Similar metabolic rate-temperature relationships after acclimation at constant and fluctuating temperatures in caterpillars of a sub-Antarctic moth
Graphical abstract
Introduction
Because most ectotherms are unable to use metabolically produced heat to regulate their body temperatures, they have developed strategies to cope with environmental temperature variation. Among the most controversial is temperature compensation or metabolic cold adaptation (Clarke, 2003). To compensate for persistent exposure to low temperature conditions, populations or species from low temperature environments are thought to maintain metabolic rates at low temperatures equivalent to those of their counterparts from warmer environments measured at higher temperatures (that is – they show complete or partial temperature compensation – Cossins and Bowler, 1987). Metabolic cold adaptation may also take the form of reduced sensitivity (i.e. lower slope) of the metabolic rate-temperature (MR-T) curve in species or populations from low temperature environments compared to their counterparts from warmer areas (Addo-Bediako et al., 2002). Reduced slopes mean that metabolic rates will not decline at low temperatures as much as they might have done had the slope of the MR-T been steeper. Maintenance of metabolic rate in low temperature conditions is thought to provide a fitness advantage because it enables growth and development to be completed (Sømme and Block, 1991). For insects, evidence for metabolic cold adaptation has been found in a range of studies, though contrary findings also exist (Chown and Gaston, 1999).
Alternatively, it has been suggested that, because maintenance metabolism represents a cost to organisms, little fitness benefit exists to metabolic cold adaptation (Clarke, 1991, Clarke, 1993, Clarke, 2003). In consequence, controversy has swirled about the theoretical basis for metabolic cold adaptation as well as the extent to which it is supported by empirical data across a range of organisms (e.g. Clarke, 1993, Chown and Gaston, 1999, Lardies et al., 2004, White et al., 2012, Gaitán-Espatia and Nespolo, 2014). For terrestrial ectotherms, notably insects, the suggestion has been made that differences in metabolic rate or rate-temperature curves among populations from low and high temperature environments might not always represent metabolic cold adaptation, but rather responses to warm, dry conditions. In this case, metabolic rate at high temperatures is reduced to decrease water loss from the respiratory system, or to conserve resources more generally (Clarke, 1993, Chown and Gaston, 1999, Terblanche et al., 2010, Schimpf et al., 2012).
Several factors may complicate interpretation of experiments designed to test the metabolic cold adaptation hypothesis. Among these, two are most significant. First, responses may differ seasonally because of the substantial differences in both day length and temperature cues experienced by individuals at different times during the growing season, which influence physiological rates (Gotthard, 2004). Second, many of the experiments that examine acclimation effects on rate-temperature curves use only constant temperatures in their acclimation treatments. By contrast, fluctuating temperatures are a characteristic feature of the natural environment, differing both in their extent and predictability (Angilletta et al., 2006, Deere and Chown, 2006, Folguera et al., 2009, Colinet et al., 2015, Marshall and Burgess, 2015). Performance curves and their components may take very different forms following exposures to constant versus fluctuating temperatures (Lalouette et al., 2011, Niehaus et al., 2012, Williams et al., 2012, Paaijmans et al., 2013, Colinet et al., 2015, Kingsolver et al., 2015). In consequence, recent overviews have argued cogently for the inclusion of fluctuating regimes in the investigation of the way in which physiological and other traits respond to temperature, especially in the context of the thermal environments experienced by the organisms (e.g. Angilletta et al., 2006, Martin and Huey, 2008, Ragland and Kingsolver, 2008, Dowd et al., 2015, Woods et al., in press). In the case of metabolic rate-temperature relationships, their overall form is typically associated with substantial increases in metabolism at high temperatures (Ruel and Ayres, 1999). Thus, in fluctuating environments, metabolic rate will be disproportionately higher during the warm periods than it is lower during the cool periods, leading to an elevation of the overall mean (Ruel and Ayres, 1999, Dowd et al., 2015). Ectotherms may thus respond to these relationships by altering the thermal sensitivity of metabolic rate, depending on the conditions they experience (Pásztor et al., 2000, Niehaus et al., 2011, Williams et al., 2012). Thus, thermal variability and its context and consequences are an essential component of any assessment of metabolic rate and rate-temperature variation, such as metabolic cold adaptation vs. water conservation.
Metabolic cold adaptation therefore needs to be explored not only in a species that occupies an environment where it might be evolutionarily advantageous, such as polar environments (Sømme and Block, 1991, Addo-Bediako et al., 2002), but using an experimental design that includes both constant and fluctuating temperatures, to account for the importance of thermal variability. Sub-Antarctic Marion Island is an especially appropriate setting for such work because it has low thermal seasonality, reducing seasonal differences in characteristics, and has been extensively characterized (Chown and Froneman, 2008). In particular, the dominant detritivore, and a keystone species in the island’s terrestrial ecosystems, is the larva of a flightless, tineid moth, Pringleophaga marioni Viette, which spends the majority of its life cycle as a caterpillar (Smith and Steenkamp, 1992, Haupt et al., 2014a), and has well-characterized thermal biology (Klok and Chown, 1997, Sinclair et al., 2004, Sinclair and Chown, 2005, Haupt et al., 2014a, Haupt et al., 2014b, Haupt et al., in press). Moreover, caterpillars are unlikely to be subject to substantial desiccation stress because of the perpetually moist litter and soil habitats they occupy, along with the consumption of moist detritus as a major food source (Crafford et al., 1986, Klok and Chown, 1997). In consequence, they have continual access to water, making water conservation unlikely a factor influencing metabolic rate (see also Klok and Chown, 1998).
Here we therefore explore temperature compensation in the metabolic rate-temperature relationship of P. marioni after acclimation at both constant and fluctuating temperatures representative of the thermal conditions of its habitat (Haupt et al., in press). We predict that sub-Antarctic species should show temperature compensation and reduced sensitivity of the rate-temperature relationship as a form of metabolic cold adaptation in response to low temperature exposures (Addo-Bediako et al., 2002). Because this species is negatively affected by prolonged exposure to even 15 °C (Haupt et al., 2014a), we expect that acclimation under fluctuating thermal regimes that approach or exceed this temperature should reduce the slope of the temperature-metabolic rate relationship to mitigate the negative metabolic consequences of these temperatures. Conversely, at low temperatures, reduced thermal sensitivity should allow the animal to maintain performance at lower temperatures (consistent with metabolic cold adaptation).
Section snippets
Study site and species collection
Marion Island (46°54′S, 37°45′E) is a small (ca. 290 km2, 1280 m maximum elevation), volcanic island which forms part of the Prince Edward Island group. The island’s environment is characterized by persistently low temperatures (mean annual ambient temperature of 6.5 °C), high precipitation (1900 mm total precipitation per year) (Chown and Froneman, 2008), and two major biomes: tundra in the low to mid elevations and polar desert at the highest elevations. P. marioni is found in most habitats
Results
Metabolic rate-temperature relationships were curvilinear in all cases, with similar forms (Fig. 1; Table 1). Constant acclimation temperatures of 5 °C, 10 °C and 15 °C had no effect on VCO2 in the initial ANCOVA analyses, whereas both test temperature and mass had a significant effect (Table 2). In addition, no interaction terms were significant, indicating no variation in slopes for VCO2. By contrast, the greater power of the orthogonal polynomial contrast analysis, incorporating the full data
Discussion
In the caterpillars of this sub-Antarctic moth, response to a short-term (7 day) acclimation treatment involving temperatures likely to be encountered in its microhabitats (Haupt et al., in press), metabolic rate (measured as VCO2) tended to be higher following acclimation to low temperature than to higher temperatures. Although statistically significant, the effect size was typically modest (3–14% on average). By contrast, no response in the slopes of the metabolic rate-temperature
Acknowledgments
Jennifer Lee, Justine Shaw, Asanda Phiri and Mashudu Mashau assisted with field work. We thank two anonymous reviewers for their helpful comments. This study was supported by National Research Foundation of South Africa Grant SNA14071475789 and the South African National Antarctic Programme.
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