Maintenance energy costs, measured by metabolic rate, scale exponentially with temperature. In temperate climates, summers contain the highest potential maintenance costs. Many organisms have evolved plasticity to modify metabolic rate in response to seasonal cues (e.g. estivation); however as climate warms, hotter summers may increase energetic demand past levels that can be mitigated by plasticity. Demand will also vary by latitude with southern summers experiencing the hottest conditions. Plethodontid salamanders are particularly vulnerable to hot summers because warm conditions often force salamanders to subterranean refugia where they are unable to forage—simultaneously reducing energy acquisition and increasing maintenance costs. We use Plethodon cinereus, a wide-ranging terrestrial salamander to determine whether this taxon can 1) modify metabolic rates based on seasonal thermal cues, 2) modify rates to future climate conditions, and 3) exhibit local adaptation in plastic responses. We collected 36 individuals from each of four populations that span 6.6C in summer mean-low temperatures (Virginia, Pennsylvania, New York, Massachusetts). We estimated thermal sensitivity in metabolic rate across four temperature regimes: home spring, home summer, year 2100 summer temperatures (+4C), and a home vs. away common garden temperature (19.7C).
Results/Conclusions
We conducted 1724 individual metabolic trials during 2017. Populations responded to changes in thermal regime, with VA lowering metabolic rate from spring to summer (67% kcal savings), PA exhibiting no seasonal plasticity, and NY and MA increasing metabolic rate from spring to summer (-32% kcal savings). Despite seasonal regulation of metabolism, there were no significant differences between summer and +4C regimes. Differences in downregulation (VA) and upregulation (NY, MA) suggest responses contextualized to local climate. Our warmest population is suppressing metabolism during the warmest time of year, likely to minimize summer maintenance costs (i.e. the compensation hypothesis). In cooler climates, cold temperature limit activity, so summer temperatures increase metabolism to prime salamanders for surface activity (i.e. the intake hypothesis). Cumulatively, our findings indicate there are constraints on the degree to which salamanders can plastically respond to thermal cues. Additionally, our findings suggest that southern populations will likely be the hardest hit by increased summer temperatures because they may not be able to lower their rates further. This suggests higher maintenance costs are probable for this species despite metabolic plasticity. If allocation to maintenance increases, reduced allocation for growth and reproduction will likely impact demographic vital rates, threatening population persistence.