Materials and Methods
All experimental procedures were carried out according to the guidelines of the animal welfare body (IvD) of the University of Groningen, and all experiments were approved by the Centrale Comissie Dierproeven of the Netherlands (CCD, license number: AVD1050020186147). common voles (Microtus arvalis ) were obtained from the Lauwersmeer area (Netherlands, 53° 24’ N, 6° 16’ E, (Gerkema et al., 1993). Tundra voles (Microtus oeconomus ) were obtained from four different regions in the Netherlands (described in van de Zande et al ., 2000). All voles in this study were indoor-bred as an outbred colony at the University of Groningen.
All voles used in this study were gestated and born under a short photoperiod (SP, 8h light:16h dark) and transferred to a long photoperiod at the day of weaning at either 21±1°C or 10±1°C. Based on photoperiodic dose-response-curves for gonadal mass from our prior research (van Rosmalen et al ., in preparation), we selected the photoperiod were maximum gonadal responses were reached, to obtain identical physiological status for both species at the start of the experiments (common voles: 16h light: 8h dark; tundra voles: 14h light:10h dark). Animals were transferred to cages (12.5 x 12.5 x 20 cm) provided with running wheels (14 cm diameter) when they were 35 days old. Ad libitum food was available for all animals until they were 40 days old. Animals were provided with water ad libitumthroughout the course of the experiments. In the work-for-food protocol (starting when animals were 40 days old), animals had to make a set number of wheel revolutions in order to receive a 45 mg grain based food pellet (630 J per pellet) (F0165; Bio-Serv, Flemington NJ, USA), using a computer controlled food dispenser (Med Associates Inc., St.Albans VT, USA). All animals started on a low workload protocol (100 revolutions/ pellet = 0.03 m/J), which is similar to ad libitum food conditions, since there were always pellets present in the cages. Half of the animals were subsequently exposed to an increasing workload paradigm in which workload was increased daily by an additional 10-30 revolutions per pellet. Detailed description of the work-for-food protocol for this experiment was published elsewhere (van Rosmalen and Hut, in review). In short, the increase in workload per day was titrated to obtain moderate individual body mass loss (0 - 0.5 gram/day) and the amount of earned pellets per day (>44 kJ/day). All voles were weighed every other day throughout the course of the experiments, in order to carefully monitor growth and to keep animals above 75% of their initial body mass (35 days old).