INTRODUCTION
Global warming is predicted to increase average ambient temperature as
well as the frequency, intensity and duration of extreme weather events
such as heat waves (Pachauri et al. 2014; Meehl & Tebaldi 2004).
While some organisms may plastically adjust or adapt to rising
temperatures over time (Somero 2010; Franks & Hoffmann 2012), heat
waves are thought to be harder to buffer and may represent an immediate
threat to survival (McKechnie & Wolf 2010; Stillman 2019). Reduced
survival can be caused by high temperatures acting on extrinsic factors
such as food, water availability (Donelson et al. 2010; Fulleret al. 2014), or predation (Lowe et al. 2021), as well as
on intrinsic mechanisms like metabolic and endocrine pathways (Fulleret al. 2020), stress responses (Telemeco & Addis 2014), and the
production of reactive oxygen species (ROS) (Paital et al. 2016).
These physiological modifications can be detrimental and may influence
important processes linked to biological age like rates of DNA
methylation (Sheldon et al. 2020), and telomere attrition
(Whittemore et al. 2019). Telomeres are protective structures
found at the end of chromosomes that can be affected by high ambient
temperatures (Fitzpatrick et al. 2019). Reduced telomere length
and increased attrition are associated with lower survival within and
across species (Haussmann et al. 2005; Wilbourn et al.2018; Tricola et al. 2018) independently from age (Bize et
al. 2010). Therefore, heat waves may impact telomere dynamics and
fitness (Zhang et al. 2018), but experimental studies testing
this possibility are limited and the magnitude and direction of results
appear to vary with temperature (Liu et al. 2022).
Temperature effects on telomeres are known to be especially important
during the sensitive early life stages of growth and development
(Metcalfe & Alonso Alvarez 2010; Entringer et al. 2011;
Boonekamp et al. 2014; Atema et al. 2015; Monaghan & Ozanne
2018; Eastwood et al. 2019) and can have very marked effects in
ectothermic organisms (Burraco et al. 2020; Dezetter et
al. 2022; Liu et al. 2022). Heat waves may increase somatic
growth and metabolic activity in ectotherms leading to higher ROS
production (Stahlschmidt et al. 2017) and associated damage with
detrimental consequences for telomere dynamics (Halliwell & Gutteridge
2015; Burraco et al. 2020). Alternatively, telomere length in
ectotherms could benefit from warmer temperatures if these push
organismal physiology closer to the optimal thermal conditions for
development (Friesen et al. 2021). Growth in endotherms is typically
influenced less by ambient temperature (Reichert & Stier 2017; Olssonet al. 2018), but telomeres may be shorter and show higher
attrition in response to increased growth and organismal stress due to
deflection from homeostasis depending on species-specific levels of heat
tolerance (Monaghan 2014; Simide et al. 2016). Interspecific
variation between ectotherms and endotherms in the effects of
temperature-induced stress on telomere dynamics might also be rooted in
other aspects of the evolutionary history of the species. Our current
understanding of this interspecific variation does not allow us to draw
conclusions on the consequences of heat waves for telomere dynamics in
the ectothermic and endothermic stages of an organism that encompasses
both thermoregulatory regimes. A test performed within a species would
therefore be very informative to gather critical evidence about the
presence of developmental intervals with different sensitivity to
physiological and molecular effects of environmental temperature on
phenotypic development. This information is particularly important to
acquire because embryos and offspring of many species are unable to
produce heat and therefore physiologically ectothermic for a period of
their existence, independently of whether they ultimately become
endothermic as adult.
Altricial birds are particularly interesting in this regard as they
transition from being ectothermic to poikilothermic to endothermic
within a few days of hatching (Sirsat et al. 2016). This
transition occurs during a period when the temperature exposure of the
chicks is influenced both by the external conditions and by the amount
of heat provided by brooding adults. Parental care is known to be
important for telomere dynamics (Blaze et al. 2015, Viblancet al. 2020, Quque et al. 2021), and parental brooding
effort in songbirds is responsive to environmental temperature during
the incubation and the nestling stage (Ardia et al. 2009; Coeet al. 2015; Mitchell et al. 2020). This means that
variation in adult brooding behavior and how this is influenced by
environmental temperature will have downstream effects on offspring
telomere length during these early stages of development. Therefore,
nestlings of altricial species provide unique opportunities to test the
effects of heat waves on telomere dynamics at different thermoregulatory
capabilities within the context of possible additional influences from
parental care behavior.
To date our understanding of parental care and ambient temperature in
explaining post-natal telomere dynamics at different thermoregulatory
stages is limited. Descriptive studies of wild zebra finchesTaeniopygia castanotis , exposed to natural variation in ambient
conditions have revealed positive correlations between ambient
temperatures and another biomarker of aging, the DNA methylation rate
(Sheldon et al. 2020), and of the latter with rates of telomere
shortening (Sheldon et al. 2021). These correlative results
suggest that thermal conditions during early life influence telomeres,
but since temperature was not manipulated in that study (Sheldonet al 2021), other factors may affect the results such as
seasonal variation in reproductive investment, parental condition, or
food availability. In an earlier study focused on the same experimental
offspring, we demonstrated that heat waves had no effect on growth or
whole body metabolic and hygric physiology of zebra finch at different
ages (Ton et al. 2021a). We also previously found that heated
nestlings had lower body temperature during the poikilothermic stage
(day 12), which may help in reducing heat stress and telomere loss by
keeping the organism further from thermal upper limits. However, these
same birds showed increased mitochondrial metabolism for leak
respiration as endothermic adults (Ton et al. 2021a). On one hand leak
respiration is the process responsible for heat production and its
increase requires higher heat dissipation with a greater risk of heat
stress and telomere attrition. On the other hand, higher leak
respiration is also known to help buffering damage from ROS, a major
mechanism underpinning telomere erosion and reduced intrinsic survival
(Halliwell & Gutteridge 2015). When rate of uncoupling increases, the
amount of ROS is expected to decrease but more heat is released. So, if
the organism experiences high environmental temperatures, this might
change the outcome of the trade-off between mitochondrial efficiency and
exposure to ROS damage. Therefore, the physiological adjustments
triggered by heat in our previous experiment may be beneficial for
telomeres as predicated under the uncoupling to survive
hypothesis (Brand 2000), or detrimental depending on the
thermoregulatory conditions. While telomeres were not considered in the
aforementioned studies, those findings (Sheldon et al 2020,
Sheldon et al 2021, Ton et al 2021a) provide testable predictions
about the likely effects of the experimental treatment on telomere
length.
To investigate the effect of environmental temperature during early
development on telomere dynamics, we exposed newly hatched zebra finches
to experimental heat waves within the nest chamber, and measured
telomeres at three ages (day 5, 12 and 80 after hatch) that provide
samples from each of the three different thermoregulatory states
(ectothermic, poikilothermic and endothermic) respectively. We expected
heat waves to cause higher attrition and shorter telomeres during the
endothermic stage due to physiological conditions being closer to the
threshold for heat stress. Conversely, we expect telomere loss to be
lower during the ectothermic and poikilothermic parts of growth when
offspring produce minimal heat and the thermal balance of the brood can
be buffered by changes in parental care. Therefore, we also videotaped
adult activity at the nest during the ectothermic stage to understand
the effects of heat waves on brooding rates, and the possible downstream
consequences for telomeres.