Introduction

Senescence – the deterioration of health and performance in old age – occurs in nearly all species (Nussey et al. , 2013). However, within species there can be considerable individual variation in the onset and rate of senescence (e.g. Lemaître et al. , 2013). Thus, individuals may be biologically older or younger than expected for their chronological age. Measuring ‘biological age’ (Baker and Sprott, 1988) is valuable, not only in regards to organismal health but also in terms of understanding fundamental concepts in ecology and evolution e.g. trade-offs in life-history strategies, or the impact of different environmental stressors (Stearns, 2008; Lemaître et al. , 2015).
Telomeres are repetitive nucleotide sequences at the ends of chromosomes, which protect the functional integrity of the genome. Due to the ‘end replication problem’ (Watson, 1972), telomeres shorten with each cell division, until a critical length is reached where cells can no longer divide (Olovnikov, 1996; Campisi, 2003) Telomeres also shorten when exposed to sources of cellular damage, such as reactive oxygen species (Von Zglinicki, 2002; Reichert and Stier, 2017). Both the number of cell-divisions and cell damage load are cumulative (i.e. age-dependent) but also variable in rate. Telomere length shortens with increasing age in a broad range of taxa (Barrett et al. , 2013; Bendix et al. , 2014; Stier et al. , 2015) but see Fairlieet al. (2016).
There is considerable empirical evidence to support the idea of telomere length being a marker of biological age. Accelerated telomere shortening occurs as an outcome of life-history or environmental conditions associated with increased cellular division and reactive oxygen species production, including developmental growth (Salomons et al. , 2009; Monaghan and Ozanne, 2018), early-life adversity (Boonekampet al. , 2014; Watson, Bolton and Monaghan, 2015) and reproductive effort (Reichert et al. , 2014; Sudyka et al. , 2014). Shorter telomeres and/or higher attrition rates are associated with increased mortality risk (e.g. Haussmann, Winkler and Vleck, 2005; Veraet al. , 2012; Fairlie et al. , 2016; Barret et al ., 2013) and ‘faster’ life histories (Haussmann et al. , 2003; Sudyka, Arct, et al. , 2019). Therefore, telomere length has been proposed as a valuable biomarker linking past life-history costs to future performance (Young, 2018).
Over the last decade there has been a rapid expansion in studies investigating the causes and consequences of telomere dynamics across a wide range of taxa and environmental situations. However, our growing awareness of the complexity of telomere dynamics raises important questions on how we interpret telomeres as a biomarker. Longitudinal studies in humans and some wild vertebrates have shown that within-individual changes in telomere length are highly variable and bidirectional (Svenson et al. , 2011; Fairlie et al. , 2016; Hoelzl et al. , 2016; Spurgin et al. , 2017; van Lieshoutet al. , 2019). Until recently, observations of telomere lengthening were often attributed to measurement error between samples collected too close in time – relative to the rate of telomere loss – to detect telomere shortening (Chen et al. , 2011; Steenstrupet al. , 2013). However, it is now recognised that the degree or frequency of observed telomere lengthening is often greater than that expected from measurement error alone (Bateson and Nettle, 2017; Spurginet al. , 2017; van Lieshout et al. , 2019).
Telomere lengthening within the same individual may be observed for a variety of reasons. First, the enzyme telomerase can restore lost telomere length (Blackburn et al. , 1989). Since telomeres shorten during cell division, telomerase remains active in cell lineages requiring greater proliferation potential, such as haematopoietic stem cells (Morrison et al. , 1996; Haussmann et al. , 2007). Telomeres can also lengthen via alternative mechanisms, independent of telomerase (see Cesare and Reddel, 2010 for a discussion). Importantly, telomere measurements may increase in subsequent assays due to changes in clonal cell composition, i.e. an increase in long-telomere cells relative to short-telomere cells. All the mechanisms explained above are relevant to the telomere dynamics of blood, the tissue most often utilised for ecological and evolutionary studies on vertebrates (Nusseyet al. , 2014). Furthermore, in mammals the proportions of circulating leucocyte cell types (with differing telomere lengths; Weng, 2012) can also change dramatically within an individual, for example in response to infection, resulting in apparent changes in overall telomere length (Beirne et al. , 2014). In birds and reptiles, blood-derived assays of telomere length overwhelmingly stem from nucleated erythrocytes (Stier et al. , 2015), and telomerase activation or turnover in haematopoietic cell lines could, in theory, create heterogeneity in measured telomere length.
The importance of (apparent) telomere lengthening in wild populations remains uncertain. Since telomere attrition occurs as a consequence of life-history or environmental stress costs, telomere lengthening may reflect investment in self-maintenance when those costs are alleviated. For example, wild edible dormice (Glis glis ) that receive supplementary food showed lengthened telomeres (Hoelzl et al. , 2016). In other wild species, changes in telomere length reflected temporal differences in environmental conditions, with lengthening coinciding with more favourable environments (e.g. Mizutani et al. , 2013; Foley et al. , 2020). Telomere dynamics can also reflect changes in parasitic pressure. For example, infection with malaria has been associated with telomere attrition in wild and captive birds (Asghar et al. , 2016), but the clearing of infections in humans can result in lengthening (Asghar et al. , 2018). The ability of telomeres to both shorten and lengthen, rather than being an irreversible one-way ratchet, suggests that we may have to rethink our interpretation of telomere dynamics. Instead of reflecting the accumulation of all past stressors and growth, telomere length may be more of a short-term marker, reflecting an individual’s current condition consequent on the challenges and trade-offs faced by an individual. However, in contrast to telomere shortening, the circumstances under which telomere lengthening occurs in natural populations remain poorly understood.
Given the fitness costs associated with shorter, or more rapidly shortening, telomeres (see above), one might expect improved fitness to be associated with telomere lengthening. Recent reviews argue that telomere dynamics are a non-causal biomarker of accumulated cellular damage – such as that occurring from oxidative stress – that subsequently impacts fitness (Simons, 2015; Young, 2018). However, there is evidence that active restoration of telomere length can impact organismal performance. First, telomere lengthening could reduce the frequency of critically short telomeres – thought to directly contribute to organismal ageing by inducing cellular senescence and apoptosis (Vera et al. , 2012; Van Deursen, 2014). Secondly, telomerase has restorative effects on cells (Cong and Shay, 2008; Criscuolo et al. , 2018). Both telomerase activity and telomere lengthening are associated with tissue regeneration (Anchelin et al. , 2011; Reichert, Bize, et al. , 2014) and telomerase overexpression in mice is beneficial to a range of health parameters (Bernardes de Jesus et al. , 2012; Simons, 2015). Conversely, active telomere lengthening could also have negative effects, such as proliferating cancers (Shay and Wright, 2011) or by diverting energy from competing traits (Young, 2018). Nonetheless, telomere lengthening has the potential to be associated with organismal performance, and this impact is not dependent on telomere length playing a causal role in organismal ageing.
In this study, we aim to determine when and why telomere lengthening occurs, and assess its association with survival, in a wild population of the facultative cooperatively breeding Seychelles warbler (Acrocephalus sechellensis ). Previous studies on this population have shown that telomeres shorten with age, and individuals with shorter telomeres are less likely to survive to the following year (Barrettet al. , 2013). Furthermore, telomere shortening is associated with various stresses in this species, including inbreeding (Bebbingtonet al. , 2016), intra-specific antagonistic interactions (Bebbington et al. , 2017) and parental care (Hammers et al. , 2019). However, telomere lengthening is often observed between successive samples taken from the same adult individual, and is greater than that expected from measurement error alone (Spurgin et al. , 2018). We predict that telomere lengthening occurs in individuals that experience reduced stress. Specifically, we predict that, for adults, telomere lengthening will be associated with reduced reproductive effort (less breeding, higher food availability and the presence of helpers) and an absence of malaria (the only known parasite in the population). We expect this relationship to be sex-specific, given that reproductive investments differ between sexes in this species (Hammers et al. , 2019; van Boheemen et al. , 2019). Furthermore, we tested whether increased survival is associated with telomere lengthening.