Reproductive strategies in female European Hamsters

European Hamsters differ from other hibernating mammals in a number of aspects. They do not depend on their body fat reserves for hibernation. Instead they build up food caches which can be used during winter. Therefore, the time and energy constraints are less severe than in other species. The result is a high reproductive output with up to four litter per year and early puberty in males and females. An additional time saving adjustment can be made by over-lapping ovulation and lactation patterns. Female European hamsters become pregnant while still lactating. Nonetheless, there is a high degree of inter-individual variation in puberty, gestation length, number of litters and litter size which probably reflects the interacting effects of condition, development and environment. With regard to body temperature changes during winter, European hamsters differ from most other hibernators. They spend more time in euthermy and show short periods of low body temperature. In the proposed project we plan to investigate interactions among female reproductive output, parental effort and developmental factors in this project. The effects of reproductive timing and investment of individual females on the number and condition of their offspring will be documented. The comparisons will be done among and within three age classes, juveniles (before first hibernation), yearlings (after first hibernation), and older females (after second hibernation). Early and late born animals and the ages at weaning can be compared. For yearlings and older individuals an additional factor would be the reproductive effort in the last season. Endocrine analyses can provide physiological and temporal information on ovarian development, gestation and lactation. To document interactions with hibernation, the results can be compared to hibernation duration, over-winter mass changes, and temperature patterns in the hibernacula. A population of hamsters in an urban environment would provide the unique opportunity to trap individuals frequently and collect detailed behavioral and endocrine data. To control for possible anthropogenic effects, we habe chosen a second colony in a more natural habitat. The study would give insight in trade-offs, that the animals make as they allocate resources for current and future reproduction.

In hibernating mammals, seasonal processes like reproduction, molt and fattening exist in a competitive process, associated with both energetic and temporal constraints. Females have to complete gestation, lactation and preparation for hibernation within a limited time period. In most species these constraints result in a very strict timing of seasonal processes and only one litter per year. European Hamsters differ from other hibernating mammals in a number of aspects. They are known to have a high reproductive output with up to three litters per year and early puberty in males and females. In this study, reproductive strategies of individual female hamsters were investigated. We aimed at determining crucial factors for reproductive success and relate them to conditional parameters, hibernation patterns and survival. Individually marked animals were captured in weekly intervals and behavioral observations were carried out throughout the active season. The results showed high individual variation in litter size and the number of litters per season. Some females successfully raised three litters, others had only one or even no litter. The timing of the active season turned out to be a determining factor for reproductive output. Females that emerged from hibernation early in spring had more litters and more overall offspring per season than those that started surface activity later. Surprisingly, early emergence was not related to the onset of gestation. Females used the first period after hibernation to improve their body condition before mating. Body mass significantly increased during that period and females spent most of the time foraging and food caching. Individuals in good physical condition were able to become estrous shortly after parturition and started the second gestation period while lactating the first litter. This postpartum estrus can be viewed as an important timesaving adjustment, especially in a hibernating species. High reproductive success had consequences for the females. They lost more mass during lactation and spent less time for food caching after weaning, than individuals with fewer offspring. As a consequence, successful females had to prolong the active season to compensate these effects. The results of this two-year study are an important basis for further investigations on reproduction and hibernation strategies in European hamsters. The analyses of endocrine parameters in the collected samples will provide more insight in the reproductive physiology in this species. The results also have an impact on conservational issues, because the European hamster is listed as an endangered species in Europe. Detailed knowledge on reproduction, hibernation and mortality is urgently required to develop effective protection and management strategies.