Discussion

In our Seychelles warbler population, a change in telomere length (∆RTL) was associated with life-history factors predicted to influence self-maintenance trade-offs - but only in females. Greater telomere shortening was observed in females with higher reproductive effort when living in areas of poorer food availability, as well as in individuals infected with malaria. Importantly, telomere lengthening was more often observed in females experiencing the reverse circumstance – i.e. those not infected with malaria and living in areas of high food availability – and those with helpers at the nest (the presence of which reduces reproductive effort per individual). These opposing changes in telomere length were not due to differences in initial telomere length or regression-to-the-mean effects. Consistent with ∆RTL being negatively correlated with stress, higher subsequent survival probabilities were associated with telomere lengthening, independently of sex and age.
We found that telomeres shortened in individuals that tested positive for malaria, but only in females. This finding is consistent with previous studies demonstrating greater erythrocyte telomere shortening in malaria-infected individuals compared to uninfected individuals (Asghar et al. , 2015; Karell et al. , 2017). While we expected malaria to affect both sexes equally, sex-specific differences in the impact of malaria on telomere length have also been observed in blue tits (Cyanistes caeruleus ; Sudyka, et al. , 2019). An emerging view is that telomere shortening is an outcome of immunological responses to infection (Giraudeau et al. , 2019). One such response – oxidative stress – is elevated in Seychelles warblers infected with malaria; albeit depending on the breeding stage (van de Crommenacker et al. , 2012). In our system, adults that test positive for malaria are in the chronic stage (i.e. late) and/or relapsed infections. Therefore, the telomere shortening we observed seems to reflect a cost of persistent infection – perhaps due to immunological responses - rather than a direct cost of parasitism, which tends to occur during the acute malarial stage (Asghar et al. , 2018). However, not knowing when malarial parasites became present or absent in blood, relative to the time of sampling, is a limitation of our observational study. Furthermore, while the frequency of reinfection/infection relapses is generally low (ca. 20% within two years; Hammers et al. , 2016), initially uninfected individuals could have undetected outbreaks within the time period of repeated samples.
We also found that telomere shortening was greater in females that produced more offspring. Moreover, the relationship only occurred when the mean food availability was low during the period of offspring production. Food limitation is expected to increase reproductive effort per unit of reproductive success, and thus increase the costs of reproduction (Harshman and Zera, 2007; Santos and Nakagawa, 2012). For example, individuals on poor-quality territories may have to work harder to meet the food demands of offspring, leading to elevated stress (see Soulsbury and Halsey, 2018). Likewise, Seychelles warblers tend to be in poorer condition (in terms of oxidative stress and body mass) when provisioning chicks, compared to other nest stages, and when occupying poorer quality territories (Van de Crommenacker et al. , 2011, 2011). There are now several experimental and observational studies which show that individuals experiencing higher reproductive effort have shorter telomeres and/or experience greater telomere shortening (recently reviewed by Sudyka, 2019). However, few of these studies have explored associations between telomeres and reproduction in the context of food availability. Thus, our finding adds novel insight into life-history framework of telomere dynamics.
The relationship between telomeres and reproductive effort was only apparent in females. This was expected, since parental effort is higher in females; in the Seychelles warbler only females incubate and they also have higher provisioning rates than males (Hammers et al. , 2019; van Boheemen et al. , 2019). Thus, females benefit more from having nest helpers (Hammers et al. , 2019) and may be more responsive to differences in food availability when caring for offspring (e.g. Low et al. , 2012). Alternatively, telomere shortening may correlate with egg production – which is associated with substantial self-maintenance costs (Visser and Lessells, 2001; Williams, 2005) – more than with provisioning effort. Likewise, male telomere shortening may be more correlated with male-specific reproductive behaviours that were not accounted for in this study. For example, Bebbington et al. (2017) showed that Seychelles warbler males, which are more involved in territory defence, have more telomere shortening with increased competition from rival males.
Telomere length increased in females that produced more offspring when experiencing higher food availability, and when the production of offspring was assisted by nest helpers. This finding supports our main prior prediction – that telomere lengthening occurs in individuals experiencing lower levels of life-history stress. The non-biological explanation – that observed telomere lengthening is a consequence of high measurement error relative to attrition rate (Steenstrup et al. , 2013) – seems unlikely for several reasons. First, the degree of within-individual telomere lengthening observed in our system is greater than that expected from measurement error alone (Spurgin et al. , 2018). Secondly, high measurement error and a lack of telomere shortening would result in no overall change (i.e. a random scatter of values around zero), whereas we observed an overall increase in telomere length consistent with our predictions. Lastly, our analysis accounted for regression-to-the-mean effects. This suggests that ‘real’ telomere lengthening (i.e. that which is not purely a consequence of measurement error) is more frequent in individuals with less stressful life-histories.
Telomere lengthening may be an outcome of lower reproductive costs associated with high food availability, cooperative breeding and absence of parasite infection. As discussed above, reduced reproductive effort is associated with the maintenance of longer telomeres and/or reduced telomere attrition. Some experimental studies that manipulated offspring number find no change or even slight telomere lengthening in treatment groups with the fewest offspring (Kotrschal, Ilmonen and Penn, 2007; Heidinger et al. , 2012; Sudyka et al. , 2014); however, these observations tend to be reported as reduced telomere shortening. Furthermore, reduced oxidative damage and telomere lengthening have also been observed in wild rodents receiving food supplements (Fletcheret al. , 2013; Hoelzl et al. , 2016). Thus, telomere lengthening may occur because plentiful food permits the allocation of energy to mechanisms involved in restoring previously lost telomere length, such as telomerase. This is a possibility in the Seychelles warbler, as high telomerase activity has been observed in the bone marrow (relative to other tissues) in adults of other bird species (Haussmann et al. , 2007). Alternatively, telomere lengthening may be associated with the more successful life-histories of higher-quality females, rather than a consequence of low life-history stress (Bauch, Becker and Verhulst, 2013; Angelier et al. , 2019). Nevertheless, our study indicates that the telomere lengthening observed in some wild populations is not necessarily random or merely an artefact of measurement error, but can instead be associated with important life-history traits and/or trade-offs.
We found that individuals with greater telomere shortening had lower survival prospects. This finding is consistent with short telomeres being negatively associated with survival in the Seychelles warbler (Barrett et al. , 2013) and in a range other wild vertebrate species (reviewed in Wilbourn et al. , 2018). Telomere shortening can directly impact survival by increasing the frequency of critically short telomeres, which can trigger cellular senescence (Kurz et al. , 2004). However, our measure of telomere length is a mean value (i.e. across chromosomes and cells) rather than a measure of the frequency of short telomeres per se (Bendix et al. , 2010). The non-causal explanation is that factors which shorten telomeres – such as oxidative stress – also cause wider cellular damage that ultimately increases mortality risk. Interestingly, we show that infection with malaria, for which we have not been able to find a survival impact in this species (Hammers et al. , 2016), may increase mortality risk via mechanisms that also shorten telomeres. Importantly, the telomere–survival relationship was not solely driven by negative effects (i.e. cellular damage and/or critically short telomeres), since individuals with lengthened telomeres had better survival prospects relative to individuals with no change in telomere length. This finding is consistent with the positive health and longevity effects of telomerase in mice (Bernardes de Jesus et al. , 2012; Simons, 2015) but contrasts with the results of Wood and Young, (2019), who found that increased telomere length was not associated with higher nestling survival in white‐browed sparrow‐weavers (Plocepasser mahali ). In our study, telomere lengthening was also associated with life-history traits known to benefit survival and longevity in the Seychelles warbler: high food availability (Brouweret al. , 2006) and helpers (Hammers et al. , 2019). This suggests that telomere lengthening may be characteristic of a strategy in which individuals make higher reproductive investments in more favourable environments, without incurring survival costs (as suggested by Hoelzl et al. , 2016).
Our study adds to the growing body of literature on the bidirectionality of within-individual telomere dynamics in ecological settings. We found that telomere lengthening can reflect good current environmental conditions and subsequently is linked to better survival prospects. Therefore, single measures of telomere length may not be a reliable indicator of damage accumulated in an individual’s past life, nor their future performance – and hence not a good biological age marker. Future studies should determine the mechanisms behind telomere lengthening observed in wild populations, and whether telomere lengthening is coordinated across multiple tissue types within individuals.