DISCUSSION
Much of the theoretical and empirical research on the mechanisms that
shape variation in body size is based on life-history theory which
suggests that individuals allocate their acquired resources between
growth, reproduction, and self-maintenance (Stearns, 1989; Zera &
Harshman, 2001; Roff & Fairbairn, 2007). In this experimental study we
examined the consequences of artificial selection for larger and smaller
parental body size and how this influences variation in offspring TL at
early age, as well as the associations between TL and recruitment,
longevity and reproductive success in two wild house sparrow
populations. First, a negative correlation between nestling TL and
tarsus length was evident under the artificial selection for both larger
and smaller tarsi (Table 3). This link between TL and structural body
size suggests that telomere dynamics might mediate a trade-off between
investment in early-life growth and long-term somatic maintenance in the
wild (Metcalfe & Monaghan 2003; Ringsby et al., 2015; Monaghan &
Ozanne, 2018). Artificial selection for larger individuals in thehigh population caused TL to decrease significantly as tarsus
length increased during the four years of selection (Fig. 3).
Additionally, there was weak evidence that TL tended to increase as
tarsus length decreased in the low population (Fig. 4). It is
possible that the artificial selection for smaller body size in adults
only caused a small change in offspring size because the proportion of
additive genetic variance may be lower for small compared to large
individuals (Charmantier et al., 2004). Thus, selecting for smaller body
size for multiple years, as in our experiment, may accumulate
individuals that are smaller than their predicted size due to for
instance malnutrition or disease caused by e.g. environmental or
parental effects (Angelier et al., 2015).
TL has been suggested as a biomarker monitoring health and stress
exposure of organisms (Monaghan, 2014; Pepper et al., 2018; Chatelain et
al., 2020), individual phenotypic quality (Bauch et al., 2013; Boonekamp
et al., 2013; Le Vaillant et al., 2015), and as an integrated
physiological marker of cumulative life‐history costs (Monaghan &
Haussmann, 2006). The prevailing negative correlation between TL and
body size documented in this study, indicates that TL is influenced by
structural growth in free-living birds, which confirms the observation
by Ringsby et al. (2015). The artificial selection pressure on body size
was accompanied by a reduction in TL that was probably not counteracted
within the nestling period by increased investment into telomere
maintenance (i.e. canalization). Early-life changes in TL have been
hypothesized to influence long-term somatic state (Eisenberg, 2011;
Boonekamp et al., 2013; Vedder et al., 2017; Criscuolo et al., 2018a).
The enzyme telomerase can elongate telomeres (Blackburn, 1991), but its
activity is assumed to be a physiologically costly process (Hatakeyama
et al., 2016; Criscuolo et al., 2018b) or with potential increased
cancer risk effects (Seluanov et al., 2018). Accordingly, somatic
telomerase activity is generally assumed to be repressed in birds (Gomes
et al., 2010), though more investigation of this is needed since some
somatic telomerase activity has been detected (Haussmann et al., 2007).
In common with other non-mammalian vertebrates, birds have nucleated
erythrocytes; thus, TLs derived from whole blood samples are mainly
measured in erythrocytes, which are normally produced in the bone
marrow. Compared to other tetrapods, avian erythrocytes have a
relatively short lifespan of 1 month with ~3% being
replaced each day (Glomski & Pica, 2016). Early-life erythrocyte TLs in
house sparrows have been estimated to 15-20 kbp (Ringsby et al., 2015),
which is thought to reflect TLs in hematopoietic stem cells (Vaziri et
al., 1994). If 50-100 bp of telomeric DNA are lost with each cell
division (Lansdorp, 1995), these early cells would have the potential of
150-400 divisions, many more than is needed for growth and maturation of
the adult house sparrow (Sidorov et al., 2009). However, increased
oxidative stress associated with acquiring and maintaining a larger body
size (Alonso-Alvarez et al., 2007) could accelerate the shortening of
telomeres significantly (Reichert & Stier, 2017) providing an
explanation for the observed negative association between size and TL
(see Fig. 1 and Fig. 5a).
The evolutionary significance of the observed changes in TL induced by
the artificial size selection will depend on the heritability of TL,
which has been shown to vary considerably among species and populations:
Among bird species, TL heritability have been shown to range from 0 to 1
(reviewed in Dugdale & Richardson, 2018), but may be relatively low in
house sparrows given the effects of growth and weather observed in this
study. We have refrained from estimating heritabilities of TL in the
present study, which would be biased by the non-random removal of
individuals during the artificial selection events (Steinsland et al.,
2014), but future studies may show whether the relationship between size
and TL is underpinned by genetic correlations (Monaghan & Ozanne,
2018).
Like most altricial passerines, the growth and survival of house sparrow
nestlings depend on early-life conditions such as habitat quality and
insect food being supplied by the parents (Anderson, 2006). Larger
sparrows have higher juvenile and adult survival (Ringsby et al., 1998;
Jensen et al., 2008), and harsh weather during the nestling period
increases juvenile mortality (Ringsby et al., 2002). The associations
between TL and both body size and the weather proxy (NAO_30) in
nestlings (Table 3) suggest that TL is determined by complex and
potentially counter-acting effects of growth, nutrition and external
factors (Angelier et al., 2015; Nettle et al., 2016). For instance,
malnutrition may lead to arrested growth, but also increased oxidative
stress and telomere attrition (Nettle et al., 2017). Also, indirect
effects of weather conditions may cause foraging stress or maternal
stress effects during breeding that negatively affect TL (Haussmann et
al., 2012; Mizutani et al. 2013), and direct effects of weather may
cause shortening of telomeres, such as thermal stress observed in e.g.
brown trout, Salmo trutta (Debes et al., 2016), dark‐eyed juncos,Junco hyemalis (Graham et al., 2019) and greater-eared bats,Myotis myotis (Foley et al., 2020). Thus, generally habitat
quality is important, with shorter telomeres in low-quality
habitats (Angelier et al., 2013;
Watson et al., 2015; Wilbourn et al., 2017). Spurgin et al. (2018) found
a positive effect of seasonal insect prey abundance on TL in Seychelles
warblers (Acrocephalus sechellensis ) when accounting for a
negative correlation with tarsus length. In the same population, the
amount of reactive oxygen metabolites in the territorial adult warblers,
was shown to be higher in low quality territories than in territories of
higher quality (van de Crommenacker et al., 2011), indicating that
oxidative stress exposure is involved in telomere shortening (von
Zglinicki, 2002). The regional NAO_30 index must be interpreted with
respect to local conditions along the northern Norwegian coast but might
be a better single proxy for the overall weather conditions by reducing
complexity and avoiding problems of model variable selection (Stenseth
et al. 2003; Hallett et al. 2004). Thus, a low NAO_30 index, which in
our study area corresponds to a combination of low temperatures, strong
winds and rainfall during a 30-day interval before TL sampling, was
found to significantly reduce TL in nestlings, when correcting for body
size (Fig. 5b). This is consistent with studies reporting shorter
telomeres because of poor nutrition, competition, or thermoregulation
(reviewed in Chatelain et al., 2020).
Natural selection against shorter telomeres may be driven by their
negative effect on immune function and longevity (Wilbourn et al. 2018)
or reduced cell replicative potential (Blackburn, 1991), while selection
against longer telomeres is thought to be due to the high energetic
costs associated with increased somatic maintenance (Eisenberg, 2011;
Vedder et al., 2017) or increased cancer susceptibility (Aviv et al.,
2017; Pepke & Eisenberg, 2020). Several ecological and epidemiological
studies have reported a negative association between TL and subsequent
mortality risk; mainly in birds (reviewed in Wilbourn et al., 2018) and
humans (reviewed in Boonekamp et al. 2013; Wang et al., 2018). This
association can be attributed to either the biomarker characteristic of
TL reflecting cumulative environmental stressors (Monaghan, 2014; Nettle
et al., 2017; Pepper et al., 2018; Angelier et al., 2018) or the direct
effect of having short telomeres leading to cellular senescence and
certain diseases (Blackburn et al., 2015; Young, 2018). However, this
correlation is not universal across tetrapods, with some studies finding
no correlation in birds (Boonekamp et al., 2014), mammals (Fairlie et
al., 2016), and reptiles (Olsson et al. 2011), or that shorter telomeres
correlate with higher survival in birds (Wood & Young, 2019), snakes
(Ujvari & Madsen, 2009), and fish (McLennan et al., 2017). Ringsby et
al. (2015) suggested that the changes in TL induced by the artificial
size selection could underpin a trade-off between body size and lifespan
if TL is related to lifespan (Heidinger et al., 2012). In this study, we
found little support for an effect of TL on short-term survival (i.e.
survival of juveniles until recruitment) after accounting for the
positive association between tarsus length and survival (Table 4). Body
size is likely to be an important component of juvenile mortality if the
mortality is mainly due to extrinsic factors (Wood & Young, 2019;
Eastwood et al., 2020), as expected in juvenile house sparrows (Ringsby
et al., 1998). The artificial selection increased the range of body
sizes across the populations, which may more clearly reveal effects of
TL on fitness. The evidence for individuals with either short or long
telomeres to have reduced mortality risk through life, controlling for
the negative effect of tarsus length, was weak (Fig. 6b). While some
correlative studies may have overlooked such weak disruptive selection
on TL, such patterns can be confounded by (unmeasured) telomerase
expression in somatic cells with high proliferation rates (Klapper et
al., 1998; Ujvari & Madsen, 2009; Cerchiara et al., 2017). However, if
TL is inversely related to telomere loss later in life (Verhulst et al.,
2013; Bauch et al., 2014), measuring TL at a later age may generate the
expected positive correlation between survival probability and TL (Wood
& Young, 2019). Alternatively, in individuals with short telomeres, TL
may be traded off against some unmeasured component of individual
quality (Wilson & Nussey, 2010). Yet, when controlling for lifespan,
short telomeres were associated with higher recruit production in thehigh population (Table 5). Telomere shortening rates in house
sparrows are unknown, but we found little evidence for directional
changes in TL across 5-17 days old nestlings (Table 2). The observation
in humans that short telomeres are associated with age-dependent
degenerative diseases and long telomeres with higher cancer incidence
rates (Aviv et al., 2017), suggests the opposite of our findings (i.e.
that both short and long TL is associated with higher mortality).
However, there is probably little or no constraints on TL imposed by
cancer or age-dependent diseases in free-living, short-lived sparrows
(Møller et al., 2017). Combined, the adaptive significance of telomere
length dynamics may be complex in wild populations with high juvenile
mortality and no individuals surviving into very old age (the oldest
house sparrow in this study survived until its 6thyear).
There was a weak negative effect of TL on reproductive success within
individuals that survived until breeding in the population in which
selection for larger size was imposed (high population, Table 5).
This might suggest that there are additional negative impacts on TL
associated with acquiring an artificially increased body size that
deviates from the optimal body size under the prevailing conditions.
Such impacts may act through increased competition when siblings are
larger (Nettle et al., 2016) and increased oxidative stress during
growth (Geiger et al., 2012). This indicates that in the highpopulation, high fitness birds were bigger and therefore had shorter
telomeres.
Telomeres were longer in male than in female house sparrows, also when
correcting for size (Table 3). We also note that males tended to have
higher LRS (Table 5), but only in the high population, where just
6 males managed to reproduce at least one recruit. There were no
sex-differences in survival in our study (Table S2.4), which has been
suggested to underlie sex-specific telomere dynamics in humans, mice,
and sand lizards (reviewed in Barrett & Richardson 2011). In similar
Norwegian house sparrow populations, Holand et al. (2016) did not find
any general sex-biased mortality or senescence patterns among adults.
However, Cleasby et al. (2010) found females to have lower juvenile
mortality than males in an English house sparrow population. In birds
and mammals, adult mortality appears to be biased towards the
heterogametic sex (Liker & Székely, 2005), which may be caused by the
potentially unmasked expression of deleterious sex-linked alleles
(Trivers, 1985; Hrdličková et al., 2012). In birds, females are the
heterogametic sex, but sexual differences in telomere dynamics have only
rarely been observed among bird species (but see Foote et al. 2011;
Bauch et al., 2020). However, unmeasured sex-specific differences in
growth dynamics (in house sparrows, Cleasby et al., 2011) or
differential telomere loss under parasite infection (in blue tits,Cyanistes caeruleus , Sudyka et al., 2019) could also generate the
observed TL sex differences.
Our study demonstrates the differential impacts of artificial body size
selection on early-life TL during the important growth phase. TL was
influenced by growth and weather and varied between sexes and
populations. Body size was an important determinant of survival, but
both short and long telomeres tended to predict lower mortality across
the populations after the range of body sizes had been artificially
increased. In individuals larger than their optimal size in the wild, TL
was reduced, which may have been associated with an increased
reproductive output. When selecting for smaller adult body size, we
observed a smaller response in fledgling size and TL, and no
relationship between TL and reproductive success. Thus, this study shows
that the relationship between body size and fitness is complex, with
larger body size giving rise to shorter TL. The fitness consequences of
this interaction are not simple, and our experimental results suggest
that evolution will optimize TL alongside phenotypic parameters.