000 Contrasted Thermal Regimes Do Not Influence Digestion and Growth Rates in a Snake from a Temperate Climate * Corresponding author; e-mail: michel@cebc.cnrs.fr. Physiological and Biochemical Zoology 83(6):000–000. 2010.  2010 by The University of Chicago. All rights reserved. 1522-2152/2010/8306-0006$15.00. DOI: 10.1086/656050 Catherine Louise Michel* Xavier Bonnet Centre d’Etudes Biologiques de Chize ´, Centre National de la Recherche Scientifique, UPR 1934, Villiers en Bois 79360, Beauvoir sur Niort, France Accepted 6/29/2010; Electronically Published 10/22/2010 ABSTRACT Temperature influences almost all life-history traits. For a pe- riod of 3 mo, we placed four groups of snakes under four contrasted thermal treatments: (1) a natural regime (NR), based on daily variations (24-h cycle); (2) an accelerated regime (AR), where the thermoperiod fluctuated rapidly (12-h cycle); (3) a slow regime (SR; 48-h cycle); and (4) a cool stable regime (ZR; no fluctuation). The mean temperature, set at 23°C, was iden- tical for the four groups. For the first three groups (NR, AR, SR), ambient temperature fluctuated between 18°C and 28°C. Relative humidity and photoperiod were constant. We recorded feeding success, digestion efficiency, growth rate, activity, and ecdysis events. Differences between groups were expected be- cause of varied exposure to the optimal temperatures, most notably in the ZR group, where the preferred body temperature for digestion (∼30°C) would not be reached. Surprisingly, there was no significant effect of the experimental treatment on feed- ing rate, digestion, body mass increase, and growth rate. Our results do not conform to the paradigm stipulating that max- imal body temperature selected by ectotherms necessarily cor- responds to the most efficient for resource assimilation and that temperature fluctuations are essential. We propose that increasing the digestive tract’s performance through body- temperature elevation trades off against elevated (parasite) en- ergy expenditure from the rest of the body. The main advantage of high body temperatures would be to reduce the amount of time necessary to assimilate prey rather than to improve the net mass gain during digestion. Introduction In most habitats, environmental temperature fluctuates and thus poses challenges to organisms, notably in temperate and cool climates (Huey 1982; Lillywhite 1987; Peterson et al. 1993; Row and Blouin-Demers 2006). Indeed, optimal body tem- peratures are required to achieve high physiological perfor- mances (Blouin-Demers et al. 2000; Seebacher and Wilson 2007; Clark et al. 2008). Some lineages of ectothermic verte- brates essentially rely on behaviors to attain optimal body tem- peratures. They can select their body temperature when a wide gradient of environmental temperatures is available; this notion generated the term “preferred body temperature” (PBT), which represents a major paradigm of thermal ecology (Reynolds and Casterlin 1979; Huey and Bennett 1987; Angilletta 2001; Ed- wards and Blouin-Demers 2007). The PBT paradigm was orig- inally based on experimental studies showing that lizards and snakes spent most of their time in a relatively narrow range of temperatures when placed into thermal gradients (Reynolds and Casterlin 1979). The concept evolved, and currently the coadaptation hypothesis offers a more complex and realistic conceptual framework that notably considers a range of rela- tionships between PBTs, life-history traits, and phylogeny (Huey and Bennett 1987; Angilletta et al. 2006). However, the PBT paradigm integrated into the broader coadaptation hy- pothesis is still useful and widely used (Angilletta 2001; Edwards and Blouin-Demers 2007). For instance, temperature-sensitive neurons involved in the precise selection of preferred temper- ature in relation with optimal performances have been recently localized in the Drosophila brain, providing strong neurophys- iological support for the central role of PBT, at least in some insects (Hamada et al. 2008). Importantly, the PBT paradigm should be considered dif- ferently with respect to broad climatic conditions, taxonomic group, individuals, and physiological state. As expected, in trop- ical areas thermoregulation is easier operationally; for instance, most tropical reptiles are able to maintain high and stable T b (∼30°C) throughout the year without overt thermoregulatory activities. The PBT concept is often irrelevant for these animals (Shine and Madsen 1996; Luiselli and Akani 2002). On the contrary, in temperate or cold-climate areas, individuals reg- ularly forsake their refuge for basking in order to reach high body temperatures (Avery 1979). It is generally assumed that the maximal T b selected by reptiles, usually expressed through a plateau following a phase of rapidly increasing T b (e.g., during basking), also corresponds to the PBT of the organism (Dawson 1975; Jobling 1981; Bennett and Beitinger 1997; Blouin-Demers et al. 2000; Angilletta et al. 2002). Under cool ambient tem- peratures, major functions such as digestion, ecdysis, foraging,