Summer acclimatization in the short-tailed field vole,Microtus agrestis

We investigated the changes that occurred in basal and noradrenaline-induced metabolic rate, body temperature and body mass in short-tailed field voles,Microtus agrestis, during exposure to naturally increasing photoperiod and ambient temperature. These parameters were first measured in winter-acclimatized voles (n=8) and then in the same voles which had been allowed to seasonally acclimatize to photoperiod and ambient temperature (6 months later). Noradrenaline induced metabolic rate, basal metabolic rate and nonshivering thermogenesis were significantly higher in winter-acclimatized compared to summer-acclimatized voles. There was a significant positive relationship between basal metabolic rate and noradrenaline-induced metabolic rate. Body mass was significantly higher in summer-acclimatized compared to winter-acclimatized voles. There was a significant positive relationship between body mass and noradrenaline-induced metabolic rate in both winter-acclimalized and summer-acclimatized voles; however, there was no relationship between basal metabolic rate and body mass in either seasonal group of voles. Body temperature after measurements of basal metabolic rate was not significantly different in the seasonal cohorts of voles. However, body temperature was significantly higher in winter-acclimatized compared to summer-acclimatized voles after injection of noradrenaline. Previously we have found that a long photoperiod was not a sufficient stimulus to reduce thermogenic capacity in winter-acclimatized voles during cold exposure, since basal metabolic rate increased to compensate for a reduction in regulatory nonshivering thermogenesis. Here we found that a combination of increased ambient temperature and photoperiod did significantly reduce thermogenic capacity in winter-acclimatized voles. This provided evidence that the two aspects of non-shivering thermogenesis, obligatory and regulatory, are stimulated by different exogenous cues. Summer acclimatization in the shorttailed field vole is manifest as a significant decrease in both basal and noradrenaline-induced metabolic rate, combined with a significant increase in body mass.Abbreviations ANCOV A analysis of covariance - BAT brown adipose tissue - BM body mass - BMR basal metabolic rate - NST non-shivering thermogenesis - NA noradrenaline - V the maximum V recorded following mass specific injection of noradrenaline - V the maximum V recorded following mass specific injection of saline - T _a ambient temperature - T _b rectal body temperature - T _1c lower critical temperature - UCP uncoupling protein - V oxygen consumption     

The feedback of exogenous steroids on LH release and ovulation in the intact female vole (Microtus agrestis).

Female voles were subjected to various regimens of subcutaneous injections of oestradiol-17beta, oestradiol benzoate and/or progesterone. Ovulation occurred in only a few of the mature females and in none of the immature animals. There was no indication of any increased LH levels in blood samples taken every 2 h for 50 h after 150 microgram oestradiol-17beta.     

Isolation and characterization of polymorphic microsatellite loci in the field vole,Microtus agrestis,and their cross‐utility in the common vole,Microtus arvalis

We developed nine polymorphic microsatellite loci for the field vole, Microtus agrestis. The number of alleles ranged from five to 15 and observed heterozygosities ranged from 0.40 to 1.00. We also tested the microsatellite loci for amplification and polymorphism in the congeneric species Microtus arvalis. Five of the nine loci were successfully analysed in this species. The microsatellite markers will be employed in studies of reproductive success and fine‐scale spatial genetic structure.     

Studies on the control of the corpus luteum in the vole, Microtus agrestis.

The lifespan of corpora lutea resulting from hormonally induced ovulations was prolonged by exogenous prolactin, concurrent lactation, or pregnancy. Treatment of mated females with bromocriptine resulted in failure of luteal function only when the drug was given before Day 6 of pregnancy. Pregnancy was dependent on the presence of the ovaries in its later stages. The results suggest that prolactin is luteotrophic in early pregnancy but that a placental luteotrophin may become effective by Day 6 of pregnancy.     

The development of social behaviour in the vole (Microtus agrestis)

Juvenile voles of the species Microtus agrestis investigated the bodies of other juveniles with the nose, by touching the dorsal surface from snout to tail. Play movements often followed such nosing contacts and augmented further contacts. The most important scent stimulating play was produced by the skin at the back of the head. The secretion of the nose and mouth region may convey sexual identity, the rump region and feet individual identity.
Young born in the spring and early summer often interacted and played, the autumn young seldom. The behaviour of overwintering sexually immature adults was similar to that of the autumn juveniles. It is suggested that this seasonal difference in the behaviour of juveniles has consequences for subsequent mature males, and that this developmental factor may have an important effect on the annual population cycle.     

Short-term and circadian rhythms in the behaviour of the vole,Microtus agrestis (L.)

Summary The activity behaviour of the vole, Microtus agrestis, has been recorded in order to investigate the relationship between short-term rhythm and circadian rhythm. A simple device was developed, allowing separate monitoring of the time spent in or outside the nest, wheel-running, eating and drinking. Under natural light conditions during summer, a distinct differentiation between a short term rhythm of eating and drinking during the day-time and a circadian rhythm of wheel-running during the night was observed. The short-term rhythm depends closely on metabolic demands (hunger, thirst, excretion). Control of these demands by an endogenous oscillation could not be substantiated. The circadian rhythm of wheel-running activity is, however, controlled by an endogenous oscillation, synchronized by light conditions. It is subjected to seasonal variations. a) The threshold of light intensity below which wheel-running occurs is lowest during summer (<0.5 lx) and is higher during spring and autum (> 5 lx). b) Wheel-running is controlled by a circadian oscillation during summer only whereas it is an integrated part of the short-term rhythm during spring and autumn (experiments during the winter have not yet been performed). Experiments gave evidence that the properties of the cage can deeply influence the amount and pattern of wheel-running activity. It is concluded that wheel-running reflects a certain level of excitation, which may be caused by different behavioural intentions. The seasonal changes of the control of wheel-running activity are discussed with respect to this assumption. The relevancy of locomotor activity patterns as usually recorded in the laboratory to reveal the physiological and ecological significance of endogenously controlled behavioural patterns is discussed.Supported by the Deutsche Forschungsgemeinschaft     

Phytosterols affect endocrinology and metabolism of the field vole (Microtus agrestis)

Phytosterols or plant sterols (PS) enter the ecosystem via pulp mill effluents. They are also consumed by the general population of developed countries in natural remedies and margarines to lower elevated serum cholesterol levels. This study screened the endocrine and enzymatic parameters of the field vole (Microtus agrestis) for the effects of subchronic PS exposure at three doses (0, 5, or 50 mg of PS kg(-1) day(-1)). PS at 5 or 50 mg kg(-1) day(-1) decreased the relative liver weight of the voles. The kidney glycogen phosphorylase activity decreased at 5 or 50 mg kg(-1) day(-1), but the liver glycogen phosphorylase activity increased at 5 mg kg(-1) day(-1). The plasma estradiol and testosterone concentrations of males were higher due to PS supplement at 5 mg kg(-1) day(-1). This can be due to increased sex steroid synthesis from PS precursors. Biotransformation enzyme activities were not affected. PS caused multiple, previously unreported effects that were more pronounced at a low dose. As 5 mg PS kg(-1) day(-1) is the recommended dose for various health products, a thorough risk assessment of the effects and interactions of PS is warranted.     

Natal dispersal in relation to population density and sex ratio in the field vole,Microtus agrestis

Summary In a sample of 240 juvenile field voles 8% of the males and 22% of the females reached sexual maturity within their natal home range. Among individuals retrapped as adults, 58% of males and 23% of females had dispersed, i.e. had moved more than one home range diameter. The mean distance moved for males (58.5 m) exceeded that for females (28.6 m). Male movement distances were negatively associated with total density, and with density of adult females, but not with male density. Female movements were not related to population density. There were no relation between sex ratio and distance moved. The distribution of distances moved for both males and females fit a geometrical distribution, suggesting the importance of competitive processes.     

Individual growth rates in natural field vole, Microtus agrestis, populations exhibiting cyclic population dynamics

Rodents that have multi-annual cycles of density are known to have flexible growth strategies, and the “Chitty effect”, whereby adults in the high-density phase of the cycle exhibit larger average body mass than during the low phase, is a well-documented feature of cyclic populations. Despite this, there have been no studies that have repeatedly monitored individual vole growth over time from all phases of a density cycle, in order to evaluate whether such variation in body size is due to differences in juvenile growth rates, differences in growth periods, or differential survival of particularly large or small voles. This study compares growth trajectories from voles during the peak, increase and crash phases of the cycle in order to evaluate whether voles are exhibiting fast or slow growth strategies. We found that although voles reach highest asymptotic weights in the peak phase and lowest asymptotes during the crash, initial growth rates were not significantly different. This suggests that voles attain larger body size during the peak phase as a result of growing for longer.     

Matrilineal kin clusters and their effect on reproductive success in the field vole Microtus agrestis

Kin dusters of philopatric females are thought to form the basicsocial organization of breeding females in various vole species.The spatial association between young breeding females and theirmothers was studied in five 2500–m² enclosures in naturalhabitats during mid- and late breeding season in summer 1992.The mothers (founder females) were introduced into the enclosuresin die beginning of the study in June, whereas the young breedingfemales were born within the enclosures. Matrilineal kinshipwas determined by using radionudides. In July-August, 58% ofyoung breeding females were philopatric, but 82% of the mother-daughterassociations broke up until August-September. Of the young femalesthat started to breed in August-September, only 12% were philopatricThus the kin clusters found were short Hved and few in numberduring the late breeding season. The reduction in the incidenceof the kin clusters coincided with an increase of populationdensity. The mean reproductive success of females breeding inkin clusters was better than that of those breeding separately.In addition, among the nonphilopatric females there was a positive,significant correlation between reproductive success and thedistance to the nearest unrelated founder female. Such correlationwas not found among the philopatric females.