Animals living in temperate zones anticipate seasonal environmental changes to adapt their biological functions, especially reproduction and metabolism. Two main physiological mechanisms have evolved for this adaptation: intrinsic long-term timing mechanisms with an oscillating period of approximately 1 year, driven by a circannual clock [1], and synchronization of biological rhythms to the sidereal year using day length (photoperiod) [2]. In mammals, the pineal hormone melatonin relays photoperiodic information to the hypothalamus to control seasonal physiology through well-defined mechanisms [3-6]. In contrast, little is known about how the circannual clock drives endogenous changes in seasonal functions. The aim of this study was to determine whether genes involved in photoperiodic time measurement (TSHβ and Dio2) and central control of reproduction (Rfrp and Kiss1) display circannual rhythms in expression under constant conditions. Male European hamsters, deprived of seasonal time cues by pinealectomy and maintenance in constant photoperiod, were selected when expressing a subjective summer or subjective winter state in their circannual cycle of body weight, temperature, and testicular size. TSHβ expression in the pars tuberalis (PT) displayed a robust circannual variation with highest level in the subjective summer state, which was positively correlated with hypothalamic Dio2 and Rfrp expression. The negative sex steroid feedback was found to act specifically on arcuate Kiss1 expression. Our findings reveal TSH as a circannual output of the PT, which in turn regulates hypothalamic neurons controlling reproductive activity. Therefore, both the circannual and the melatonin signals converge on PT TSHβ expression to synchronize seasonal biological activity.

Although the European hamster is probably the fastest-declining Eurasian mammal, its IUCN RedList status is still Least Concern. In addition to the huge distribution area, this categorization is based on the assumptions (1) that the decline affects only Western Europe, where (2) modern agriculture has led to (3) an increase in the mortality of the species. Since mortality-reducing protection measures in Western Europe have been unable to stop the decline, we reviewed the literature from 1765 to the present and reappraised the situation. We found support for none of these assumptions. The species has also vanished from more than 75% of its range in Central and Eastern Europe. In 48 of 85 Russian, Belarussian, Ukrainian and Moldovan provinces, its relative occurrence has decreased. It is now rare in 42 provinces and extinct in 8. Mortality has not increased, but the reproduction rate has shrunk since 1954 throughout the distribution area. Today the reproduction rate is only 23% of that between 1914 and 1935. Taking into account the mortality of this prey species, 1 female today raises only 0.5 females for next year's reproduction. The extrapolation of the literature data points to an extinction of the species between 2020 and 2038. We strongly recommend (1) changing the status of the European hamster on the IUCN Red List from Least Concern at least to Vulnerable or even Endangered and (2) supporting scientific research on the reproduction of European hamsters as a protection measure. Global threats such as climate change, light pollution or (in the past) fur trapping are more likely to be the ultimate reason for the decline of this species than modern agriculture.

Many species synchronize reproductive behavior with a particular phase of the lunar cycle to increase reproductive success. In humans, a lunar influence on reproductive behavior remains controversial, although the human menstrual cycle has a period close to that of the lunar cycle. Here, we analyzed long-term menstrual recordings of individual women with distinct methods for biological rhythm analysis. We show that women’s menstrual cycles with a period longer than 27 days were intermittently synchronous with the Moon’s luminance and/or gravimetric cycles. With age and upon exposure to artificial nocturnal light, menstrual cycles shortened and lost this synchrony. We hypothesize that in ancient times, human reproductive behavior was synchronous with the Moon but that our modern lifestyles have changed reproductive physiology and behavior.

This study investigated in male European hamsters (Cricetus cricetus ) whether entrainment of circannual rhythms follows the principles of the nonparametric entrainment model. In 2 experiments the times of the year when long (LP) or short photoperiod (SP) are able to synchronize the reproductive cycle were determined, by recording phase response curves (PRCs). A total of 28 groups of 10 hamsters were synchronized by SP, before being subjected to 2 converse experiments: a) 14 groups were transferred to constant LP, only interrupted by SP for 1 month (SP-pulse), the pulse being increasingly delayed between groups by 2 weeks or 1 month steps; and b) the remaining 14 groups stayed in constant SP interrupted by LP for 1 month (LP-pulse) at different phases of the cycle. In a 3rd experiment 5 groups of 10 European hamsters were subjected to constant LP interrupted by 1-month SP-pulses in regular non-365-day zeitgeber intervals (circannual T-cycles) differing between groups (c). The reproductive state was checked every 2 or 4 weeks. The PRCs revealed that an SP-pulse had a very strong phase-resetting capability of -180 degrees to at least +81 degrees in subjective summer (a). During subjective winter when the animals hibernate, a SP-pulse had only weak effectiveness (a) whereas an LP-pulse advanced the circannual clock by up to +41 degrees (b). In the latter conditions a further advance of up to +156 degrees was achieved by the decrease in photoperiod at the return to SP conditions, which terminated the reproductive phase already after 4 to 5 weeks. In different circannual T-cycles the animals entrained for at least 2 cycles (c). In conclusion, 1) the circannual rhythm of European hamsters can be entrained by one photoperiodic signal per cycle, 2) the decrease in photoperiod is most important for its synchronization, and 3) as in circadian clocks the resetting of circannual clocks follows the principles of the nonparametric entrainment model.