichment of Annotations Tool’ analysis of age-related CpGs primarily revealed hypermethylation of promoter regions linked to development and transcription factor activity, particularly via changes in methylation of PRC2 targets sites. Critically, our epigenetic clock was able to predict broad age categories in wild mice and increased over chronological time in 75% of individuals. This and similar clock developments in other short-lived wild systems, that can be bred in captivity, will enhance our ability to conduct experimental manipulations of ageing in ecology and evolution. ### Competing Interest Statement The Regents of the University of California are the sole owner of patents and patent applications directed at epigenetic biomarkers and the mammalian methylation array for which Steve Horvath is a named inventor; SH is a founder and paid consultant of the non-profit Epigenetic Clock Development Foundation that licenses these patents. SH is a Principal Investigator at the Altos Labs, Cambridge Institute of Science, a biomedical company that works on rejuvenation.

Uncovering the genomic bases of phenotypic adaptation is a major goal in biology, but this has been hard to achieve for complex behavioural traits. Here we leverage the repeated, independent evolution of obligate cavity nesting in birds to test the hypothesis that pressure to compete for a limited breeding resource has facilitated convergent evolution in behaviour, hormones and gene expression. We used an integrative approach, combining aggression assays in the field, testosterone measures and transcriptome-wide analyses of the brain in wild-captured females and males. Our experimental design compared species pairs across five avian families, each including one obligate cavity-nesting species and a related species with a more flexible nest strategy. We find behavioural convergence, with higher levels of territorial aggression in obligate cavity nesters, particularly among females. Across species, levels of testosterone in circulation were not associated with nest strategy nor aggression. Phylogenetic analyses of individual genes and co-regulated gene networks revealed more shared patterns of brain gene expression than expected by drift, although the scope of convergent gene expression evolution was limited to a small percentage of the genome. When comparing our results to other studies that did not use phylogenetic methods, we suggest that accounting for shared evolutionary history may reduce the number of genes inferred as convergently evolving. Altogether, we find that behavioural convergence in response to shared ecological pressures is associated with largely independent evolution of gene expression across different avian families, punctuated by a narrow set of convergently evolving genes.

Caloric restriction (CR) modifies lifespan and aging biology in animal models. The Comprehensive Assessment of Long-Term Effects of Reducing Intake of Energy (CALERIE™) 2 trial tested translation of these findings to humans. CALERIE™ randomized healthy, nonobese men and premenopausal women (age 21-50y; BMI 22.0-27.9 kg/m2), to 25% CR or ad-libitum (AL) control (2:1) for 2 years. Prior analyses of CALERIE™ participants' blood chemistries, immunology, and epigenetic data suggest the 2-year CR intervention slowed biological aging. Here, we extend these analyses to test effects of CR on telomere length (TL) attrition. TL was quantified in blood samples collected at baseline, 12-, and 24-months by quantitative PCR (absolute TL; aTL) and a published DNA-methylation algorithm (DNAmTL). Intent-to-treat analysis found no significant differences in TL attrition across the first year, although there were trends toward increased attrition in the CR group for both aTL and DNAmTL measurements. When accounting for adherence heterogeneity with an Effect-of-Treatment-on-the-Treated analysis, greater CR dose was associated with increased DNAmTL attrition during the baseline to 12-month weight-loss period. By contrast, both CR group status and increased CR were associated with reduced aTL attrition over the month 12 to month 24 weight maintenance period. No differences were observed when considering TL change across the study duration from baseline to 24-months, leaving it unclear whether CR-related effects reflect long-term detriments to telomere fidelity, a hormesis-like adaptation to decreased energy availability, or measurement error and insufficient statistical power. Unraveling these trends will be a focus of future CALERIE™ analyses and trials.

Telomere length (TL) is an important biomarker of cellular aging, yet its links with health outcomes may be complicated by use of different tissues. We evaluated within- and between-individual variability in TL and quality metrics of DNA across five tissues using a cross-sectional dataset ranging from 8 to 70 years (N=197). DNA was extracted from all tissue cells using the Gentra Puregene DNA Extraction Kit. Absolute TL (aTL) in kilobase pairs was measured in buccal epithelial cells, saliva, dried blood spots (DBS), buffy coat, and peripheral blood mononuclear cells (PBMCs) using qPCR. aTL significantly shortened with age for all tissues except saliva and buffy coat, although buffy coat was available for a restricted age range (8 to 15 years). aTL did not significantly differ across blood-based tissues (DBS, buffy coat, PBMC), which had significantly longer aTL than buccal cells and saliva. Additionally, aTL was significantly correlated for the majority of tissue pairs, with partial Spearman’s correlations controlling for age and sex ranging from ⍴ = 0.18 to 0.51. We also measured quality metrics of DNA including integrity, purity, and quantity of extracted DNA from all tissues and explored whether controlling for DNA metrics improved predictions of aTL. We found significant tissue variation: DNA from blood-based tissues had high DNA integrity, more acceptable A260/280 and A260/230 values, and greater extracted DNA concentrations compared to buccal cells and saliva. Longer aTL was associated with lower DNA integrity, higher extracted DNA concentrations, and higher A260/230, particularly for saliva. Model comparisons suggested that incorporation of quality DNA metrics improves models of TL, although relevant metrics vary by tissue. These findings highlight the merits of using blood-based tissues and suggest that incorporation of quality DNA metrics as control variables in population-based studies can improve TL predictions, especially for more variable tissues like buccal and saliva.

Enabled by surface-mediated equilibration, physical vapour deposition can create high-density stable glasses comparable with liquid-quenched glasses aged for millions of years. Deposition is often performed at various rates and temperatures on rigid substrates to control the glass properties. Here we demonstrate that on soft, rubbery substrates, surface-mediated equilibration is enhanced up to 170 nm away from the interface, forming stable glasses with densities up to 2.5% higher than liquid-quenched glasses within 2.5 h of deposition. Gaining similar properties on rigid substrates would require 10 million times slower deposition, taking ~3,000 years. Controlling the modulus of the rubbery substrate provides control over the glass structure and density at constant deposition conditions. These results underscore the significance of substrate elasticity in manipulating the properties of the mobile surface layer and thus the glass structure and properties, allowing access to deeper states of the energy landscape without prohibitively slow deposition rates.

Biologists aim to explain patterns of growth, reproduction and ageing that characterize life histories, yet we are just beginning to understand the proximate mechanisms that generate this diversity. Existing research in this area has focused on telomeres but has generally overlooked the telomere's most direct mediator, the shelterin protein complex. Shelterin proteins physically interact with the telomere to shape its shortening and repair. They also regulate metabolism and immune function, suggesting a potential role in life history variation in the wild. However, research on shelterin proteins is uncommon outside of biomolecular work. Intraspecific analyses can play an important role in resolving these unknowns because they reveal subtle variation in life history within and among populations. Here, we assessed ecogeographic variation in shelterin protein abundance across eight populations of tree swallow (Tachycineta bicolor) with previously documented variation in environmental and life history traits. Using the blood gene expression of four shelterin proteins in 12-day-old nestlings, we tested the hypothesis that shelterin protein gene expression varies latitudinally and in relation to both telomere length and life history. Shelterin protein gene expression differed among populations and tracked non-linear variation in latitude: nestlings from mid-latitudes expressed nearly double the shelterin mRNA on average than those at more northern and southern sites. However, telomere length was not significantly related to latitude. We next assessed whether telomere length and shelterin protein gene expression correlate with 12-day-old body mass and wing length, two proxies of nestling growth linked to future fecundity and survival. We found that body mass and wing length correlated more strongly (and significantly) with shelterin protein gene expression than with telomere length. These results highlight telomere regulatory shelterin proteins as potential mediators of life history variation among populations. Together with existing research linking shelterin proteins and life history variation within populations, these ecogeographic patterns underscore the need for continued integration of ecology, evolution and telomere biology, which together will advance understanding of the drivers of life history variation in nature.

Social species often share with conspecifics the responsibilities of finding food, defending against predators or caring for young. Within a social group, individuals' roles can be influenced by age, sex and personality. Black-capped chickadees, Poecile atricapillus, are nonmigratory passerine birds that spend their winters living in social flocks. Due to the intense metabolic stress placed on chickadees by low winter temperatures, the role of flocking in resource localization and allocation among flockmates is an important aspect of individual fitness, especially during winter. Understanding changes in flock structure in response to the environment may provide insight into the consequences of social behaviour variability on overwinter survival. Here, we used social network analysis to quantify the effects of ambient temperature on social foraging behaviour in winter flocks of black-capped chickadees over two winters in Massachusetts, U.S.A. Contrary to our expectations, we found that two measures of social connection were higher on warm days than on cold days. We found no evidence that sex or dominance rank influenced birds' behavioural responses to temperature. Furthermore, contrary to previous research, we found male-biased sex ratios in most of our flocks. Our findings that birds had more and stronger social connections on warm days and that an individual's response to temperature was not related to their status in a flock beg further investigation as to the specific costs and benefits of flocking in chickadees and other birds.

Aging is characterized by functional decline occurring alongside changes to several hallmarks of aging. One of the hallmarks includes attrition of repeated DNA sequences found at the ends of chromosomes called telomeres. While telomere attrition is linked to morbidity and mortality, whether and how it causally contributes to lifelong rates of functional decline is unclear. In this review, we propose the shelterin-telomere hypothesis of life history, in which telomere-binding shelterin proteins translate telomere attrition into a range of physiological outcomes, the extent of which may be modulated by currently understudied variation in shelterin protein levels. Shelterin proteins may expand the breadth and timing of consequences of telomere attrition, e.g., by translating early life adversity into acceleration of the aging process. We consider how the pleiotropic roles of shelterin proteins provide novel insights into natural variation in physiology, life history, and lifespan. We highlight key open questions that encourage the integrative, organismal study of shelterin proteins that enhances our understanding of the contribution of the telomere system to aging.

Telomere attrition is a proposed hallmark of aging. To evaluate the association of telomere length (TL) with chronological age across the human lifespan, we conducted a systematic review and meta-analysis of 414 study samples comprising 743,019 individuals aged 0-112 years. We examined both cross-sectional and longitudinal data, and evaluated the impact of various biological and methodological factors including sex, health status, tissue types, DNA extraction procedures, and TL measurement methods. The pooled corrected correlation between TL and age from cross-sectional samples was 0.19 (95%CI: 0.22 to 0.15), which weakened with increased chronological age ( beta = 0.003, p < 0.001). Z-score change rates of TL across the lifespan showed a gradual decrease in shortening rate until around age 50 and remained at a relatively stable rate towards the elderly period. A greater attrition rate was observed in longitudinal than cross-sectional evaluations. For TL measured in base pairs, the median change rate of TL was 23 bp/year in cross-sectional samples and 38 bp/ year in longitudinal samples. Methodological factors including TL measurement methods and tissue types impacted the TL-age correlation, while sex or disease status did not. This meta-analysis revealed the non-linear shortening trend of TL across the human lifespan and provides a reference value for future studies. Results also highlight the importance of methodological considerations when using TL as an aging biomarker.

Parasites are a strong selective force that can influence fitness-related traits. The length of chromosome-capping telomeres can be used to assess the long-term costs of parasitism, as telomere loss accelerates in response to environmental stressors and often precedes poorer survival prospects. Here, we explored the sex-specific effects of ectoparasite removal on morphology and telomere length in nestling tree swallows (Tachycineta bicolor). To do so, we experimentally removed blow fly (Protocalliphora spp.) larvae from nests using Permethrin, a broad-spectrum insecticide. Compared to water-treated controls, insecticide treatment of nests had a sex-biased effect on blood telomere length: ectoparasite removal resulted in significantly longer telomeres in males but not females. While this treatment did not influence nestling body mass, it was associated with reduced feather development regardless of sex. This may reflect a relaxed pressure to fledge quickly in the absence of parasites, or alternatively, could be a negative side effect of permethrin on morphology. Exploring robust sex-specific telomere dynamics in response to early-life environmental pressures such as parasitism will shed light on sexual dimorphism in adult life histories and aging.

Increasingly frequent and intense heatwaves generate new challenges for many organisms. Our understanding of the ecological predictors of thermal vulnerability is improving, yet, at least in endotherms, we are still only beginning to understand one critical component of predicting resilience: exactly how do wild animals cope with sub-lethal heat? In wild endotherms, most prior work focuses on one or a few traits, leaving uncertainty about organismal consequences of heatwaves. Here, we experimentally generated a 2.8°C heatwave for free-living nestling tree swallows (Tachycineta bicolor). Over a week-long period coinciding with the peak of post-natal growth, we quantified a suite of traits to test the hypotheses that (a) behavioral or (b) physiological responses may be sufficient for coping with inescapable heat. Heat-exposed nestlings increased panting and decreased huddling, but treatment effects on panting dissipated over time, even though heat-induced temperatures remained elevated. Physiologically, we found no effects of heat on: gene expression of three heat shock proteins in blood, muscle, and three brain regions; secretion of circulating corticosterone at baseline or in response to handling; and telomere length. Moreover, heat had a positive effect on growth and a marginal, but not significant, positive effect on subsequent recruitment. These results suggest that nestlings were generally buffered from deleterious effects of heat, with one exception: heat-exposed nestlings exhibited lower gene expression for superoxide dismutase, a key antioxidant defense. Despite this one apparent cost, our thorough organismal investigation indicates general resilience to a heatwave that may, in part, stem from behavioral buffering and acclimation. Our approach provides a mechanistic framework that we hope will improve understanding of species persistence in the face of climate change.

Identifying factors that influence the stability of DNA methylation measurements across biological replicates is of critical importance in basic and clinical research. Using a within-person between-group experimental design (n = 31, number of observations = 192), we report the stability of biological replicates over a variety of unique temporal scenarios, both in the absence and presence of acute psychosocial stress, and between individuals who have experienced early life adversity (ELA) and non-exposed individuals. We found that varying time intervals, acute stress, and ELA exposure influenced the stability of repeated DNA methylation measurements. In the absence of acute stress, probes were less stable as time passed; however, stress exerted a stabilizing influence on probes over longer time intervals. Compared to non-exposed individuals, ELA-exposed individuals had significantly lower probe stability immediately following acute stress. Additionally, we found that across all scenarios, probes used in most epigenetic-based algorithms for estimating epigenetic age or immune cell proportions had average or below-average stability, except for the Principal Component and DunedinPACE epigenetic ageing clocks, which were enriched for more stable probes. Finally, using highly stable probes in the absence of stress, we identified multiple probes that were hypomethylated in the presence of acute stress, regardless of ELA status. Two hypomethylated probes are located near the transcription start site of the glutathione-disulfide reductase gene (GSR), which has previously been shown to be an integral part of the stress response to environmental toxins. We discuss implications for future studies concerning the reliability and reproducibility of DNA methylation measurements. Abbreviations: DNAm – DNA methylation, CpG − 5’-cytosine-phosphate-guanine-3,’ ICC – Interclass correlation coefficient, ELA – Early-life adversity, PBMCs – Peripheral blood mononuclear cells, mQTL – Methylation quantitative trait loci, TSS – Transcription start site, GSR – Glutathione-disulfide reductase gene, TSST – Trier social stress test, PC – Principal component.

Purpose: To evaluate the association between elements of the built environment and physician well-being using a scoping review. Background: Physicians currently report low work satisfaction, high burnout and depression. The built environment has been shown to affect well-being in the general population, but its relationship to physician well-being is not well understood. Gaining a better understanding of this relationship will allow for better evidence-based design for physician well-being. Methods: A scoping review of the literature was conducted of Scopus and PubMed databases using key words for (1) environmental variables such as “environmental design”; (2) physician terms, such as “doctor”; and (3) outcome measures of well-being, such as “burnout.” Studies were included if they had at least one measure of the built environment and one measure of physician well-being. Of 1,723 abstracts screened, 146 full-text articles were reviewed, leaving 16 articles to be included for data extraction. Results: Included studies were from all around the world, except for, notably, the United States. Robust interpretation of the data was challenging because of the lack of standardization in the assessment of environmental factors and physician well-being. Excessive noise was the variable most linked to well-being. Suitable surroundings, including lighting, furniture, and art, were also linked with satisfaction, but it was unclear whether there was an association with well-being. Conclusion: Understanding how environmental factors affect physician well-being is paramount but considerably understudied. Standardization of research approach should be developed in order to produce more robust research to create evidence-based designs for physician well-being.