Effects of internal and external energy reserves on hibernation in Common hamsters

Hibernating animals have adopted two strategies to meet the energetic demands during winter: they can store energy either internally as body fat or externally as food caches. Both strategies of energy allocation are not mutually exclusive and can be combined depending on fat-storing capacity, reproductive effort, the availability of storable food items, and the size and composition of food hoards. Food caching species can have a longer breeding period than other hibernating species due to the less limited nature of external energy reserves. Accordingly, female Common hamsters (Cricetus cricetus) raise up to three litters per season. Nonetheless, there is a high inter- individual variation in timing of puberty, number of litters per season, and litter size, which probably reflects the interacting effects of physical condition, current and previous reproductive effort, and hibernation patterns. Furthermore, previous studies demonstrated high individual variation in body temperature patterns during winter both under natural and semi-natural conditions. However, by reducing the time spent in torpor, as expressed in hypometabolism and hypothermia, over-wintering strategies in these animals are coupled with higher energetic costs that have to be compensated by sufficient food intake. The variability of external energy reserves may allow the hamsters to adjust the time spent in torpor according to the food stores available. The proposed research project will combine field studies including an experimental approach with experiments in constant condition chambers to standardize or manipulate body fat content and the availability and quality of food caches. We plan to investigate relationships among reproductive performance, foraging strategies, body fat content, and hibernation patterns in free-ranging Common hamsters in Vienna. Furthermore, genetic analyses will provide information on potential heritable overwintering traits. Females with high previous reproductive effort and correspondingly low body fat in autumn are assumed to show increased food-hoarding behavior during the postbreeding period and a longer latency to the onset of torpor compared to individuals that had more time to accumulate body fat reserves. Effects of food availability on overwinter strategies will be tested by providing supplemental food during the postbreeding period for both individuals with high and low reproductive effort in the previous breeding season. The proportion of the supplemental food digested can be estimated by analyzing the individual fecal samples (n-alkane method). If the expression of torpor is minimized, we expect supplemented females to remain at normal body temperatures for most of the winter. In addition, Common hamsters will be kept in constant condition chambers during winter. In these experiments we are able to manipulate body fat content and the quantity and quality of food caches, respectively. Effects of body fat and the availability of food hoards on the time spent torpid and the response of the hamsters to changes in the quality of food caches will be analyzed. The proposed research program will provide insight in the use or avoidance of torpor and its adjustment with available internal or external energy reserves. Furthermore, knowledge on interactions among reproduction, foraging behavior, and hibernation is essential to improve conservation plans for this endangered species.

Facultative hibernators accumulate not only body fat as internal energy reserves but also rely on food stores as external reserves during winter. The aim of this research project was to investigate effects of food store quantity and quality on hibernation patterns in common hamsters (Cricetus cricetus). Individual body fat proportions and fat composition before and after winter were determined to compare the efficiency of different overwintering strategies. Hibernating small mammals use deep torpor, characterized by metabolic depression and decreased body temperature, to save energy and survive for several months in their hibernacula. Despite evident benefits, deep torpor can also have negative effects such as reduced immune capacity, oxidative stress and neuronal impairment. Sufficient food stores could allow the animals to minimize the time spent in deep torpor during winter. We therefore manipulated the availability and quality of food hoards in hamsters kept in climate chambers under controlled conditions. The animals used artificial burrows with different chambers that were accessible from outside via lids. Hamsters, which received daily food portions but were provided from storing food, were more likely to hibernate than individuals with access to food hoards. The quality of stored food also played an important role in that hamsters, which received sunflower seeds, high in fat content, particularly polyunsaturated fatty acids (PUFAs), strongly reduced the time spent in torpor. In contrast to other hibernators, which usually retain PUFAs und oxidize monounsaturated or saturated fatty acids, PUFA levels in common hamsters decreased during winter, indicating the mobilization of PUFAs during hibernation in this species. Field studies demonstrated unique sex differences in free-ranging common hamsters in that, in contrast to all other hibernators, females delayed hibernation onset until up to January and thus hibernated for shorter periods than males, although the time spent in the hibernacula did not differ between the sexes. Food supplementation in autumn, shortly before hibernation onset, shortened hibernation duration in males, while in females no effects on hibernation performance were detected. However, supplemented females gained a temporal advantage by earlier emergence and onset of reproductive activity in the following spring. This has been shown to positively affect reproductive success. Sex differences in overwintering strategies were also reflected in foraging behavior and body fat accumulation before winter. We developed and validated a method for calculating body fat proportions non-invasively. Genetic analyses revealed no evidence for heritable overwintering traits. In conclusion the results showed that common hamsters adjust torpor expression in relation to available external energy reserves and underline the high plasticity of overwintering strategies in this endangered species.