Contrasted Thermal Regimes Do Not Influence Digestion and Growth Rates in a Snake from a Temperate Climate

Abstract

Temperature influences almost all life-history traits. For a period 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 identical 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 because of varied exposure to the optimal temperatures, most notably in the ZR group, where the preferred body temperature for digestion (approximately 30°C) would not be reached. Surprisingly, there was no significant effect of the experimental treatment on feeding rate, digestion, body mass increase, and growth rate. Our results do not conform to the paradigm stipulating that maximal body temperature selected by ectotherms necessarily corresponds 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) energy 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.

Shelter availability, stress level, and digestive performances in the aspic viper

“…However, the negative value of crude assimilation rate recorded in the snakes that regurgitated (−8%) was greater than expected, as unfed (i.e. starving) snakes have been shown to display only -0.5% of body mass loss under similar conditions (Michel and Bonnet, 2010). This revealed a new stress-induced cost, possibly as a result of the loss of body fluids and/or the energy demand to initiate digestion (McCue, 2006).…”

“…Previous studies provided important background (Saint Girons, 1957;Bonnet et al, 1999;Bonnet et al, 2000;Bonnet et al, 2001;Bonnet et al, 2002;Bonnet, 2011;Ladyman et al, 2003;Zuffi et al, 2009;Michel and Bonnet, 2010). Aspic vipers [Vipera aspis (Linnaeus 1758)] feed mainly on rodents and digestion is impeded when body temperature falls below 15°C (Naulleau, 1983).…”

“…Experiment 2 provided a sufficient amount of time (16days) to monitor digestion: the production of feces requires 1week on average (Michel and Bonnet, 2010). We considered the date for the first feces as an index of transit time; we also counted the other feces and noted their date of production.…”

“…For example, a 20g mouse inducing a 4g increase in snake mass after digestion provides a value of 20% (for details, see Michel and Bonnet, 2010). We emphasize that such a calculation does not provide accurate values of digestion efficiency (e.g.…”

“…Importantly, the strong variations in the crude estimate of digestive performance caused by different timing of regurgitation (see below) were more crucial to assess digestive efficiency than any other sources of variation [e.g. presence of a heat source (Michel and Bonnet, 2010)]; therefore, we essentially focused on this key factor.…”