Predation risk, stress, and life history tactics in the edible dormouse

The exact timing of producing offspring is an important factor for an animal`s lifetime reproductive success. In seasonal environments reproduction is typically linked to the spring and summer season, when food availability is high. However, not all food resources are that easy to predict. Various terrestrial ecosystems are characterised by "pulsed resources", i.e., occasional, short periods of resource superabundance. The edible dormouse (Glis glis), a small, arboreal hibernator, shows several remarkable adaptations to fluctuations in seed production of trees, and life-history characteristics related to pulsed resource exploitation. Dormice litter only once per year in July/ August, which is extremely late in the active season, compared to other hibernators. Reproduction just in time with the availability of ripe, high-caloric seeds on tree branches apparently optimizes survival and pre-hibernation fattening of their young. The disadvantage of this highly specialized adaptation is that in years without beechnuts or acorns the survival of juveniles would be very low. Hence, dormice skip reproduction in years without seed production. It seems that the availability of energy-rich food (seed buds) in spring represents an environmental signal to which dormice adjust their reproduction. At present, it is entirely unclear, however, by which pathway this signal translates into reproductive success or failure. Here, we propose to test the hypothesis that effects of food quality on reproduction in dormice are linked to a factor that has never been considered before in this context, their exposure to predation pressure. Specifically, we hypothesize that access to energy-rich food (in years of mast seeding) allows the animals to minimize foraging time and hence exposure to predators, which can be a major stressor. We suggest that dormice are an excellent model to study the `Chronic Stress Hypothesis` which predicts that the stress profile of an animal, and its physiological consequences, result from simultaneous effects of both food requirements and predation pressures, caused by the trade-off between the need to forage and to avoid predators. We therefore plan to measure the time spent foraging (via transponder systems) and the concentration of faecal cortisol metabolites in a dormouse population in the Vienna Woods. For dormice, which forage in the canopy of woods, the major predators are nocturnal birds of prey, namely owls. One possible avenue of evading these predators altogether - at least in years of reproduction skipping - would be evasion by retreat into underground burrows. Due to the combination of estivation and hibernation, dormice can spend up to > 10 months per year in dormancy. Thus, they spend much more time in hypometabolism than climatic conditions or food resources would demand. These results led us to hypothesize that the primary cause for estivation is not reduction of energy expenditure but predator avoidance. We plan to record body temperature in free-ranging dormice during summer seasons under different environmental conditions. In a supplemental feeding field experiment we plan to investigate the effect of seed availability on the occurrence of estivation, the daily time spent foraging, concentration of cortisol metabolites, and reproduction. Further, we will investigate the consequences of the combination of hibernation, estivation, and reproduction skipping on life history characteristics of dormice, specifically on ageing. We suggest that the major cause for slowed ageing and increased longevity in dormice is their extensive use of hibernation (on average, 9 months per year) and occasional periods of estivation. These factors may interact with reproduction skipping in an additive or even non-additive way. To determine the effects of both dormancy and reproduction on rates of aging, we plan to measure seasonal changes the length of telomeres, the DNA sequences that cap and protect the ends of eukaryotic chromosomes

The main goal of this research project was to understand the complex impact of hibernation on life history strategies. Our study species, the edible dormouse (Glis glis, Gliridea, Rodentia), is a small mammal (~ 80 -120 g) raising a single litter with, on average, five juveniles per year. Further, they are known to switch from their arboreal lifestyle during the active season to underground hibernacula during winter, where they hibernate for on average eight months every year. Their life expectancy can reach up to 13 years, which is extraordinary for a mammal of this small body size. In comparison with the life history strategy of other species, the edible dormouse can be scaled with much larger mammals such as e.g., small ungulates or even the wild boar, which shows a comparable longevity and litter size. Thus, we hypothesized that hibernation might be an important key process, affecting the interactions between aging, stress and reproduction. Edible dormice are strongly adapted to the availability of pulsed resources and reproduce only in so-called beech mast years, i.e., in years with high availability of beech seeds. In our study we could measure body temperature in dormice under mast and mast failure conditions. Because of this study we were able to prove that free-living dormice can extend the hibernation duration to up to 11.4 months in mast failure years. In more detail, dormice with high body mass entered hibernation already in early summer (i.e. estivation), shortly after emerging from the last hibernation season. This extraordinary long hibernation duration has never been observed before in any free-ranging mammal. Animals in less than optimal body condition, however, stayed active but showed high frequencies of short torpor use (an energy saving mechanism) during the active season. Further, active animals switch to more constant and alternative habitats and food resources (e.g. pine cones) in these mast failure years. Interestingly, we could not detect that stress had an impact of reproduction or timing of hibernation in this species. We concluded that minimising the risk of predation, while hibernating in secure burrows deep under ground, leads to an increased survival rate in this species. The increase in survival probability, however, might allow the animals to invest in cell repair and maintenance mechanisms. To test this we measured the relative length of telomeres (the endcaps of chromosomes) in dormice, which are known to correlate with future survival in some species. We found that dormice surprisingly elongated their relative telomere length during the active season with increasing age. This contradicts the present understanding of aging mechanisms and we suggest that dormice, or hibernators in general, could be an interesting new model for research on aging.