Abstract
Environmental stress induced by natural and anthropogenic processes including climate change may threaten the productivity of species and persistence of populations. Ectotherms can potentially cope with stressful conditions such as extremes in temperature by exhibiting physiological plasticity. Amphibian larvae experiencing stressful environments display altered thyroid hormone (TH) status with potential implications for physiological traits and acclimation capacity. We investigated how developmental temperature (Tdev) and altered TH levels (simulating proximate effects of environmental stress) influence the standard metabolic rate (SMR), body condition (BC), and thermal tolerance in metamorphic and post-metamorphic anuran larvae of the common frog (Rana temporaria) reared at five constant temperatures (14–28 °C). At metamorphosis, larvae that developed at higher temperatures had higher maximum thermal limits but narrower ranges in thermal tolerance. Mean CTmax was 37.63 °C ± 0.14 (low TH), 36.49 °C ± 0.31 (control), and 36.43 °C ± 0.68 (high TH) in larvae acclimated to different temperatures. Larvae were able to acclimate to higher Tdev by adjusting their thermal tolerance, but not their SMR, and this effect was not impaired by altered TH levels. BC was reduced by 80% (metamorphic) and by 85% (post-metamorphic) at highest Tdev. The effect of stressful larval conditions (i.e., different developmental temperatures and, to some extent, altered TH levels) on SMR and particularly on BC at the onset of metamorphosis was carried over to froglets at the end of metamorphic climax. This has far reaching consequences, since body condition at metamorphosis is known to determine metamorphic success and, thus, is indirectly linked to individual fitness in later life stages.
Highlights
Climate change has profound and diverse effects on organisms and is altering aquatic and terrestrial systems worldwide (IPCC 2014; Deutsch et al 2015)
standard metabolic rate (SMR) increased in all treatments except of animals which were reared at 14 °C at low thyroid hormone (TH) level and for those which were reared at 28 °C and high TH levels
Our results suggest that both larvae and froglets 7 days after completion of metamorphosis of R. temporaria are not able to acclimate to higher developmental temperatures by adjusting their maintenance energy costs (i.e., SMR) and that this capacity is impaired by the interaction between higher temperature and altered TH status to some extent as often elicited by environmental stressors
Summary
Climate change has profound and diverse effects on organisms and is altering aquatic and terrestrial systems worldwide (IPCC 2014; Deutsch et al 2015). Journal of Comparative Physiology B (2020) 190:297–315 including heat hardening, thermal compensation, and acclimation (Bullock 1955; Prosser 1955; Hazel and Prosser 1974; Huey et al 1999; Angilletta et al 2006). These different forms of thermally induced plasticity allow individuals to potentially acclimate to the complex temporal or spatial heterogeneity in environmental temperatures (Angilletta 2009; Angilletta et al 2010; Jessop et al 2018) and increase resilience of ectothermic animals to climate change (Seebacher et al 2015; Little and Seebacher 2016; Lillywhite 2016). A major challenge that ectotherms face in variable thermal environments is the maintenance of appropriate energy metabolism (e.g., the standard metabolic rate, SMR) (Angilletta et al 2002; Little and Seebacher 2016) which is determined by measuring rates of O2 consumption at rest and represents the energy required to cover minimum physiological functions (Rowe et al 1998; Beck and Congdon 2003)
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