Reversal of photoschedule in spring does not prevent photorefractoriness in Siberian hamsters

Individual Differences in the Phenotypic Flexibility of Basal Metabolic Rate in Siberian Hamsters Are Consistent on Short- and Long-Term Timescales

Basal metabolic rate (BMR) correlates with the cost of life in endothermic animals. It usually differs consistently among individuals in a population, but it may be adjusted in response to predictable or unpredictable changes in the environment. The phenotypic flexibility of BMR is considered an adaptation to living in a stochastic environment; however, whether it is also repeatable it is still unexplored. Assuming that variations in phenotypic flexibility are evolutionarily important, we hypothesized that they are consistently different among individuals. We predicted that not only BMR but also its flexibility in response to changes in ambient temperature (T_a) are repeatable on short- and long-term timescales. To examine this, we acclimated Siberian hamsters (Phodopus sungorus) for 100 d to winterlike and then to summerlike conditions, and after each acclimation we exposed them interchangeably to 10° and 28°C for 14 d. The difference in BMR measured after each exposure defined an individual's phenotypic flexibility (ΔBMR). BMR was repeatable within and among seasons. It was also flexible in both seasons, but in winter this flexibility was lower in individuals responding to seasonal changes than in nonresponding ones. When we accounted for individual responsiveness, the repeatability of ΔBMR was significant in winter (τ = 0.48, P = 0.01) and in summer (τ = 0.55, P = 0.005). Finally, the flexibility of BMR in response to changes in T_a was also repeatable on a long-term timescale, that is, among seasons (τ = 0.31, P = 0.008). Our results indicate the evolutionary importance of the phenotypic flexibility of energy metabolism and suggest that it may be subject to selection.

Phenotypic flexibility of energetics in acclimated Siberian hamsters has a narrower scope in winter than in summer

As photoperiod shortens with the approach of winter, small mammals should reduce their energy expenditure to survive periods of food limitation. However, within seasons, animals should balance their energy budgets as abiotic conditions change, sometimes unpredictably; cold spells should increase heat production, while warm spells should do the opposite. Therefore, we addressed specific questions about the possible interactions between seasonal acclimatization and the intra-seasonal phenotypic flexibility of metabolic rate. We hypothesized that phenotypic flexibility in small mammals differs seasonally and is greater in summer than in winter, and predicted that seasonal adjustments in energetics, which are driven by photoperiod, overwhelm the influence of variations in the thermal environment. We measured body mass, basal metabolic rate (BMR), facultative non-shivering thermogenesis (fNST), body temperature, and calculated minimum thermal conductance in Siberian hamsters Phodopus sungorus. Animals were acclimated to winter-like, and then to summer-like conditions and, within each season, were exposed twice, for 3 weeks to 10, 20 or 28 °C. We used differences between values measured after these short acclimation periods as a measure of the scope of phenotypic flexibility. After winter acclimation, hamsters were lighter, had lower whole animal BMR, higher fNST than in summer, and developed heterothermy. After these short acclimations to the above-mentioned temperatures, hamsters showed reversible changes in BMR and fNST; however, these traits were less flexible in winter than in summer. We conclude that seasonal acclimation affects hamster responses to intra-seasonal variations in the thermal environment. We argue that understanding seasonal changes in phenotypic flexibility is crucial for predicting the biological consequences of global climate changes.

Effects of prolonged acclimation to intermediate photoperiod and photo-schedule reversal in photosensitive golden hamsters

We investigated the effect of prolonged acclimation to 12 hr of light and photo-schedule reversal during the time of photosensitivity in golden hamsters (Mesocricetus auratus). Before the experiments, animals were housed under natural photoperiod and then transferred to 12L:12D (light 12 hr:dark 12 hr) in autumn for 12 weeks. After 4 weeks of acclimation, photo-schedule was reversed (12D:12L). First experiments were done after 4 weeks of acclimation to an ambient temperature (T(a)) of 23 degrees C and a 12L:12D photo-schedule. We examined the daily variations in brown adipose tissue (BAT) capacity for nonshivering thermogenesis (NST). Noradrenaline (NA) injections were given every 4 hr while BAT temperature (T(BAT)) and preferred ambient temperature (PT(a)) were monitored continuously and simultaneously in a thermal gradient system. Then, we investigated the effect of light-dark cycle reversal on a daily rhythm of NST. The hamsters were acclimated to the photo-schedule reversed by 12 hr and the same T(a). After 4 and 8 weeks of acclimation to a reversed photo-schedule, the experiments were repeated. We found that the daily rhythm of the response to NA was entrained to the new light-dark cycle after 4 weeks of acclimation to a reversed photo-schedule. Maximum effect of NA was always recorded during the light phase and in the latter part of the dark phase of the day. NA-induced increase in T(BAT) was correlated with the decrease in PT(a), and was also inversely correlated with pre-injection T(BAT). These data imply that the daily rhythm of the capacity for NST opposes the daily rhythm of body temperature (T(b)). After 8 weeks of acclimation to the reversed photo-schedule, the rhythmicity of the response to NA disappeared, and the daily fluctuations in T(BAT) were the smallest. This lack of rhythm may be a physiological adaptation to winter conditions when the daily amplitude of T(b) rhythm is markedly reduced and, as a consequence, NST capacity does not vary within the day. Moreover, after 8 weeks of acclimation to reversed photo-schedule, NST capacity decreased while response to saline increased. During the experiments, hamsters were photosensitive and were changing to their winter status. However, because of the lack of cold during acclimation, the capacity for NST did not increase. Increased responsiveness to saline, indicating an increase in stress-induced thermogenesis, might be advantageous for "fight or flight" reaction.