1 INTRODUCTION

Phenotypic plasticity is key to persisting in novel conditions. Plasticity in behavioural, physiological and morphological phenotypes can increase survival and mitigate the negative effects of stressful environments (Dingemanse & Wolf, 2013; Kayes et al., 2009; Seebacher et al., 2015). With anthropogenic climate change posing the biggest threat to global biodiversity (Bellard et al., 2012), the role of trait plasticity in buffering animals against novel thermal environments has received increasing attention (Bodensteiner et al., 2021; Pottier et al., 2022). At present, this attention has focused primarily on the impact of alterations to mean diurnal temperatures, which has left a major gap in our understanding of the role of nighttime temperatures and thermal fluctuations in driving plasticity.

Terrestrial ectotherms are predicted to be some of the most adversely impacted taxa due to their biological dependence on temperature regimes (Deutsch et al., 2008). Global patterns in fisheries (Roessig et al., 2004), amphibian biodiversity (Pounds et al., 2006) and lizard local extinction (Sinervo et al., 2010) indicate that the impacts of climate change are already being felt. A recent global analysis of reptiles predicted that 16%–30% of the 538 species examined will go extinct by 2070 if climate change continues on current trajectories (Román-Palacios & Wiens, 2020). While responses to changing conditions will be both taxon and context dependent, understanding fundamental patterns between temperature and phenotypes is core in understanding responses to changes in climate.

Variable thermal regimes may be particularly relevant at the diel level, as temperature declines at night compared with day are the most predictable, ubiquitous form of variation (Kefford et al., 2022). Under climatic warming, evening temperatures are predicted to increase approximately 1.4× faster than daytime temperatures (Sadok & Jagadish, 2020; Sillmann et al., 2013). This differential heating rate has raised concern in the agricultural sector, as declines in crop yield are linked to increased respiration resulting from higher night-time temperatures (Lesjak & Calderini, 2017; Sadok & Jagadish, 2020; Zhu et al., 2020). Such findings are as concerning for ectotherms, as like plants most diurnal species have limited thermoregulatory capacity during night-time periods. Thus, increased temperatures could have strong fitness implications for ectotherms.

Within animal biology, few studies have investigated the relationship of physiology and night-time thermal quiescence for diurnal animals. Studies that that have decoupled night-time temperature effects from day-time thermoregulatory opportunity have shown changes in evening temperature have strong effects on life history strategy, reproduction and possibly fitness (Brusch et al., 2020; Clarke & Zani, 2012; Rutschmann et al., 2021). Such findings are not unsurprising given the important role that periods of thermal rest have for ectotherms, and the limited thermoregulatory opportunities available at night (Nordberg & Schwarzkopf, 2019). Thus, rest period refuge selection in ectotherms is often thermally mediated (Huey et al., 1989). Hotter environmental conditions force ectotherms to select cooler refuges (Huey et al., 1989; Kearney, 2002), while under thermally restrictive environments, animals may select hotter or more exposed sites to gain thermoregulatory opportunity (Huey et al., 1989; Rowe et al., 2020; Stark et al., 2023). Voluntary hypothermia by lizards in the evening is often posited to minimize costly and unnecessary metabolic expenditure (Flesch et al., 2017; Kidman et al., 2023; Nordberg & McKnight, 2023; Regal, 1967). For example, decreased evening temperatures of free ranging bluetongue lizards (Tiliqua scincoides) reduced energy expenditure by 14% due to lower evening body temperatures (Christian et al., 2003). Similarly, periods of thermal rest afforded by evening temperature drops likely provide important periods of cellular recovery (Kefford et al., 2022). However, while thermal factors often drive retreat selection, animals must balance thermal benefits against biotic factors such as site competition (Yang et al., 2012) or predation risk (Amo et al., 2007). Evidently, diurnal animals expend considerable effort in selecting an evening retreat, likely to optimize their temperature-sensitive physiologies while avoiding potentially costly movement during evening (though see opportunistic evening activity; Gordon et al., 2010a, 2010b).

Potentially, reversible thermal plasticity (hereafter, acclimation) can mitigate the impacts of warming nighttime temperatures in the same way it is thought to help animals accommodate natural, short-term fluctuations in air temperatures (e.g. acclimation as an adaptation for variable thermal environments; Angilletta, 2009). Across field and lab studies, it is apparent that thermal acclimation in most fitness-relevant traits exists, including sprint speed (Kubisch et al., 2016) and enzyme function (Guderley & Seebacher, 2011). However, integrating findings from the field and lab is difficult given the inherent differences in study design. Laboratory studies of acclimation commonly employ constant temperature regimes compared to more complex thermal semi-natural regimes (Colinet et al., 2015; Crickenberger et al., 2020; Sørensen et al., 2016). Alternatively, manipulations of daytime duration of thermoregulatory opportunities focus on the overall time at preferred temperatures versus in thermal quiescence (Anderson et al., 2023; Caldwell et al., 2017; Schwanz, 2016; Sun et al., 2022). Importantly, thermal fluctuations in invertebrates have been shown to increase performance measures if test temperatures are within a tolerable range (Colinet et al., 2015; Overgaard & Sørensen, 2008; Sørensen et al., 2020). Similarly, fluctuations that allow animals reprieve from stressful thermal extremes are crucial periods of cellular recovery (Colinet et al., 2015). Within vertebrates, American alligators housed in constant temperature treatments have slower growth and altered biochemical activity relative to those that are housed in fluctuating temperatures (Price et al., 2017). While many vertebrate studies have investigated the effects of variable temperatures on embryonic development and developmental plasticity (Raynal et al., 2022; Zhang et al., 2023), as well as manipulated basking availability (Aubret & Shine, 2010; Caldwell et al., 2017; Halstead & Schwanz, 2015; Heathcote et al., 2014; Schwanz, 2016; Wapstra et al., 2010), few studies have considered the role of variable temperatures, particularly nighttime reprieves, on reversible acclimation of performance physiology (Anderson et al., 2023; Sun et al., 2022).

Our experiment aimed to examine the role that daily temperature variation has on thermal and performance phenotypes in an Australian lizard, the Jacky lizard (Amphibolurus muricatus). In this study, we acclimated jacky lizards under a Hot Constant (35°C), Cold Constant (20°C) and a 12-h Alternating (35°C/20°C) treatment for 30 days and measured acclimation responses in their metabolic rate, sprinting performance, thermal preferences and thermal limits. We predicted that, following patterns observed in the literature (Kubisch et al., 2023), acclimation to temperature extremes (Hot Constant and Cold Constant) will cause animals to adopt different thermal phenotypes. Hot Constant animals are predicted to have greater sprint performance and hotter optima, as well as higher thermal limits, but lower preferences and metabolic optima compared to their Cold Constant counterparts. We predict that Alternating animals will adopt a phenotype intermediate of the two extremes. Sustained exposure of animals to preferred temperatures in the Hot Constant treatment will result in unnecessary energy expenditure, and decreased body condition (mass scaled to size).

中文翻译：

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两全其美：适应波动的环境可以带来优势，并最大限度地降低恒定环境的成本

1 简介

表型可塑性是在新条件下持续存在的关键。行为、生理和形态表型的可塑性可以提高生存率并减轻压力环境的负面影响（Dingemanse & Wolf,  2013；Kayes et al.,  2009；Seebacher et al.,  2015）。随着人为气候变化对全球生物多样性构成最大威胁（Bellard 等，  2012），性状可塑性在缓冲动物免受新热环境影响方面的作用受到越来越多的关注（Bodensteiner 等，  2021；Pottier 等，  2022 ） ）。目前，人们的注意力主要集中在平均昼夜温度变化的影响上，这在我们对夜间温度和热波动在驱动可塑性中的作用的理解上留下了重大空白。

由于陆地变温动物对温度状况的生物依赖，预计它们将成为受影响最严重的类群之一（Deutsch 等，  2008）。全球渔业模式（Roessig 等人，  2004 年）、两栖动物生物多样性（Pounds 等人，  2006 年）和蜥蜴局部灭绝（Sinervo 等人，  2010 年）表明，人们已经感受到了气候变化的影响。最近对全球爬行动物进行的一项分析预测，如果气候变化继续按照目前的轨迹发展，所研究的 538 个物种中的 16%–30% 将在 2070 年灭绝（Román-Palacios & Wiens，  2020）。虽然对不断变化的条件的反应将取决于分类单元和环境，但了解温度和表型之间的基本模式是理解对气候变化的反应的核心。

可变的热状态可能与昼夜水平特别相关，因为与白天相比，夜间温度下降是最可预测、最普遍的变化形式（Kefford 等人，  2022）。在气候变暖的情况下，预计夜间气温的升高速度比白天快约 1.4 倍（Sadok & Jagadish，  2020；Sillmann 等人，  2013）。这种加热速率的差异引起了农业部门的关注，因为作物产量下降与夜间气温升高导致呼吸作用增加有关（Lesjak & Calderini,  2017；Sadok & Jagadish,  2020；Zhu et al.,  2020）。这些发现与变温动物有关，就像植物一样，大多数昼夜物种在夜间的温度调节能力有限。因此，温度升高可能对变温动物的健康产生强烈影响。

在动物生物学中，很少有研究调查昼间动物的生理学和夜间热静止的关系。将夜间温度影响与白天体温调节机会脱钩的研究表明，夜间温度的变化对生活史策略、繁殖和可能的适应性有很大影响（Brusch 等人，  2020 年；Clarke 和 Zani，  2012 年；Rutschmann 等人）等，  2021）。考虑到热休息期对变温动物的重要作用，以及夜间可用的体温调节机会有限，这样的发现并不奇怪（Nordberg & Schwarzkopf，  2019）。因此，变温动物的休息期避难所选择通常是由热介导的（Huey et al.,  1989）。较热的环境条件迫使变温动物选择较凉爽的避难所（Huey et al.,  1989；Kearney,  2002），而在热限制环境下，动物可能会选择较热或更暴露的地点以获得体温调节机会（Huey et al.,  1989；Rowe et al., 1989）。等人，  2020；斯塔克等人，  2023）。蜥蜴在晚上的自愿低温通常被认为是为了最大限度地减少昂贵和不必要的代谢支出（Flesch 等人，  2017 年；Kidman 等人，  2023 年；Nordberg & McKnight，  2023 年；Regal，  1967 年）。例如，自由放养的蓝舌蜥蜴 ( Tiliqua scincoides ) 夜间温度降低，由于夜间体温降低，能量消耗减少了 14%（Christian 等，  2003）。同样，夜间气温下降带来的热休息期可能提供细胞恢复的重要时期（Kefford 等人，  2022）。然而，虽然热因素通常会驱动撤退选择，但动物必须平衡热效益与生物因素，例如场地竞争（Yang 等，  2012）或捕食风险（Amo 等，  2007）。显然，昼行动物在选择晚间休养地方面付出了相当大的努力，可能会优化其对温度敏感的生理机能，同时避免在夜间进行可能代价高昂的运动（尽管参见机会性晚间活动；Gordon et al.,  2010a , 2010b）。

潜在的，可逆的热可塑性（以下称为驯化）可以减轻夜间气温变暖的影响，就像人们认为可以帮助动物适应自然的、短期的气温波动一样（例如，驯化作为对可变热环境的适应；Angilletta ，  2009）。在现场和实验室研究中，很明显，大多数与健身相关的特征都存在热适应，包括冲刺速度（K​​ubisch 等人，  2016）和酶功能（Guderley 和 Seebacher，  2011）。然而，鉴于研究设计的固有差异，整合现场和实验室的研究结果很困难。与更复杂的热半自然状态相比，驯化的实验室研究通常采用恒温状态（Colinet 等人，  2015；Crickenberger 等人，  2020；Sørensen 等人，  2016）。或者，对体温调节机会的白天持续时间的控制侧重于首选温度与热静止状态下的总时间（Anderson 等人，  2023；Caldwell 等人，  2017；Schwanz，  2016；Sun 等人，  2022）。重要的是，如果测试温度在可容忍的范围内，无脊椎动物的热波动已被证明可以提高性能指标（Colinet 等人，  2015；Overgaard & Sørensen，  2008；Sørensen 等人，  2020）。同样，让动物从极端高温压力中解脱出来的波动是细胞恢复的关键时期（Colinet et al.,  2015）。在脊椎动物中，相对于在波动温度下饲养的美洲短吻鳄，在恒温处理下饲养的美洲短吻鳄生长速度较慢，生化活性也发生了改变（Price 等，  2017）。虽然许多脊椎动物研究已经调查了可变温度对胚胎发育和发育可塑性的影响（Raynal 等人，  2022 年；Zhang 等人，  2023 年），以及操纵晒太阳的可用性（Aubret 和 Shine，  2010 年；Caldwell 等人，2010 年）。 ，  2017；Halstead & Schwanz，  2015；Heathcote 等人，  2014；Schwanz，2016；Wapstra 等人，  2010），很少有研究考虑可变温度，特别是夜间缓和，对性能生理学可逆适应的作用（Anderson等人，  2023；Sun 等人，  2022）。

我们的实验旨在研究每日温度变化对澳大利亚蜥蜴 Jacky 蜥蜴（Amphibolurus muricatus）的热和性能表型的影响。在这项研究中，我们让 Jacky 蜥蜴在高温恒定 (35°C)、低温恒定 (20°C) 和 12 小时交替 (35°C/20°C) 处理下适应 30 天，并测量它们的适应反应。代谢率、短跑表现、热偏好和热极限。我们预测，根据文献中观察到的模式（Kubisch 等人，  2023），对极端温度（高温常数和低温常数）的适应将导致动物采用不同的热表型。与冷恒定动物相比，预计热恒定动物具有更好的冲刺性能和更热的最佳状态，以及更高的热极限，但偏好和代谢最佳状态较低。我们预测交替动物将采用两个极端的中间表型。在高温恒定治疗中，动物持续暴露于首选温度将导致不必要的能量消耗，并降低身体状况（质量按尺寸缩放）。