Variation in winter metabolic reduction between sympatric amphibians Luděk Podhajský, Lumír Gvoždík ⁎ Institute of Vertebrate Biology CAS, Květná 8, 60365 Brno, Czech Republic abstract article info Article history: Received 31 May 2016 Received in revised form 28 June 2016 Accepted 6 July 2016 Available online 12 July 2016 Distribution and abundance of temperate ectotherms is determined, in part, by the depletion of their limited caloric reserves during wintering. The magnitude of winter energy drain depends on the species-specific capacity to seasonally modify the minimal maintenance costs. We examined seasonal variation of minimum oxygen consumption between two newt species, Ichthyosaura alpestris and Lissotriton vulgaris. Oxygen consumption was measured in both species during their active season (daily temperature range = 12–22 °C) and wintering period (4 °C) at 4 °C and 8 °C. The seasonal reduction in metabolic rates differed between species and experimen- tal temperatures. Wintering newts reduced their metabolic rates at 4 °C and 8 °C in I. alpestris, but only at 8 °C in L. vulgaris. Both species reduced the thermal sensitivity of oxygen consumption during wintering. Theoretical calculations of winter depletion of caloric reserves under various thermal conditions revealed that seasonal metabolic reduction is more effective in I. alpestris than in L. vulgaris, and its effectiveness will increase with the proportion of warmer days during wintering period. The variation in winter metabolic reduction between sympatric newt species potentially contributes to their distribution patterns and population dynamics under climate change. © 2016 Elsevier Inc. All rights reserved. Keywords: Caloric reserves Ichthyosaura Lissotriton Metabolic rate Newt Oxygen consumption Respirometry Salamander Thermal sensitivity Wintering 1. Introduction In temperate ectotherms, a species' occurrence in a given habitat depends on thermal conditions not only during their active season but also during wintering. Ectotherms cope with low temperatures using behavioral and physiological mechanisms (Storey and Storey, 1988; Ultsch, 1989; Tattersall and Boutilier, 1997, 1999; Tattersall and Ultsch, 2008; Costanzo and Lee, 2013; Sinclair, 2015). The most im- portant behavioral response to winter conditions is temperature- dependent microhabitat selection, whereas physiological mechanisms involve freeze thermal tolerance, thermal dependence of metabolic rates, and its seasonal reduction (suppression) patterns. Projections of winter climate change predict the increase in both mean and variation in air temperature (IPCC, 2013), which may accelerate the energy con- sumption of wintering individuals (Williams et al., 2015), and thereby affecting their spring body condition and reproductive success (Irwin and Lee, 2000; Reading, 2007; Bosch et al., 2010; Kristín and Gvoždík, 2014a). Hence, the capacity to modify thermal dependence of metabolic rates during wintering will play an increasingly important role in ther- mal requirements of temperate ectotherms. Winter metabolic reduction may take two forms (Sinclair, 2015). First, metabolic rates decrease equally across temperatures. Because the winter metabolic rate is an individually repeatable trait (Kristín and Gvoždík, 2014a), the magnitude of this shift determines overall energy consumption in wintering individuals. Second, the thermal sen- sitivity shift increases or decreases metabolic rates, depending on body temperature. The increased thermal sensitivity should be advantageous at low temperatures but thermal variation results in higher energy costs at elevated temperatures. Accordingly, less thermally sensitive metabolic rates during overwintering should be favored under thermally variable conditions. Although depressed winter metabolism has been studied for more than a century (Bohr, 1900; Krogh, 1904), both forms of winter metabolic reduction have received limited attention in ecto- thermic terrestrial vertebrates. We examined variation in the winter metabolic reduction in sympatric alpine newts (Ichthyosaura alpestris) and smooth newts (Lissotriton vulgaris). Distribution areas of both species largely overlap in Europe. However, I. alpestris primarily inhabits forest areas, while L. vulgaris occurs in a variety of landscapes (Griffiths, 1996). Forests generally provide milder and less variable thermal conditions than open habitats (Morecroft et al., 1998), which is consistent with newt thermal requirements and thermoregulatory efforts during the active season (Balogová and Gvoždík, 2015). Accordingly, we investigated if both species vary in their winter metabolic reduction, which may contribute to their unequal habitat breadths. Following predictions mentioned above, the habitat generalist, L. vulgaris, should have less thermally sensitive metabolic rates than I. alpestris in winter. In turn, the prolonged newt wintering period in a forest habitat (L. Gvoždík, Comparative Biochemistry and Physiology, Part A 201 (2016) 110–114 ⁎ Corresponding author at: Institute of Vertebrate Biology CAS, Research Facility “Studenec”, Studenec 122, 67502 Koněšín, Czech Republic. E-mail address: gvozdik@brno.cas.cz (L. Gvoždík). Contents lists available at ScienceDirect Comparative Biochemistry and Physiology, Part A journal homepage: www.elsevier.com/locate/cbpa http://dx.doi.org/10.1016/j.cbpa.2016.07.003 1095-6433/© 2016 Elsevier Inc. All rights reserved.