An experimental demonstration that early-life competitive disadvantage accelerates telomere loss

An early-life challenge: becoming an older sibling in wild mandrills

In monotocous mammals, most individuals experience the birth of a younger sibling. This period may induce losses in maternal care and can be physiologically, energetically and emotionally challenging for the older sibling, yet has rarely been studied in wild primates. We used behavioural data collected from a natural population of mandrills to investigate changes in maternal care and mother-juvenile relationship throughout the transition to siblinghood (TTS), by comparing juveniles who recently experienced the birth of a younger sibling, to juveniles who did not. We found that the TTS was associated with an abrupt cessation of the weaning process for the juvenile, and to a decrease in maternal affiliation. Juveniles' reactions were sex-specific, as males associated less with their mother, while females tended to groom their mother more often after the birth of their sibling. Despite the substantial loss of maternal care, juveniles did not show an increase in conflict or anxiety-related behaviours. This study contributes to explain why short interbirth intervals often pose a risk to juveniles' survival in monotocous primates. Our results contrast existing studies and further highlight the importance of examining the TTS in species and populations with various life histories and ecologies.

A future food boom rescues the negative effects of early-life adversity on adult lifespan in a small mammal

Early-life adversity, even when transient, can have lasting effects on individual phenotypes and reduce lifespan across species. If these effects can be mitigated by a high-quality later-life environment, then differences in future resources may explain variable resilience to early-life adversity. Using data from over 1000 wild North American red squirrels, we tested the hypothesis that the costs of early-life adversity for adult lifespan could be offset by later-life food abundance. We identified six adversities that reduced juvenile survival in the first year of life, though only one-birth date-had continued independent effects on adult lifespan. We then built a weighted early-life adversity (wELA) index integrating the sum of adversities and their effect sizes. Greater weighted early-life adversity predicted shorter adult lifespans in males and females, but a naturally occurring food boom in the second year of life ameliorated this effect. Experimental food supplementation did not replicate this pattern, despite increasing lifespan, indicating that the buffering effect of a future food boom may hinge on more than an increase in available calories. Our results suggest a non-deterministic role of early-life conditions for later-life phenotype, highlighting the importance of evaluating the consequences of early-life adversity in the context of an animal's entire life course.

Why and when should organisms elongate their telomeres? Elaborations on the 'excess resources elongation' and 'last resort elongation' hypotheses

Telomere length and telomere shortening are thought to be critical cellular attributes and processes that are related to an individual's life span and fitness. The general pattern across most taxa is that after birth telomere length gradually decreases with age. Telomere protection and restoration mechanisms are usually assumed to reduce the rate of shortening or at most keep telomere length constant. However, here we have compiled a list of 26 articles showing that there is an increasing number of studies reporting apparent elongation of telomeres (i.e., a net increase in TL from timet to timet+1) often in a considerable proportion of the individuals studied. Moreover, the few studies which have studied telomere elongation in detail show that increases in telomere length are unlikely to be due to measurement error alone. In this article, we argue that episodes of telomere elongation deserve more attention as they could reflect individual strategies to optimise life histories and maximise fitness, which may not be reflected in the overall telomere dynamics patterns. We propose that patterns of telomere (net) elongation may be partly determined by other factors than those causing telomere shortening, and therefore deserve analyses specifically targeted to investigate the occurrence of telomere elongation. We elaborate on two ecological hypotheses that have been proposed to explain patterns of telomere elongation (the 'excess resources elongation' and the 'last resort elongation' hypothesis) and we discuss the current evidence for (or against) these hypotheses and propose ways to test them.

Genomic approaches to mitigating genetic diversity loss in declining populations

The accelerating pace of global biodiversity loss is exacerbated by habitat fragmentation and subsequent inbreeding in small populations. To address this problem, conservation practitioners often turn to assisted breeding programmes with the aim of enhancing genetic diversity in declining populations. Although genomic information is infrequently included in these efforts, it has the potential to significantly enhance the success of such programmes. In this study, we showcase the value of genomic approaches for increasing genetic diversity in assisted breeding efforts, specifically focusing on a highly inbred population of Western burrowing owls. To maximize genetic diversity in the resulting offspring, we begin by creating an optimal pairing decision tree based on sex, kinship and patterns of homozygosity across the genome. To evaluate the effectiveness of our strategy, we compare genetic diversity, brood size and nestling success rates between optimized and non-optimized pairs. Additionally, we leverage recently discovered correlations between telomere length and fitness across species to investigate whether genomic optimization could have long-term fitness benefits. Our results indicate that pairing individuals with contrasting patterns of homozygosity across the genome is an effective way to increase genetic diversity in offspring. Although short-term field-based metrics of success did not differ significantly between optimized and non-optimized pairs, offspring from optimized pairs had significantly longer telomeres, suggesting that genetic optimization can help reduce the risk of inbreeding depression. These findings underscore the importance of genomic tools for informing efforts to preserve the adaptive potential of small, inbred populations at risk of further decline.

Alternative telomere maintenance mechanism in Alligator sinensis provides insights into aging evolution

Lifespan is a life-history trait that undergoes natural selection. Telomeres are hallmarks of aging, and shortening rate predicts species lifespan, making telomere maintenance mechanisms throughout different lifespans a worthy topic for study. Alligators are suitable for the exploration of anti-aging molecular mechanisms, because they exhibit low or even negligible mortality in adults and no significant telomere shortening. Telomerase reverse transcriptase (TERT) expression is absent in the adult Alligator sinensis, as in humans. Selection analyses on telomere maintenance genes indicated that ATM, FANCE, SAMHD1, HMBOX1, NAT10, and MAP3K4 experienced positive selection on A. sinensis. Repressed pleiotropic ATM kinase in A. sinensis suggests their fitness optimum shift. In ATM downstream, Alternative Lengthening of Telomeres (ALT)-related genes were clustered in a higher expression pattern in A. sinensis, which covers 10-15% of human cancers showing no telomerase activities. In summary, we demonstrated how telomere shortening, telomerase activities, and ALT contributed to anti-aging strategies.

Early-life seasonal, weather and social effects on telomere length in a wild mammal

Early-life environmental conditions can provide a source of individual variation in life-history strategies and senescence patterns. Conditions experienced in early life can be quantified by measuring telomere length, which can act as a biomarker of survival probability in some species. Here, we investigate whether seasonal changes, weather conditions and group size are associated with early-life and/or early-adulthood telomere length in a wild population of European badgers (Meles meles). We found substantial intra-annual changes in telomere length during the first 3 years of life, where within-individual effects showed shorter telomere lengths in the winter following the first spring and a trend for longer telomere lengths in the second spring compared to the first winter. In terms of weather conditions, cubs born in warmer, wetter springs with low rainfall variability had longer early-life (3-12 months old) telomeres. Additionally, cubs born in groups with more cubs had marginally longer early-life telomeres, providing no evidence of resource constraint from cub competition. We also found that the positive association between early-life telomere length and cub survival probability remained when social and weather variables were included. Finally, after sexual maturity, in early adulthood (i.e., 12-36 months) we found no significant association between same-sex adult group size and telomere length (i.e., no effect of intrasexual competition). Overall, we show that controlling for seasonal effects, which are linked to food availability, is important in telomere length analyses, and that variation in telomere length in badgers reflects early-life conditions and also predicts first year cub survival.

The telomere regulatory gene POT1 responds to stress and predicts performance in nature: Implications for telomeres and life history evolution

Telomeres are emerging as correlates of fitness-related traits and may be important mediators of ecologically relevant variation in life history strategies. Growing evidence suggests that telomere dynamics can be more predictive of performance than length itself, but very little work considers how telomere regulatory mechanisms respond to environmental challenges or influence performance in nature. Here, we combine observational and experimental data sets from free-living tree swallows (Tachycineta bicolor) to assess how performance is predicted by the telomere regulatory gene POT1, which encodes a shelterin protein that sterically blocks telomerase from repairing the telomere. First, we show that lower POT1 gene expression in the blood was associated with higher female quality, that is, earlier breeding and heavier body mass. We next challenged mothers with an immune stressor (lipopolysaccharide injection) that led to "sickness" in mothers and 24 h of food restriction in their offspring. While POT1 did not respond to maternal injection, females with lower constitutive POT1 gene expression were better able to maintain feeding rates following treatment. Maternal injection also generated a 1-day stressor for chicks, which responded with lower POT1 gene expression and elongated telomeres. Other putatively stress-responsive mechanisms (i.e., glucocorticoids, antioxidants) showed marginal responses in stress-exposed chicks. Model comparisons indicated that POT1 mRNA abundance was a largely better predictor of performance than telomere dynamics, indicating that telomere regulators may be powerful modulators of variation in life history strategies.

Habitat restoration weakens negative environmental effects on telomere dynamics

Habitat quality can have far-reaching effects on organismal fitness, an issue of concern given the current scale of habitat degradation. Many temperate upland streams have reduced nutrient levels due to human activity. Nutrient restoration confers benefits in terms of invertebrate food availability and subsequent fish growth rates. Here we test whether these mitigation measures also affect the rate of cellular ageing of the fish, measured in terms of the telomeres that cap the ends of eukaryotic chromosomes. We equally distributed Atlantic salmon eggs from the same 30 focal families into 10 human-impacted oligotrophic streams in northern Scotland. Nutrient levels in five of the streams were restored by simulating the deposition of a small number of adult Atlantic salmon Salmo salar carcasses at the end of the spawning period, while five reference streams were left as controls. Telomere lengths and expression of the telomerase reverse transcriptase (TERT) gene that may act to lengthen telomeres were then measured in the young fish when 15 months old. While TERT expression was unrelated to any of the measured variables, telomere lengths were shorter in salmon living at higher densities and in areas with a lower availability of the preferred substrate (cobbles and boulders). However, the adverse effects of these habitat features were much reduced in the streams receiving nutrients. These results suggest that adverse environmental pressures are weakened when nutrients are restored, presumably because the resulting increase in food supply reduces levels of both competition and stress.

Within-individual repeatability in telomere length: A meta-analysis in nonmammalian vertebrates

Telomere length is increasingly used as a biomarker of long-term somatic state and future survival prospects. While most studies have overlooked this aspect, biological interpretations based on a given telomere length will benefit from considering the level of within-individual repeatability of telomere length through time. Therefore, we conducted a meta-analysis on 74 longitudinal studies in nonmammalian vertebrates, with the aim to establish the current pattern of within-individual repeatability in telomere length and to identify the methodological (e.g., qPCR/TRF) and biological factors (e.g., age class, phylogeny) that may affect it. While the median within-individual repeatability of telomere length was moderate to high (R = 0.55; 95% CI: 0.05-0.95; N = 82), marked heterogeneity between studies was evident. Measurement method affected the repeatability estimate strongly, with TRF-based studies exhibiting high repeatability (R = 0.80; 95% CI: 0.34-0.96; N = 25), while repeatability of qPCR-based studies was markedly lower and more variable (R = 0.46; 95% CI: 0.04-0.82; N = 57). While phylogeny explained some variance in repeatability, phylogenetic signal was not significant (λ = 0.32; 95% CI: 0.00-0.83). None of the biological factors investigated here significantly explained variation in the repeatability of telomere length, being potentially obscured by methodological differences. Our meta-analysis highlights the high variability in within-individual repeatability estimates between studies and the need to put more effort into separating technical and biological explanations. This is important to better understand to what extent biological factors can affect the repeatability of telomere length and thus the interpretation of telomere length data.

Epithelial Cell Adhesion Molecule (EpCAM) Expression Can Be Modulated via NFκB

The epithelial cell adhesion molecule (EpCAM) is considered an essential proliferation signature in cancer. In the current research study, qPCR induced expression of EpCAM was noted in acute lymphoblastic leukemia (ALL) cases. Costunolide, a sesquiterpene lactone found in crepe ginger and lettuce, is a medicinal herb with anticancer properties. Expression of EpCAM and its downstream target genes (Myc and TERT) wasdownregulated upon treatment with costunolide in Jurkat cells. A significant change in the telomere length of Jurkat cells was not noted at 72 h of costunolide treatment. An in silico study revealed hydrophobic interactions between EpCAM extracellular domain and Myc bHLH with costunolide. Reduced expression of NFκB, a transcription factor of EpCAM, Myc, and TERT in costunolide-treated Jurkat cells, suggested that costunolide inhibits gene expression by targeting NFκB and its downstream targets. Overall, the study proposes that costunolide could be a promising therapeutic biomolecule for leukemia.

Stressors interact across generations to influence offspring telomeres and survival

Parental stress often has long-term consequences for offspring. However, the mechanisms underlying these effects and how they are shaped by conditions offspring subsequently experience are poorly understood. Telomeres, which often shorten in response to stress and predict longevity, may contribute to, and/or reflect these cross-generational effects. Traditionally, parental stress is expected to have negative effects on offspring telomeres, but experimental studies in captive animals suggest that these effects may depend on the subsequent conditions that offspring experience. Yet, the degree to which parental stress influences and interacts with stress experienced by offspring to affect offspring telomeres and survival in free-living organisms is unknown. To assess this, we experimentally manipulated the stress exposure of free-living parent and offspring house sparrows (Passer domesticus). We found a weak, initial, negative effect of parental stress on offspring telomeres, but this effect was no longer evident at the end of post-natal development. Instead, the effects of parental stress depended on the natural sources of stress that offspring experienced during post-natal development whereby some outcomes were improved under more stressful rearing conditions. Thus, the effects of parental stress on offspring telomeres and survival are context-dependent and may involve compensatory mechanisms of potential benefit under some circumstances.

Effects of the social environment on vertebrate fitness and health in nature: Moving beyond the stress axis

Social interactions are a ubiquitous feature of the lives of vertebrate species. These may be cooperative or competitive, and shape the dynamics of social systems, with profound effects on individual behavior, physiology, fitness, and health. On one hand, a wealth of studies on humans, laboratory animal models, and captive species have focused on understanding the relationships between social interactions and individual health within the context of disease and pathology. On the other, ecological studies are attempting an understanding of how social interactions shape individual phenotypes in the wild, and the consequences this entails in terms of adaptation. Whereas numerous studies in wild vertebrates have focused on the relationships between social environments and the stress axis, much remains to be done in understanding how socially-related activation of the stress axis coordinates other key physiological functions related to health. Here, we review the state of our current knowledge on the effects that social interactions may have on other markers of vertebrate fitness and health. Building upon complementary findings from the biomedical and ecological fields, we identify 6 key physiological functions (cellular metabolism, oxidative stress, cellular senescence, immunity, brain function, and the regulation of biological rhythms) which are intimately related to the stress axis, and likely directly affected by social interactions. Our goal is a holistic understanding of how social environments affect vertebrate fitness and health in the wild. Whereas both social interactions and social environments are recognized as important sources of phenotypic variation, their consequences on vertebrate fitness, and the adaptive nature of social-stress-induced phenotypes, remain unclear. Social flexibility, or the ability of an animal to change its social behavior with resulting changes in social systems in response to fluctuating environments, has emerged as a critical underlying factor that may buffer the beneficial and detrimental effects of social environments on vertebrate fitness and health.

Causes and consequences of variation in early-life telomere length in a bird metapopulation

Environmental conditions during early-life development can have lasting effects shaping individual heterogeneity in fitness and fitness-related traits. The length of telomeres, the DNA sequences protecting chromosome ends, may be affected by early-life conditions, and telomere length (TL) has been associated with individual performance within some wild animal populations. Thus, knowledge of the mechanisms that generate variation in TL, and the relationship between TL and fitness, is important in understanding the role of telomeres in ecology and life-history evolution. Here, we investigate how environmental conditions and morphological traits are associated with early-life blood TL and if TL predicts natal dispersal probability or components of fitness in 2746 wild house sparrow (Passer domesticus) nestlings from two populations sampled across 20 years (1994-2013). We retrieved weather data and we monitored population fluctuations, individual survival, and reproductive output using field observations and genetic pedigrees. We found a negative effect of population density on TL, but only in one of the populations. There was a curvilinear association between TL and the maximum daily North Atlantic Oscillation index during incubation, suggesting that there are optimal weather conditions that result in the longest TL. Dispersers tended to have shorter telomeres than non-dispersers. TL did not predict survival, but we found a tendency for individuals with short telomeres to have higher annual reproductive success. Our study showed how early-life TL is shaped by effects of growth, weather conditions, and population density, supporting that environmental stressors negatively affect TL in wild populations. In addition, shorter telomeres may be associated with a faster pace-of-life, as individuals with higher dispersal rates and annual reproduction tended to have shorter early-life TL.

How does early-life adversity shape telomere dynamics during adulthood? Problems and paradigms

Although early-life adversity has been associated with negative consequences during adulthood, growing evidence shows that such adversity can also lead to subsequent stress resilience and positive fitness outcomes. Telomere dynamics are relevant in this context because of the link with developmental conditions and longevity. However, few studies have assessed whether the effects of early-life adversity on developmental telomere dynamics may relate to adult telomere dynamics. We propose that the potential links between early-life adversity and adult telomere dynamics could be driven by developmental constraints (the Constraint hypothesis), by the nature/severity of developmental adversity (the Resilience hypothesis), or by developmental-mediated changes in individual life-history strategies (the Pace of Life hypothesis). We discuss these non-mutually exclusive hypotheses, explore future research directions, and propose specific studies to test these hypotheses. Our article aims to expand our understanding of the evolutionary role of developmental conditions on adult telomere dynamics, stress resilience and ageing.

A multi-tissue view on telomere dynamics and postnatal growth

Trade-offs between growth and self-maintenance are common in nature, such that early-life effects on growth can generate lasting consequences on survival and longevity. Telomeres-putative biomarkers of self-maintenance-may link early growth with these later phenotypic effects, but evidence for growth-telomere trade-offs is mixed. Null or even positive relationships between growth and telomeres may be driven by heterogeneity in resource availability or invariable allocation towards telomere maintenance within a population. We used nestling tree swallows (Tachycineta bicolor) to assess the directionality and timing of relationships between growth and telomere length in several tissues. We focused on two important phases of growth: first, the peak of postnatal growth occurring around 6 days old when nestlings grow by ~33% in a single day, and subsequently, the later phase of growth occurring as body mass plateaus near adult size at 12 days old. We quantified telomere attrition in blood during postnatal growth, as well as telomere length in the blood, brain, adrenals, and liver at 12 days old. Growth was unrelated to telomere length in the liver and telomere dynamics in blood. However, brain telomere length was positively correlated with peak growth, and adrenal telomere length was positively related to later growth, particularly for chicks that had experienced a temporary stressor. These observations suggest that variation in resource availability may mask trade-offs, generating positive correlations between growth and telomere length at the population level. They also provide insights into complex relationships between growth and self-maintenance that can be revealed by looking in multiple tissues.

Phenotypic differences among wild passerine nestlings in relation to early-life rearing environment

Subtle changes in stress physiology during critical developmental stages have been linked to long-term fitness; however, the biological processes and phenotypic responses to early-life rearing environments, such as anthropogenic land use conditions, have not been fully evaluated in insectivorous birds. We manipulated Tree Swallow (Tachycineta bicolor (Vieillot, 1808)) brood sizes at sites with contrasting agricultural land use to assess phenotypic changes in body condition and genetic and physiological biomarkers of stress during the sensitive nestling growth phase. We predicted that nestling swallows raised on cropland-dominated sites, especially those in enlarged broods, would have lower body condition, shorter telomeres, and higher feather corticosterone than nestlings raised in smaller broods at grassland sites. Body condition was highest among nestlings raised in reduced broods but was unrelated to land use. Telomere lengths tended to be shorter in nestlings from enlarged broods and at cropland sites. Corticosterone was not related to any factor. Locally abundant insect populations associated with wetlands may have dampened the effects and (or) parent swallows assumed higher costs of reproduction rather than passing these costs to nestlings. Results suggest that food or other environmental stressors could reduce fledgling survival via telomere shortening; a hypothesis that requires further investigation due to its potential importance to population viability in multiple declining aerial insectivore species.

Exposure to food insecurity increases energy storage and reduces somatic maintenance in European starlings (Sturnus vulgaris)

Birds exposed to food insecurity-defined as temporally variable access to food-respond adaptively by storing more energy. To do this, they may reduce energy allocation to other functions such as somatic maintenance and repair. To investigate this trade-off, we exposed juvenile European starlings (Sturnus vulgaris, n = 69) to 19 weeks of either uninterrupted food availability or a regime where food was unpredictably unavailable for a 5-h period on 5 days each week. Our measures of energy storage were mass and fat scores. Our measures of somatic maintenance were the growth rate of a plucked feather, and erythrocyte telomere length (TL), measured by analysis of the terminal restriction fragment. The insecure birds were heavier than the controls, by an amount that varied over time. They also had higher fat scores. We found no evidence that they consumed more food overall, though our food consumption data were incomplete. Plucked feathers regrew more slowly in the insecure birds. TL was reduced in the insecure birds, specifically, in the longer percentiles of the within-individual TL distribution. We conclude that increased energy storage in response to food insecurity is achieved at the expense of investment in somatic maintenance and repair.

Growth acceleration results in faster telomere shortening later in life

There is a wealth of evidence for a lifespan penalty when environmental conditions influence an individual's growth trajectory, such that growth rate is accelerated to attain a target size within a limited time period. Given this empirically demonstrated relationship between accelerated growth and lifespan, and the links between lifespan and telomere dynamics, increased telomere loss could underpin this growth-lifespan trade. We experimentally modified the growth trajectory of nestling zebra finches (Taeniopygia guttata), inducing a group of nestlings to accelerate their growth between 7 and 15 days of age, the main phase of body growth. We then sequentially measured their telomere length in red blood cells at various time points from 7 days to full adulthood (120 days). Accelerated growth between 7 and 15 days was not associated with a detectable increase in telomere shortening during this period compared with controls. However, only in the treatment group induced to show growth acceleration was the rate of growth during the experimental period positively related to the amount of telomere shortening between 15 and 120 days. Our findings provide evidence of a long-term influence of growth rate on later-life telomere shortening, but only when individuals have accelerated growth in response to environmental circumstances.

Repeated exposure to challenging environmental conditions influences telomere dynamics across adult life as predicted by changes in mortality risk

The effects of stress exposure are likely to vary depending on life-stage and stressor. While it has been postulated that mild stress exposure may have beneficial effects, the duration of such effects and the underlying mechanisms are unclear. While the long-term effects of early-life stress are relatively well studied, we know much less about the effects of exposure in adulthood since the early- and adult-life environments are often similar. We previously reported that repeated experimental exposure to a relatively mild stressor in female zebra finches, first experienced in young adulthood, initially had no effect on mortality risk, reduced mortality in middle age, but the apparently beneficial effects disappeared in old age. We show here that this is underpinned by differences between the control and stress-exposed group in the pattern of telomere change, with stress-exposed birds showing reduced telomere loss in middle adulthood. We thereby provide novel experimental evidence that telomere dynamics play a key role linking stress resilience and aging.

Developing Recommendations for Cumulative Endpoints and Lifetime Use for Research Animals

Research animals are important for scientific advancement, and therefore, their long-term welfare needs to be monitored to not only minimize suffering, but to provide positive affective states and experiences. Currently, there is limited guidance in countries around the world on cumulative and experimental endpoints. This paper aims to explore current opinions and institutional strategies regarding cumulative use and endpoints through a scoping survey and review of current regulations and welfare assessment tools, and ultimately to provide recommendations for assessment of cumulative and lifetime use of research animals. The survey found that only 36% of respondents indicated that their institution had cumulative use endpoint policies in place, but these policies may be informal and/or vary by species. Most respondents supported more specific guidelines but expressed concerns about formal policies that may limit their ability to make case-by-case decisions. The wide diversity in how research animals are used makes it difficult for specific policies to be implemented. Endpoint decisions should be made in an objective manner using standardized welfare assessment tools. Future research should focus on robust, efficient welfare assessment tools that can be used to support planning and recommendations for cumulative endpoints and lifetime use of research and teaching animals.

Birds of different morphs use slightly different strategies to achieve similar reproductive performance following heatwave exposure

Responses to extreme climatic events may differ between individuals of distinct morphs which differ in life-history strategies, resulting in climate change 'winners' and 'losers' within species. We examined the reproductive performance and carry-over effects on offspring of black- and red-headed Gouldian finches Erythrura gouldiae after exposure to simulated heatwaves of moderate or severe intensity. We expected black-headed pairs' reproductive performance to decline after the severe heatwave because only the condition of black-headed females deteriorates during such a heatwave. Supporting the fact that Gouldian finches of different morphs use alternative reproductive strategies, we found that black-headed females initiated egg-laying a month earlier than red-headed females after experiencing a severe heatwave. We also found that this severe heatwave resulted in shorter spermatozoa in males irrespective of their morph. Despite these effects associated with heatwave intensity, the overall reproductive performance of both morphs was not affected by this factor, which was possibly due to an increased nestling provisioning rate by parents after exposure to the severe heatwave. However, offspring still bore the cost of parental exposure to the severe heatwave, as they showed a reduced condition (lower plasma antioxidant capacity and transient lower breathing rate) and higher oxidative damage (at least in fledglings with black-headed parents). These results suggest that inter-morph phenotypic variability in the Gouldian finch does not result in clear differences in reproductive performance following heatwave exposure, despite basal phenotypic differences between morphs. Whether animals using alternative reproductive strategies are, in the end, differently affected by climate changes will likely depend on the capacity of their offspring to recover from altered developmental conditions.

Early maternal separation is not associated with changes in telomere length in domestic kittens (Felis catus)

OBJECTIVE

Studies of multiple species have found that adverse early life experiences, including childhood trauma and maternal separation, can result in accelerated telomere shortening. The objective of this study was to determine if premature separation from the mother affected telomere length in domestic kittens (Felis catus). Subjects were 42 orphaned kittens and 10 mother-reared kittens from local animal rescue groups and shelters. DNA was extracted from whole blood collected from kittens at approximately 1 week and 2 months of age. Telomere length was assessed by qPCR (quantitative polymerase chain reaction) from a total of 86 samples and expressed as a ratio of telomere PCR relative to a single copy gene PCR (T/S).

RESULTS

A generalized linear mixed model found there were no detectable differences in telomere length based on survival (F 1, 76.2 = 3.35, p = 0.07), orphan status (F 1, 56.5 = 0.44, p = 0.51), time point (F 1, 43.5 = 0.19, p = 0.67), or the interaction between orphan status and time (F 1, 43.5 = 0.86, p = 0.36). Although in other species telomere shortening is commonly associated with aging, even early in life, we did not find evidence for telomere shortening by two months of age. Our results suggest that the experience of early maternal separation in domestic cats who are subsequently hand-reared by humans does not accelerate telomere shortening compared to mother-reared kittens, at least in the first few months of life.

Individual telomere dynamics and their links to life history in a viviparous lizard

Emerging patterns suggest telomere dynamics and life history are fundamentally linked in endotherms through life-history traits that mediate the processes underlying telomere attrition. Unlike endotherms, ectotherms maintain the ability to lengthen somatic telomeres throughout life and the link between life-history strategies and ectotherm telomere dynamics is unknown. In a well-characterized model system (Niveoscincus ocellatus), we used long-term longitudinal data to study telomere dynamics across climatically divergent populations. We found longer telomeres in individuals from the cool highlands than those from the warm lowlands at birth and as adults. The key determinant of adult telomere length across populations was telomere length at birth, with population-specific effects of age and growth on adult telomere length. The reproductive effort had no proximate effect on telomere length in either population. Maternal factors influenced telomere length at birth in the warm lowlands but not the cool highlands. Our results demonstrate that life-history traits can have pervasive and context-dependent effects on telomere dynamics in ectotherms both within and between populations. We argue that these telomere dynamics may reflect the populations' different life histories, with the slow-growing cool highland population investing more into telomere lengthening compared to the earlier-maturing warm lowland population.

Exercise training has morph-specific effects on telomere, body condition and growth dynamics in a color-polymorphic lizard

Alternative reproductive tactics (ARTs) are correlated suites of sexually selected traits that are likely to impose differential physiological costs on different individuals. While moderate activity might be beneficial, animals living in the wild often work at the margins of their resources and performance limits. Individuals using ARTs may have divergent capacities for activity. When pushed beyond their respective capacities, they may experience condition loss, oxidative stress, and molecular damage that must be repaired with limited resources. We used the Australian painted dragon lizard that exhibits color polymorphism as a model to experimentally test the effect of exercise on body condition, growth, reactive oxygen species (ROS) and telomere dynamics - a potential marker of stress and aging and a correlate of longevity. For most males, ROS levels tended to be lower with greater exercise; however, males with yellow throat patches - or bibs - had higher ROS levels than non-bibbed males. At the highest level of exercise, bibbed males exhibited telomere loss, while non-bibbed males gained telomere length; the opposite pattern was observed in the no-exercise controls. Growth was positively related to food intake but negatively correlated with telomere length at the end of the experiment. Body condition was not related to food intake but was positively correlated with increases in telomere length. These results, along with our previous work, suggest that aggressive - territory holding - bibbed males suffer physiological costs that may reduce longevity compared with non-bibbed males with superior postcopulatory traits.

DNA Integrity Estimated via the Comet Assay Reflects Oxidative Stress and Competitive Disadvantage in Developing Birds

AbstractIncreases in DNA degradation have been detected in numerous situations in which organisms are exposed to pollutants. However, outside of the ecotoxicological literature, few studies have investigated whether there exists important variation in DNA integrity in free-living, healthy animals. Using the alkaline version of the comet assay to estimate DNA integrity in blood samples, we aimed to evaluate whether DNA integrity during early life is associated with nestlings' age, body mass, within-brood status, and oxidative stress using nestlings from a wild population of spotless starlings (Sturnus unicolor) as a model. We found important levels of variation in DNA integrity, suggesting the possibility that DNA integrity may have implications for offspring fitness. DNA integrity was dependent on the developmental stage, being lower at hatching than at the end of the nestling period. DNA integrity was also negatively related to the levels of oxidative damage at hatching and positively associated with wing length at fledging. In addition, position within the size hierarchy of the brood at fledging explained differences in DNA integrity, with higher levels in core than in marginal nestlings. Finally, despite extensive within-individual variation along nestling's age, we found DNA integrity during early life to be moderately repeatable within broods. Hence, DNA integrity in early life appears to be mainly affected by environmental factors, such as natural stressors. Our results suggest that measuring the variation in DNA integrity may be a fruitful approach for the assessment of individual fitness in natural populations and can be applied to studies in developmental biology and ecology.

Interplays between pre- and post-natal environments affect early-life mortality, body mass and telomere dynamics in the wild

Early-life conditions are crucial determinants of phenotype and fitness. The effects of pre- and post-natal conditions on fitness prospects have been widely studied but their interactive effects have received less attention. In birds, asynchronous hatching creates challenging developmental conditions for the last-hatched chicks, but differential allocation in last-laid eggs might help to compensate this initial handicap. The relative importance and potential interaction between pre- and post-hatching developmental conditions for different fitness components remains mostly unknown. We manipulated hatching order in wild pied flycatchers (Ficedula hypoleuca), creating three groups: natural asynchrony (last-laid eggs hatching last), reversed asynchrony (last-laid eggs hatching first) and hatching synchrony (all eggs hatching at once). We examined the effects of these manipulations on early-life survival, growth and telomere length, a potential cellular biomarker of fitness prospects. Mortality was mostly affected by hatching order, with last-hatched chicks being more likely to die. Early-life telomere dynamics and growth were influenced by the interplays between laying and hatching order. Last-laid but first-hatched chicks were heavier but had shorter telomeres 5 days after hatching than their siblings, indicating rapid early growth with potential adverse consequences on telomere length. Synchronous chicks did not suffer any apparent cost of hatching synchronously. Impaired phenotypes only occurred when reversing the natural hatching order (i.e. developmental mismatch), suggesting that maternal investment in last-laid eggs might indeed counterbalance the initial handicap of last-hatched chicks. Our experimental study thus highlights that potential interplays between pre- and post-natal environments are likely to shape fitness prospects in the wild.

Telomere dynamics from hatching to sexual maturity and maternal effects in the 'multivariate egg'

Avian eggs contain a large number of molecules deposited by the mother that provide the embryo with energy but also potentially influence its development via the effects of maternally derived hormones and antibodies: the avian egg is thus 'multivariate'. Multivariate effects on offspring phenotype were evaluated in a study on captive zebra finches, by simultaneously manipulating maternally derived antibodies (MAb) by lipopolysaccharide (LPS) treatment of mothers and injection of testosterone into the egg yolk. LPS treatment had a positive effect on body mass growth at 30 days after hatching and immune response at sexual maturity, while egg testosterone treatment positively influenced immune response at fledging and courtship behaviour in sexually mature male offspring. Maternal effects are known to modulate offspring telomere length (TL). However, the multivariate effects of egg-derived maternal components on offspring telomere dynamics from hatching to sexual maturity are undefined. Here, we tested: (1) the effects of LPS and testosterone treatments on TL from hatching to sexual maturity (day 82); (2) how LPS treatment modulated TL over reproduction in adult females; and (3) the relationship between maternal and offspring TL. We predicted that TL would be shorter in LPS fledglings (as a cost of faster growth) and that TL would be longer in sexually mature adults after yolk testosterone treatment (as a proxy of individual quality). In adult females, there was an overall negative relationship between laying and rearing investments and TL, this relationship was weaker in LPS-treated females. In chicks, there was an overall negative effect of LPS treatment on TL measured at fledging and sexual maturity (day 25-82). In addition, at fledging, there was a Sex×LPS×Testosterone interaction, suggesting the existence of antagonistic effects of our treatments. Our data partially support the hypothesis that telomeres are proxies of individual quality and that individual differences in TL are established very early in life.

Estimation of environmental, genetic and parental age at conception effects on telomere length in a wild mammal

Understanding individual variation in fitness-related traits requires separating the environmental and genetic determinants. Telomeres are protective caps at the ends of chromosomes that are thought to be a biomarker of senescence as their length predicts mortality risk and reflect the physiological consequences of environmental conditions. The relative contribution of genetic and environmental factors to individual variation in telomere length is, however, unclear, yet important for understanding its evolutionary dynamics. In particular, the evidence for transgenerational effects, in terms of parental age at conception, on telomere length is mixed. Here, we investigate the heritability of telomere length, using the 'animal model', and parental age at conception effects on offspring telomere length in a wild population of European badgers (Meles meles). Although we found no heritability of telomere length and low evolvability (<0.001), our power to detect heritability was low and a repeatability of 2% across individual lifetimes provides a low upper limit to ordinary narrow-sense heritability. However, year (32%) and cohort (3%) explained greater proportions of the phenotypic variance in telomere length, excluding qPCR plate and row variances. There was no support for cross-sectional or within-individual parental age at conception effects on offspring telomere length. Our results indicate a lack of transgenerational effects through parental age at conception and a low potential for evolutionary change in telomere length in this population. Instead, we provide evidence that individual variation in telomere length is largely driven by environmental variation in this wild mammal.

Early life starvation has stronger intra-generational than transgenerational effects on key life-history traits and consumption measures in a sawfly

Resource availability during development shapes not only adult phenotype but also the phenotype of subsequent offspring. When resources are absent and periods of starvation occur in early life, such developmental stress often influences key life-history traits in a way that benefits individuals and their offspring when facing further bouts of starvation. Here we investigated the impacts of different starvation regimes during larval development on life-history traits and measures of consumption in the turnip sawfly, Athalia rosae (Hymenoptera: Tenthredinidae). We then assessed whether offspring of starved and non-starved parents differed in their own life-history if reared in conditions that either matched that of their parents or were a mismatch. Early life starvation effects were more pronounced within than across generations in A. rosae, with negative impacts on adult body mass and increases in developmental time, but no effects on adult longevity in either generation. We found some evidence of higher growth rates in larvae having experienced starvation, although this did not ameliorate the overall negative effect of larval starvation on adult size. However, further work is necessary to disentangle the effects of larval size and instar from those of starvation treatment. Finally, we found weak evidence for transgenerational effects on larval growth, with intra-generational larval starvation experience being more decisive for life-history traits. Our study demonstrates that intra-generational effects of starvation are stronger than transgenerational effects on life-history traits and consumption measures in A. rosae.

Early-life telomere length predicts lifespan and lifetime reproductive success in a wild bird

Poor conditions during early development can initiate trade-offs that favour current survival at the expense of somatic maintenance and subsequently, future reproduction. However, the mechanisms that link early and late life-history are largely unknown. Recently it has been suggested that telomeres, the nucleoprotein structures at the terminal end of chromosomes, could link early-life conditions to lifespan and fitness. In wild purple-crowned fairy-wrens, we combined measurements of nestling telomere length (TL) with detailed life-history data to investigate whether early-life TL predicts fitness prospects. Our study differs from previous studies in the completeness of our fitness estimates in a highly philopatric population. The association between TL and survival was age-dependent with early-life TL having a positive effect on lifespan only among individuals that survived their first year. Early-life TL was not associated with the probability or age of gaining a breeding position. Interestingly, early-life TL was positively related to breeding duration, contribution to population growth and lifetime reproductive success because of their association with lifespan. Thus, early-life TL, which reflects growth, accumulated early-life stress and inherited TL, predicted fitness in birds that reached adulthood but not noticeably among fledglings. These findings suggest that a lack of investment in somatic maintenance during development particularly affects late life performance. This study demonstrates that factors in early-life are related to fitness prospects through lifespan, and suggests that the study of telomeres may provide insight into the underlying physiological mechanisms linking early- and late-life performance and trade-offs across a lifetime.

Oxidative status and telomere length are related to somatic and physiological maturation in chicks of European starlings (Sturnus vulgaris)

Telomere length can be considered as an indicator of an organism's somatic state, long telomeres reflecting higher energy investment in self-maintenance. Early-life is a period of intense investment in somatic growth and in physiological maturation but how this is reflected in telomere length remains unclear. Using European starling chicks we tested: (i) how telomere length measured at asymptotic mass is related to proxies of somatic growth and physiological maturity in 17-day-old nestlings; (ii) how telomere length measured at 17 days then predicts the changes in somatic and physiological maturity occurring in fledglings (between 17 and 21 days); (iii) how growth and telomere length co-vary when chicks are under experimentally good (fed) growth conditions. Depending on environmental conditions, our data suggest links between somatic growth, physiological maturation and body maintenance parameters (positive with oxidative stress and negative with telomere length) in nestlings. Telomere length measured at day 17 predicted a subsequent change in physiological maturation variables observed in fledglings, but only in second-brood chicks: chicks with shorter telomeres had a higher pre-fledging rate of increase in haematocrit and haemoglobin content and a greater decrease in reticulocyte count. Finally, food supplementation of chicks did not change telomere length compared with that in control siblings. Our results suggest that physiological maturation prior to fledging may occur at the expense of telomere length but only when environmental conditions are sub-optimal.

Intergenerational effects on offspring telomere length: interactions among maternal age, stress exposure and offspring sex

Offspring produced by older parents often have reduced longevity, termed the Lansing effect. Because adults usually have similar-aged mates, it is difficult to separate effects of maternal and paternal age, and environmental circumstances are also likely to influence offspring outcomes. The mechanisms underlying the Lansing effect are poorly understood. Variation in telomere length and loss, particularly in early life, is linked to longevity in many vertebrates, and therefore changes in offspring telomere dynamics could be very important in this context. We examined the effect of maternal age and environment on offspring telomere length in zebra finches. We kept mothers under either control (ad libitum food) or more challenging (unpredictable food) circumstances and experimentally minimized paternal age and mate choice effects. Irrespective of the maternal environment, there was a substantial negative effect of maternal age on offspring telomere length, evident in longitudinal and cross-sectional comparisons (average of 39% shorter). Furthermore, in young mothers, sons reared by challenged mothers had significantly shorter telomere lengths than sons reared by control mothers. This effect disappeared when the mothers were old, and was absent in daughters. These findings highlight the importance of telomere dynamics as inter-generational mediators of the evolutionary processes determining optimal age-specific reproductive effort and sex allocation.

The environmental and genetic determinants of chick telomere length in Tree Swallows (Tachycineta bicolor)

Conditions during early life can have dramatic effects on adult characteristics and fitness. However, we still know little about the mechanisms that mediate these relationships. Telomere shortening is one possibility. Telomeres are long sequences of DNA that protect the ends of chromosomes. They shorten naturally throughout an individual's life, and individuals with short telomeres tend to have poorer health and reduced survival. Given this connection between telomere length (TL) and fitness, natural selection should favor individuals that are able to retain longer telomeres for a greater portion of their lives. However, the ability of natural selection to act on TL depends on the extent to which genetic and environmental factors influence TL. In this study, we experimentally enlarged broods of Tree Swallows (Tachycineta bicolor) to test the effects of demanding early-life conditions on TL, while simultaneously cross-fostering chicks to estimate heritable genetic influences on TL. In addition, we estimated the effects of parental age and chick sex on chick TL. We found that TL is highly heritable in Tree Swallow chicks, and that the maternal genetic basis for TL is stronger than is the paternal genetic basis. In contrast, the experimental manipulation of brood size had only a weak effect on chick TL, suggesting that the role of environmental factors in influencing TL early in life is limited. There was no effect of chick sex or parental age on chick TL. While these results are consistent with those reported in some studies, they are in conflict with others. These disparate conclusions might be attributable to the inherent complexity of telomere dynamics playing out differently in different populations or to study-specific variation in the age at which subjects were measured.

Artificial light at night does not affect telomere shortening in a developing free-living songbird: A field experiment: Artificial light at night and telomere dynamics

Artificial light at night (ALAN) is an increasingly pervasive anthropogenic disturbance factor. ALAN can seriously disrupt physiological systems that follow circadian rhythms, and may be particularly influential early in life, when developmental trajectories are sensitive to stressful conditions. Using great tits (Parus major) as a model species, we experimentally examined how ALAN affects physiological stress in developing nestlings. We used a repeated-measure design to assess effects of ALAN on telomere shortening, body mass, tarsus length and body condition. Telomeres are repetitive nucleotide sequences that protect chromosomes from damage and malfunction. Early-life telomere shortening can be accelerated by environmental stressors, and has been linked to later-life declines in survival and reproduction. We also assayed nitric oxide, as an additional metric of physiological stress, and determined fledging success. Change in body condition between day 8 and 15 differed according to treatment. Nestlings exposed to ALAN displayed a trend towards a decline in condition, whereas control nestlings displayed a trend towards increased condition. This pattern was driven by a greater increase in tarsus length relative to mass in nestlings exposed to ALAN. Nestlings in poorer condition and nestlings that were smaller than their nest mates had shorter telomeres. However, exposure to ALAN was unrelated to telomere shortening, and also had no effect on nitric oxide concentrations or fledging success. Thus, exposure to ALAN may not have led to sufficient stress to induce telomere shortening. Indeed, plasticity in other physiological systems could allow nestlings to maintain telomere length despite moderate stress. Alternatively, the cascade of physiological and behavioral responses associated with light exposure may have no net effect on telomere dynamics.

Developmental history, energetic state and choice impulsivity in European starlings, Sturnus vulgaris

Impulsivity—the extent to which a reward is devalued by the amount of time until it is realized—can be affected by an individual’s current energetic state and long-term developmental history. In European starlings (Sturnus vulgaris), a previous study found that birds that were lighter for their skeletal size, and birds that had undergone greater shortening of erythrocyte telomeres over the course of development, were more impulsive as adults. Here, we studied the impulsivity of a separate cohort of 29 starlings hand-reared under different combinations of food amount and begging effort. The task involved repeated choice between a key yielding one pellet after 3 s and another key yielding two pellets after 8 s. Impulsivity was operationalised as the proportion of choices for the short-delay option. We found striking variation in impulsivity. We did not replicate the results of the previous study concerning developmental telomere attrition, though combining all the evidence to date in a meta-analysis did support that robustness of that association. We also found that early-life conditions and mass for skeletal size interacted in predicting impulsivity. Specifically, birds that had experienced the combination of high begging effort and low food amount were less impulsive than other groups, and the usual negative relationship between impulsivity and body mass was abolished in birds that had experienced high begging effort. We discuss methodological differences between our study and studies that measure impulsivity using an adjusting-delay procedure.

Heritability of telomere variation: it is all about the environment!

Individual differences in telomere length have been linked to survival and senescence. Understanding the heritability of telomere length can provide important insight into individual differences and facilitate our understanding of the evolution of telomeres. However, to gain accurate and meaningful estimates of telomere heritability it is vital that the impact of the environment, and how this may vary, is understood and accounted for. The aim of this review is to raise awareness of this important, but much under-appreciated point. We outline the factors known to impact telomere length and discuss the fact that telomere length is a trait that changes with age. We highlight statistical methods that can separate genetic from environmental effects and control for confounding variables. We then review how well previous studies in vertebrate populations including humans have taken these factors into account. We argue that studies to date either use methodological techniques that confound environmental and genetic effects, or use appropriate methods but lack sufficient power to fully separate these components. We discuss potential solutions. We conclude that we need larger studies, which also span longer time periods, to account for changing environmental effects, if we are to determine meaningful estimates of the genetic component of telomere length.

This article is part of the theme issue ‘Understanding diversity in telomere dynamics'.

Harsh conditions during early development influence telomere length in an altricial passerine: Links with oxidative stress and corticosteroids

Stress during early development can induce substantial long-term effects in organisms. In the case of birds, despite growth compensations, nestlings reared under harsh conditions typically show reduced survival chances in adulthood. It has been proposed that environmental early-life stressors could affect longevity via effects on telomere length, possibly mediated through oxidative stress. However, the link between these processes is not clear. In this study, we experimentally manipulated brood size in spotless starlings (Sturnus unicolor) to test the causal relationship between early stress, oxidative and corticosterone-mediated stress and telomere shortening. Our results show that experimentally enlarged brood sizes led to a reduction in morphometric development on nestlings, the effect being stronger for females than males. Additionally, basal corticosterone levels increased with increasing brood size in female nestlings. Neither plasma antioxidant status nor malondialdehyde levels (a marker of lipid peroxidation) were affected by experimental brood size, although the levels of a key intracellular antioxidant (glutathione) decreased with increasing brood size. We found that the treatment showed a quadratic effect on nestling telomere lengths: these were shortened either by increases or by decreases in the original brood size. Our study provides experimental evidence for a link between developmental stress and telomere length, but does not support a direct causal link of this reduction with corticosterone or oxidative stress. We suggest that future studies should focus on how telomere length responds to additional markers of allostatic load.

Sex-Specific Associations between Telomere Dynamics and Oxidative Status in Adult and Nestling Pied Flycatchers

Oxidative stress can contribute to an acceleration of telomere erosion, leading to cellular senescence and aging. Increased investment in reproduction is known to accelerate senescence, generally resulting in reduced future reproductive potential and survival. To better understand the role played by oxidative status and telomere dynamics in the conflict between maintenance and reproduction, it is important to determine how these factors are related in parents and their offspring. We investigated the relationship between oxidative status and telomere measurements in pied flycatchers (Ficedula hypoleuca). Total antioxidant status (TAS) in plasma, total levels of glutathione in red blood cells (RBCs), and oxidative damage in plasma lipids (malondialdehyde [MDA]) were assessed in both parents and nestlings. Telomeres were measured in RBCs in adults. Our results showed sex differences in oxidative variables in adults that are likely to be mediated by sex steroids, with testosterone and estrogens increasing and reducing, respectively, the production of reactive oxygen and nitrogen species. We found a negative association between telomere length (TL) and MDA in adults in the previous season. Moreover, TL was positively associated with TAS in females, while telomere shortening (ΔTL) correlated positively with MDA in males in the current year. These associations could be reflecting differences between sexes in reproductive physiology. We found a positive correlation between parental ΔTL and nestling MDA, an example of how parental physiological aging could affect offspring quality in terms of oxidative stress that highlights the constraints imposed by higher rates of ΔTL during reproduction and rearing.

Chronological age, biological age, and individual variation in the stress response in the European starling: a follow-up study

The strength of the avian stress response declines with age. A recently published study of European starlings (Sturnus vulgaris) found that a marker of biological age predicted the strength of the stress response even in individuals of the same chronological age. Specifically, birds that had experienced greater developmental telomere attrition (DTA) showed a lower peak corticosterone (CORT) response to an acute stressor, and more rapid recovery of CORT levels towards baseline. Here, we performed a follow-up study using the same capture-handling-restraint stressor in a separate cohort of starlings that had been subjected to a developmental manipulation of food availability and begging effort. We measured the CORT response at two different age points (4 and 18 months). Our data suggest a decline in the strength of the CORT response with chronological age: peak CORT was lower at the second age point, and there was relatively more reduction in CORT between 15 and 30 min. Individual consistency between the two age points was low, but there were modest familial effects on baseline and peak CORT. The manipulation of begging effort affected the stress response (specifically, the reduction in CORT between 15 and 30 min) in an age-dependent manner. However, we did not replicate the associations with DTA observed in the earlier study. We meta-analysed the data from the present and the earlier study combined, and found some support for the conclusions of the earlier paper.