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Telomere length and telomere shortening predict survival in many organisms. This raises the question of the contribution of genetic and environmental effects to variation in these traits, which is still poorly known, particularly for telomere shortening. We used experimental (cross-fostering) and statistical (quantitative genetic “animal models”) means to disentangle and estimate genetic and environmental contributions to telomere length variation in pedigreed free-living jackdaws (Corvus monedula). Telomere length was measured twice in nestlings, at ages 4 (n = 715) and 29 days (n = 474), using telomere restriction fragment (TRF) analysis, adapted to exclude interstitial telomeric sequences. Telomere length shortened significantly over the nestling period (10.4 ± 0.3 bp day–1) and was highly phenotypically (rP = 0.95 ± 0.01) and genetically (rG > 0.99 ± 0.01) correlated within individuals. Additive genetic effects explained a major part of telomere length variation among individuals, with its heritability estimated at h2 = 0.74 on average. We note that TRF-based studies reported higher heritabilities than qPCR-based studies, and we discuss possible explanations. Parent–offspring regressions yielded similar heritability estimates for mothers and fathers when accounting for changes in paternal telomere length over life. Year effects explained a small but significant part of telomere length variation. Heritable variation for telomere shortening was low (h2 = 0.09 ± 0.11). The difference in heritability between telomere length (high) and telomere shortening (low) agrees with evolutionary theory, in that telomere shortening has stronger fitness consequences in this population. Despite the high heritability of telomere length, its evolvability, which scales the additive genetic variance by mean telomere length, was on average 0.48%. Hence, evolutionary change of telomere length due to selection is likely to be slow.

Variation in developmental conditions is known to affect fitness in later life, but the mechanisms underlying this relationship remain elusive. We previously found in jackdawsCorvus monedulathat larger eggs resulted in larger nestlings up to fledging. Through a cross-foster experiment of complete clutches we tested whether this association can be attributed to egg size per se, or to more proficient parents producing larger eggs and larger nestlings, with the latter effect being more or less independent of egg size. Due to other manipulations post-hatching, we primarily investigated effects on nestling mass on day 5, which we show to predict survival until fledging. We introduce a new statistical approach to compare the competing hypotheses and conclude that 92% of the association between egg size and nestling mass is attributed to a direct effect of egg size, and that this relationship is not affected by variation in other parental quality. Intriguingly, the effect of egg size on day 5 nestling mass was steeper (1.7 g/cm3) than the effect of egg size on day 1 hatchling mass (0.7 g/cm3). Early growth is exponential, and the difference in effect size may therefore be explained by hatchlings from large eggs being further in their development at hatching. The direct effect of egg size on nestling mass raises the question what causes egg size variation in jackdaws.

Life history theory suggests a trade-off between costly activities such as breeding and migration and somatic self-maintenance. However, how the short-term cost of parental effort is expressed in species with a slow pace-of-life is not well understood. Also, investigating carry-over effects of migration is most meaningful when comparing migratory strategies within the same population, but this has rarely been done. We explore this hypothesis in a long-lived, pelagic seabird, the Cory's Shearwater, Calonectris borealis, where males display partial migration. By manipulating reproductive effort and taking advantage of the natural variation in migratory strategy, we investigate whether early reproductive failure and migratory strategy had implications on the physical condition of males on return to the colony the following year. We experimentally induced breeding failure from mid-incubation, tracked the over-winter movements of these males and of males that invested in parental effort, and assessed innate immunity, stress, and residual body mass the following year. Early breeding failure resulted in earlier return to the colony among all males, associated with greater probability of reproductive success. Residents had a lower tail feather fault bar intensity, an indicator of stress during the non-breeding period, compared to migrants. Reproductive effort and migratory strategy had no impact on physiological condition otherwise. Our results provide evidence that in species with a slow-pace of life, such as the Cory's Shearwater, somatic maintenance is prioritised, with the costs of reproduction and migration paid in delayed arrival date. FCT - Fundação para a Ciência e Tecnologia info:eu-repo/semantics/publishedVersion

The assumption that reproductive effort decreases somatic state, accelerating ageing, is central to our understanding of life‐history variation. Maximal reproductive effort early in life is predicted to be maladaptive by accelerating ageing disproportionally, decreasing fitness.Optimality theory predicts that reproductive effort is restrained early in life to balance the fitness contribution of reproduction against the survival cost induced by the reproductive effort. When adaptive, the level of reproductive restraint is predicted to be inversely linked to the remaining life expectancy, potentially resulting in a terminal effort in the last period of reproduction.Experimental tests of the reproductive restraint hypothesis require manipulation of somatic state and subsequent investigation of reproductive effort and residual life span. To our knowledge the available evidence remains inconclusive, and hence reproductive restraint remains to be demonstrated.We modulated somatic state through a lifelong brood size manipulation in wild jackdaws and measured its consequences for age‐dependent mortality and reproductive success.The assumption that lifelong increased brood size reduced somatic state was supported: Birds rearing enlarged broods showed subsequent increased rate of actuarial senescence, resulting in reduced residual life span.The treatment induced a reproductive response in later seasons: Egg volume and nestling survival were higher in subsequent seasons in the increased versus reduced broods' treatment group. We detected these increases in egg volume and nestling survival despite the expectation that in the absence of a change in reproductive effort, the reduced somatic state indicated by the increased mortality rate would result in lower reproductive output. This leads us to conclude that the higher reproductive success we observed was the result of higher reproductive effort.Our findings show that reproductive effort negatively covaries with remaining life expectancy, supporting optimality theory and confirming reproductive restraint as a key factor underpinning life‐history variation., Early life reproductive restraint could be advantageous when it promotes health. This study compares reproductive success of jackdaws among accelerated versus decelerated ageing groups. Reproductive success increased in the accelerated ageing group, demonstrating reproductive restraint. Reproductive restraint provides an explanation for the commonly observed increase in reproductive success with age. Photo credit: Jelle Boonekamp.

Mercury is a heavy metal, which is pervasive and persistent in the marine environment. It bioaccumulates within organisms and biomagnifies in the marine food chain. Due to its high toxicity, mercury contamination is a major concern for wildlife and human health. Telomere length is a biomarker of aging and health, because it predicts survival, making it a potential tool to investigate sublethal effects of mercury contamination. However, the relationship between telomeres and mercury contamination is unclear. We measured feather mercury concentration in Cory's Shearwaters Calonectris borealis, long-lived seabirds and top predators, between 9 and 35 years of age and related it to telomere length in erythrocytes. Cory's Shearwaters with higher mercury concentrations had shorter telomeres and the effect was sex-dependent, reaching significance in males only. This may be explained by the fact that males have longer telomeres and higher and more variable mercury concentrations than females in this population. The mercury effect on telomere length was stronger on longer telomeres in the genome within individuals. We discuss the hypotheses that the negative correlation could either be a direct effect of mercury on telomere shortening and/or a consequence of variation in phenotypic quality among individuals that results in a covariation between mercury contamination and telomere length. Fundação para a Ciência e Tecnologia - FCT info:eu-repo/semantics/publishedVersion

Telomeres are protective caps at the end of chromosomes, and their length is positively correlated with individual health and lifespan across taxa. Longitudinal studies have provided mixed results regarding the within-individual repeatability of telomere length. While some studies suggest telomere length to be highly dynamic and sensitive to resource-demanding or stressful conditions, others suggest that between-individual differences are mostly present from birth and relatively little affected by the later environment. This dichotomy could arise from differences between species, but also from methodological issues. In our study, we used the highly reliable Terminal Restriction Fragment analysis method to measure telomeres over a 10-year period in adults of a long-lived seabird, the common tern (Sterna hirundo). Telomeres shortened with age within individuals. The individual repeatability of age-dependent telomere length was high (>0.53), and independent of the measurement interval (i.e., one vs. six years). A small (R-2 = .01), but significant part of the between-individual variation in telomere length was, however, explained by the number of fledglings produced in the previous year, while reproduction in years prior to the previous year had no effect. We confirmed that age-dependent telomere length predicted an individual's remaining lifespan. Overall, our study suggests that the majority of between-individual variation in adult telomere length is consistent across adult life, and that a smaller part of the variation can be explained by dynamic factors, such as reproduction.

Telomere length (TL) and shortening rate predict survival in many organisms. Evolutionary dynamics of TL in response to survival selection depend on the presence of genetic variation that selection can act upon. However, the amount of standing genetic variation is poorly known for both TL and TL shortening rate, and has not been studied for both traits in combination in a wild vertebrate. We used experimental (cross-fostering) and statistical (animal models) means to disentangle and estimate genetic and environmental contributions to TL variation in pedigreed free-living jackdaws (Corvus monedula). TL was measured twice early in life (age- and interval-standardized), when shortening is highest, using the high-precision TRF technique, adapted to exclude interstitial telomeric sequences. TL shortened significantly during the nestling phase (10.4 bp/day), was highly repeatable within individuals (R=0.97) and genetically correlated between the two ages (rG>0.99). Additive genetic effects explained the major part of TL variation between individuals, with heritability on average estimated ath2=0.74. Parent- offspring regressions yielded similar estimates for the sexes when accounting for changes in paternal TL over life. Cohort effects explained a small but significant part of TL variation. Heritable variation for telomere shortening was negligible. Despite the high heritability of TL, its evolvability, which scales the additive genetic variance by mean TL, was close to zero. Hence evolutionary change of TL is likely to be slow despite significant selection.

The relationship between growth and age-specific telomere length, as a proxy of somatic state, is increasingly investigated, but observed patterns vary and a predictive framework is lacking. We outline expectations based on the assumption that telomere maintenance is costly and argue that individual heterogeneity in resource acquisition is predicted to lead to positive covariance between growth and telomere length. However, canalization of resource allocation to the trait with a larger effect on fitness, rendering that trait relatively invariant, can cause the absence of covariance. In a case study of common tern (Sterna hirundo) chicks, in which hatching order is the main determinant of variation in resource acquisition within broods, we find that body mass, but not telomere length or attrition, varies with hatching order. Moreover, body mass and growth positively predict survival to fledging, whereas telomere length and attrition do not. Using a novel statistical method to quantify standardized variance in plasticity, we estimate between-individual variation in telomere attrition to be only 12% of that of growth. Consistent with the relative invariance of telomere attrition, we find no correlation between age-specific body mass or growth and telomere attrition. We suggest that common tern chicks prioritize investment in long-term somatic state (as indicated by canalization of telomere maintenance) over immediate survival benefits of growth as part of an efficient brood reduction strategy that benefits the parents. As such, interspecific variation in the growth-telomere length relationship may be explained by the extent to which parents benefit from rapid mortality of excess offspring.

Telomere length (TL) is a candidate biomarker of ageing and phenotypic quality, but little is known of the (physiological) causes of TL variation. We previously showed that individual common terns Sterna hirundo with high reproductive success had short telomeres independent of age, and this pattern was particularly strong in the longer telomeres of the within-individual TL distribution. To test whether this relation can be attributed to effects of reproductive effort, we investigated baseline corticosterone in relation to reproductive success (number of fledglings) and TL. In this context, we assume that variation in baseline corticosterone can be interpreted as index of energy expenditure and allostatic load. Males with higher corticosterone levels during incubation, compared between and within individuals, achieved higher reproductive success and had shorter telomeres. The effect on telomeres was more pronounced in corticosterone measured later in incubation and in the longer telomeres of the within-individual TL distribution. Female corticosterone level during incubation was neither related to reproductive success nor to TL. That we observed these effects only in males mirrors different parental roles during reproduction in the common tern, where males do most of the chick provisioning. The negative association between reproductive success and TL suggests individual differences in reproductive effort as reflected in, or mediated by, baseline corticosterone. We see this result as a promising step towards unravelling the physiological causes of variation in TL and the costs of reproduction.

Evidence for transgenerational effects of senescence, whereby offspring from older parents have a reduced lifetime reproductive success, is increasing. Such effects could arise from compromised germline maintenance in old parents, potentially reflected in reduced telomere length in their offspring. We test the relationship between parental age and offspring early-life telomere length in a natural population of common terns and find a significant negative correlation between paternal age and offspring telomere length. Offspring telomere length is reduced by 35 base pairs for each additional year of paternal age. We find no correlation with maternal age. These results fit with the idea of compromised germline maintenance in males, whose germline stem cells require continued division.

Oxidative stress shortens telomeres in cell culture, but whether oxidative stress explains variation in telomere shortening in vivo at physiological oxidative stress levels is not well known. We therefore tested for correlations between six oxidative stress markers and telomere attrition in nestling birds (jackdaws Corvus monedula ) that show a high rate of telomere attrition in early life. Telomere attrition was measured between ages 5 and 30 days, and was highly variable (average telomere loss: 323 bp, CV = 45%). Oxidative stress markers were measured in blood at age 20 days and included markers of oxidative damage (TBARS, dROMs and GSSG) and markers of antioxidant protection (GSH, redox state, uric acid). Variation in telomere attrition was not significantly related to these oxidative stress markers (| r | ≤ 0.08, n = 87). This finding raises the question whether oxidative stress accelerates telomere attrition in vivo . The accumulation of telomere attrition over time depends both on the number of cell divisions and on the number of base pairs lost per DNA replication and, based on our findings, we suggest that in a growing animal cell proliferation, dynamics may be more important for explaining variation in telomere attrition than oxidative stress.

Telomere length (TL) predicts health and survival across taxa. Variation in TL between individuals is thought to be largely of genetic origin, but telomere inheritance is unusual, because zygotes already express a TL phenotype, the TL of the parental gametes. Offspring TL changes with paternal age in many species including humans, presumably through age-related TL changes in sperm, suggesting an epigenetic inheritance mechanism. However, present evidence is based on cross-sectional analyses, and age at reproduction is confounded with between-father variation in TL. Furthermore, the quantitative importance of epigenetic TL inheritance is unknown. Using longitudinal data of free-living jackdaws Corvus monedula, we show that erythrocyte TL of subsequent offspring decreases with parental age within individual fathers, but not mothers. By cross-fostering eggs, we confirmed the paternal age effect to be independent of paternal age dependent care. Epigenetic inheritance accounted for a minimum of 34% of the variance in offspring TL that was explained by paternal TL. This is a minimum estimate, because it ignores the epigenetic component in paternal TL variation and sperm TL heterogeneity within ejaculates. Our results indicate an important epigenetic component in the heritability of TL with potential consequences for offspring fitness prospects., Author summary Telomeres are DNA-protein structures at chromosome ends and a short telomere length predicts reduced survival in humans, birds and other organisms. Variation in telomere length between individuals is thought to be largely of genetic origin, but telomere inheritance may be unusual because not only genes regulating telomere length are inherited, but a fertilised cell already has a telomere length (from the parental gametes). Using long-term individual-based data of jackdaw families (a small corvid species), we found that as fathers aged, they produced chicks with shorter telomeres. This suggests that telomere length inheritance has an epigenetic component. To investigate to what extent telomere length in the fertilised cell affects telomere length after birth, we compared telomere length over years within fathers with the telomere length of their consecutive offspring. This epigenetic component explained a substantial part (≥ one third) of the telomere length inheritance; whereas there was no such effect of maternal telomere length. The sex difference fits the idea that lifelong sperm formation leads to change in telomere length of the sperm cells, whereas female gametes are all formed before birth and their telomere length does not change over time.

The use of haematophagous bugs (Triatominae) for minimally-invasive blood sampling has increasingly gained interest. We developed a small bug-containing dummy egg (size: 25 × 19 [mm]) for stress-free blood sampling during incubation in Common Swifts (Apus apus) and potentially even smaller bird species. Our design expands on an application of a method previously used successfully on larger birds. In our study, 40 % of blood-sampling attempts were successful. Success was highest in the early breeding season, higher at noon than later in the day and unaffected by nest infestation with ectoparasitic louse-flies (Crataerina pallida). We recommend this method for blood-sampling birds without trapping during the sensitive period of incubation and encourage its application in small bird species.

We investigated the reliability of buccal swabs for molecular sex determination in very young nestlings (0-14 days old) and adults of the Common Swift (Apus apus) compared to the results from blood samples of the same individuals. Sex determination based on buccal swabs mat- ched the result of sex determination based on blood samples in 46 out of 47 nestlings (98 %) and in all 10 adults (100 %). Therefore, we consider that buccal swab sampling is a reliable noninvasive method to obtain DNA for sex determination in swifts. We recommend buccal swabbing as an alternative to blood sampling in future genetic studies in birds.

In vertebrates little is known about causes and correlates of differences in individual body condition. This is, however, essential if we are to gain an understanding of environmental influences on wild populations. To track changes in individual physiological state we analysed blood metabolite concentrations in breeding common terns Sterna hirundo. We selected birds to contrast sex, breeding experience, sampling year and nutritional state and collected repeated samples from individuals during incubation. Unlike cholesterol, triglyceride and uric acid levels strongly correlated with nutritional state. While cholesterol levels in males remained stable throughout incubation, comparatively lower levels were measured in females directly after egg-laying. In 2006, only highly experienced females showed increased cholesterol concentrations by mid-incubation, whereas less experienced females maintained reduced levels and were apparently unable to recover. In 2007, all females showed an increase in cholesterol by mid-incubation. Triglyceride levels did not differ between sexes, but concentrations were significantly higher in more experienced birds and were overall higher in 2007 than in 2006. Uric acid concentrations did not differ significantly. The variation in cholesterol and triglyceride levels can be explained by differences in foraging conditions, as food abundance, onset of breeding and mean clutch size indicate 2007 to be the superior year. Thus, environmental conditions and breeding experience play an important role for cholesterol and triglyceride levels as well as the physiological state.

Many ecological studies use stress hormones to assess the condition, health or disturbance levels of wild organisms. Common blood sampling protocols for this research involve trapping individuals and taking blood within three minutes to obtain a “baseline” for analysis of stress hormones (“conventional method”). In some situations it may be difficult to get an accurate measure of baseline values; therefore, alternative sampling techniques may be preferable. We compared corticosterone levels in samples taken via a newly developed, minimally invasive blood sampling technique with corticosterone levels in blood taken via the conventional method. We collected samples from incubating adult common terns Sterna hirundo via blood sucking bugs (Heteroptera, Triatominae) contained in “dummy eggs” (“bug method”) and compared measured corticosterone concentrations to concentrations in blood taken from the same birds using the conventional method. We found no significant differences in mean or variance of baseline corticosterone levels between samples collected via the different methods. This suggests that the bug method offers a viable alternative for hormone sampling.

In long-lived vertebrates, individuals generally visit potential breeding areas or populations during one or more seasons before reproducing for the first time. During these years of prospecting, they select a future breeding site, colony, or mate and improve various skills and their physical condition to meet the requirements of reproduction. One precondition of successful reproduction is arrival in time on the breeding grounds. Here, we study the intricate links among the date of initial spring arrival, body mass, sex, and the age of first breeding in the common tern Sterna hirundo , a long-lived migratory colonial seabird. The study is based on a unique, individual-based, long-term dataset of sexed birds, marked with transponders, which allow recording their individual arrival, overall attendance, and clutch initiation remotely and automatically year by year over the entire lifetime at the natal colony site. We show that the seasonal date of initial arrival at the breeding grounds predicts the individual age at first reproduction, which mostly occurs years later. Late first-time arrivals remain delayed birds throughout subsequent years. Our findings reveal that timing of arrival at the site of reproduction and timing of reproduction itself are coherent parameters of individual quality, which are linked with the prospects of the breeding career and may have consequences for fitness.

Telomeres, DNA-protein structures at chromosome ends, shorten with age, and telomere length has been linked to age-related diseases and survival. In vitro studies revealed that the shortest telomeres trigger cell senescence, but whether the shortest telomeres are also the best biomarker of ageing is not known. We measured telomeres in erythrocytes of wild common terns Sterna hirundo using terminal restriction fragment analysis. This yields a distribution of telomere lengths for each sample, and we investigated how different telomere subpopulations (percentiles) varied in their relation to age and fitness proxies. Longer telomeres within a genome lost more base pairs with age and were better predictors of survival than shorter telomeres. Likewise, fitness proxies such as arrival date at the breeding grounds and reproductive success were best predicted by telomere length at the higher percentiles. Our finding that longer telomeres within a genome predict fitness components better than the shorter telomeres indicates that they are a more informative ageing biomarker. This finding contrasts with the fact that cell senescence is triggered by the shortest telomeres. We suggest that this paradox arises, because longer telomeres lose more base pairs per unit time and thus better reflect the various forms of stress that accelerate telomere shortening, and that telomeres primarily function as biomarker because their shortening reflects cumulative effects of various stressors rather than reflecting telomere-induced cell senescence.

Telomere length is associated with cellular senescence, lifestyle and ageing. Short telomeres indicate poor health in humans and reduced life expectancy in several bird species, but little is known about telomeres in relation to phenotypic quality in wild animals. We investigated telomere lengths in erythrocytes of known-age common terns ( Sterna hirundo ), a migratory seabird, in relation to arrival date and reproductive performance. Cross-sectional data revealed that, independent of age, individuals with short telomeres performed better: they arrived and reproduced earlier in the season and had more chicks in the nest. The latter effect was stronger the older the brood and stronger in males, which do most of the chick provisioning. Longitudinal data confirmed this pattern: compared with birds that lost their brood, birds that raised chicks beyond the 10th nestling day experienced higher telomere attrition from one year to the next. However, more detailed analysis revealed that the least and most successful individuals lost the fewest base pairs compared with birds with intermediate success. Our results suggest that reproductive success is achieved at the expense of telomeres, but that individual heterogeneity in susceptibility to such detrimental effects is important, as indicated by low telomere loss in the most successful birds.