DISCUSSION
In the last decade there has been an increasing interest in understanding the ecological and physiological pathways affected by extreme weather events, and their consequences for fitness (Thorntonet al. 2014). If there are critical windows in which telomere dynamics are affected in a way that has lifelong effects, the timing of these extreme weather events becomes very important. Our experimental increase in nest temperature caused no overall change in telomere length but does suggest that the pattern of consistent decline in telomere length is altered in the chicks in the heat treated nests. While telomeres of the control nestlings had a reduction, the experimental group showed no changes during the first 12 days of life. This is particularly important because diminished telomere shortening has been highlighted as predictor of increased lifespan (Haussmann et al . 2003) and may reflect lower exposure to damage from ROS. Therefore, our results match with our predictions and suggest that a warmer environment during post-natal growth may produce transient benefits in 12 day old ectothermic/poikilothermic nestlings.
The telomere dynamic we found between age 5 and 12 for heated nestlings is consistent with other information generated in the same experimental trial used in this study. Ton et al. (2021a) found no changes in metabolic rates or growth caused by heat, and detected a lower body temperature during the poikilothermic stage. The absence of acceleration in growth and metabolic rates associated with lower body temperature is expected to have no adverse consequences for stress and ROS production. Indeed an increase in coupling activity during proton leak as found in our previous study (Ton et al . 2021a) is hypothesized to reduce costs from ROS (Brand 2000) that may be triggered independently from variation in metabolic or growth rates; this could potentially prevent oxidative damage to telomeres. Therefore, the absence of detrimental treatment effects for telomeres that we detected here matches with theoretical predictions generated from previous tests on the same subjects (Ton et al. 2021a). Additionally, a recent study (in which telomeres were not measured) indicated that moderate warming above current temperature averages experienced during ontogeny and growth can be beneficial by increasing the survival of ectothermic organisms, (Liuet al. 2022). Conversely, other studies documented detrimental effects of heat waves (Zhang et al. 2018; Axelsson et al.2020), where lizards were exposed to stressful temperatures that exceeded the field average by 6 oC had shorter telomeres compared to those kept at cooler thermal conditions (Zhanget al. 2018). Our experiment involved much less severe heat treatment, since we kept birds within ranges that reflect current temperatures experienced in nature during their breeding season (Griffith et al. 2016).
The effects of warming for telomere dynamics during some part of nestling growth but not at other ages may be explained by the thermoregulatory context. Nestlings are ectothermic initially after hatching and gradually switch to endothermy (Sirsat et al. 2016), when they can reach operational temperatures approaching 42C (McKechnie & Wolf 2019). Our experiment suggests that bringing ambient temperatures as close as possible to the physiological optima of ectothermic nestlings (Friesen et al. 2021) provides some resistance to the causes of telomere attrition. Of course it is possible that other age-associated physiological changes may be involved, but these findings are consistent with a study that demonstrated telomere elongation in lizards kept at warmer temperatures (Fitzpatrick et al. 2019) and longer telomeres in ectothermic organisms in the heat (Burraco et al. 2020). This might also involve a higher expression of telomerase, as predicted for ectotherms growing in warmer environments (Olsson et al. 2018). Telomerase functions as a repair mechanism for telomeres and shows higher activity during the post-natal stage in ectothermic than in endothermic species (Olsson, Wapstra, & Friesen, 2018). Our within species test show that experimental heat waves reduced telomere attrition on chicks of zebra finch during their ectothermic stage. However, post-natal stages of songbirds that grow in enclosed or cavity nests are ectothermic only for 8-16 days in their life (Cheng & Martin 2012) and there may be disagreement on whether they should be considered as a true ectotherm during this short window. Still, the relationship between telomere dynamics and heat waves during the first days after hatch aligns with that found for true ectothermic species (Fitzpatrick et al. 2019; Burraco et al. 2020). This result is particularly meaningful for birds because, in contrast to other endothermic vertebrates, their embryos grow outside of the stable thermal environment of the mother. Indeed, early life stages of birds are subjected to considerable temperature fluctuations due to variation in parental care (Martinet al. 2007), and this variation can have important downstream consequences for development and metabolism (Ton & Martin 2017).
Here we also investigated the presence of complex strategies whereby parent birds may buffer offspring to a degree in early life. We found that our heat treatment significantly reduced brooding effort at the nest. This is not a novel finding (Mitchell et al. 2020) but it is important in the present context because a cross-fostering experiment found parental care to drive telomere dynamics independently of genetic background (Viblanc et al. 2020), and in that study, the role of brooding behavior as a potential causal agent was not tested. The lower rate of brooding we demonstrated for our heated nests have been suggested to reflect energy benefits for the parents (Yom-Tov & Hilborn 1981; Bryan & Bryant 1999), but tests during incubation show that, as temperature increases, these savings appear to be negligible (Tonet al. 2021b, Vleck 1981). Instead, lowering brooding at high temperatures may be a strategy aimed at buffering potential risks of thermal stress for the adults and for their offspring. However, we found shorter telomeres associated to lower brooding effort for the heated nestlings at age 12, which may reflect differences in parental quality and heat tolerance. As already stated, the apparent effect of brooding on telomere length that we detected needs to be interpreted with caution, but encourages future studies to elucidate if lowering brooding effort during extreme heat may yield higher chances of surviving and therefore future reproduction for the parents, while also reducing telomere length and future survival for the young.
Finally, we also found a negative correlation between telomere length and body mass independently of treatment and age, and this is important because both telomere and body size have been shown to be heritable (Jensen et al. 2003; Boonekamp et al. 2020), and may influence bird fitness (Ringsby et al. 2015). Similar results were previously generated in a selection experiment aimed at manipulating tarsus length which is related to structural size (Ringsbyet al. 2015), and in a descriptive study on the same species used here (Sheldon et al. 2021). Similar to the findings of another study (Boonekamp et al. 2020), variation in telomere length in our analysis did not appear to be associated with differences in rates of growth. This may be because in captivity with food available ad libitum, all chicks were growing at their optimal rate and did not experience the accelerated growth that is associated with higher telomere attrition (Monaghan & Ozanne 2018).
Our study highlights the complexity of the mechanisms underlying variation in telomere length by finding amelioration of telomere attrition caused by heat during the ectothermic stage of an endothermic species. This kind of effect has been found in other correlative and experimental studies (Pipoly et al. 2013; Liu et al. 2022) but the consequences for parental care and its potential contribution to the thermal environment and to telomere dynamics and outcomes for individuals over the life course is an avenue of research still in its infancy that deserves further investigation.