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.