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.