Disproportionate eruption of maxillary and mandibular incisors in the long-tailed ground squirrel

The surface of the maxillary and mandibular incisors of Spermophilus undulatus long-tailed ground squirrels, including those born in the current year and those that have hibernated (trapped one month or later after hibernation) is studied. The presence of daily growth increments on the incisors’ surface allows the evaluation of the eruption rate of the incisors; a specific change in the character of the growth increments corresponds to winter hibernation (hibernation zone), which serves as the time mark. Ratio between the eruption rates of the maxillary and mandibular incisors typical for rodents is found in young-of-the-year and some animals after hibernation. In these animals the eruption rate of the mandibular incisors is higher than the eruption rate of the maxillary incisors and can be taken as proportional to their length. In individuals that have hibernated and show proportional eruption of the incisors, the proportions of the total length of the incisor formed before hibernation zone are equal for the maxillary and mandibular incisors. In the individuals that also have hibernated and show the ratio between the total length of the maxillary and mandibular incisors typical for rodents, the eruption rate of the mandibular incisor is equal to or less than the eruption rate of the maxillary incisor and the proportion of the incisor formed before hibernation is greater in the mandibular incisor than in the maxillary. This disproportionate pattern of incisor eruption is not typical for rodents and is a result of inequal attrition of the maxillary and mandibular incisors, which ultimately results in the normal ratio of the total length of the maxillary and mandibular incisors.     

Incisor growth rate in rodents and the record of the entire annual cycle in the incisors of Marmota baibacina centralis

The lower incisors of 80 specimens of Marmota baibacina of 1?C10 years old killed in spring in the highland of the Tien Shan Mountains were investigated. On the surface of all the incisors, regular daily increments and ??hibernation zones?? were revealed. The mean width of increments was used as an indicator of the incisor growth rate before hibernation. We found that the mean width did not depend on the sex of animals, locality, and incisor length, but depended on animal age. The increment width significantly decreased, and the number of the increments formed before hibernation significantly increased with age. This means that the growth rate of the incisors and the intensity of their attrition decrease with the animals?? age. The decrease in the incisor growth rate with age is a manifestation of the well-known decrease with age in the intensity of all growth processes in mammals. As a result of this decrease, in the majority of old marmots under study, zones of hibernation were revealed in the basal as well as in the apical parts of the incisor. The zone in the basal part formed during the latest hibernation and the zone in the apical part corresponded to the previous one. Thus, the incisor preserved the record of the entire year of animal life.     

Dynamics of incisor growth and daily increments on the incisor surface in three species of small rodents

The lower and upper incisors has been studied in striped field mice (Apodemus agrarius), yellow-necked mice (A. flavicollis), and migratory hamsters (Cricetulus migratorius) trapped in the Volgograd Region from April to March. The incisors of all animals have been found to have a striated surface, with series of slight ridges and grooves. As shown by analyzing the incisors of animals labeled with tetracycline, these are daily increments of dentin. The total numbers of increments (the period of complete tooth renewal) in the upper and lower incisors of the same animal are similar, but the degrees of their distinctness can be different. The number of daily increments increases but their width (daily growth rate) decreases with age even in adult animals and, in addition, varies by seasons. These two trends—the decrease in tooth increment with age and its seasonal variation—have proved to mask each other when an uneven-aged sample of animals trapped over several months is studied. Therefore, the season of death of an animal cannot be reliably estimated from the width of daily tooth increment unless the age of this animal is taken into account. The increment width in young animals in autumn may be the same as in older animals in spring and summer. In attempts to use the incisor surface sculpture for determining the season of animal death, it is necessary to separate the young from adults (at least by the criterion of incisor length). The decrease of increment width with age and its seasonal variation in the absence of obvious changes in the diet and pattern of incisor attrition indicate that the incisor growth rate is subject to age-dependent and seasonal changes, as is the growth rate of the organism as a whole. The distinctness and internal structure of daily increments also vary depending on animal age and species, which is due apparently to differences in the circadian rhythms of incisor growth. The presence of one or, less frequently, several grooves within a daily increment, which has occurred in all species studied, is evidence for not only circadian but also ultradian rhythms in the growth of incisors.     

Hibernation patterns of Turkish hamsters: influence of sex and ambient temperature

Turkish hamsters (Mesocricetus brandti) are a model organism for studies of hibernation, yet a detailed account of their torpor characteristics has not been undertaken. This study employed continuous telemetric monitoring of body temperature (T _b) in hibernating male and female Turkish hamsters at ambient temperatures (T _as) of 5 and 13 °C to precisely characterize torpor bout depth, duration, and frequency, as well as rates of entry into and arousal from torpor. Hamsters generated brief intervals of short (<12 h), shallow test bouts (T _b > 20 °C), followed by deep torpor bouts lasting 4–6 days at T _a = 5 °C and 2–3 days at T _a = 13 °C. Females at T _a = 5 °C had longer bouts than males, but maintained higher torpor T _b; there were no sex differences at T _a = 13 °C. Neither body mass loss nor food intake differed between the two T _as. Hamsters entered torpor primarily during the scotophase (subjective night), but timing of arousals was highly variable. Hamsters at both T _as generated short, shallow torpor bouts between deep bouts, suggesting that this species may be capable of both hibernation and daily torpor.     

On the growth of incisors in ground squirrels (Spermophilus) during hibernation

The growth of incisors was studied in two Spermophilus parryii and two S. undulatus ground squirrels with DS-1922L temperature data loggers implanted in the peritoneal cavity, which were kept under laboratory conditions. Daily increments on the incisors surface were similar to those in other species of ground squirrels, but they were less distinct and regular than in wild-living conspecific individuals from the same region. Two S. parryii and one S. undulatus ground squirrels entered hibernation and successfully overwintered. Despite some anomalies in their incisors, changes in body temperature during hibernation (recorded by the data loggers) had an effect on the pattern of their growth, resulting in the formation of a “hibernation zone” on the incisor surface. The number of narrow increments within this zone roughly corresponded to the number of alternating periods of torpor and euthermia during hibernation. This could be regarded as evidence that the incisors of the animals studied continued growing throughout hibernation, including the period of deep hibernation, with the rhythm of their growth coinciding with the rhythm of changes in body temperature. The effect of spontaneous trauma of an upper incisor on the growth of other incisors is described.     

Cold resistance of the brain during hibernation. II. Na-K-activated ATPase

It was found that the activity of the Na-K-ATPase from the cerebral cortex of awake hamsters and rats was not different between 5 ° C and 38 ° C. The enzyme from the hibernating hamsters showed increased activity between 5 ° C and 10 ° C over that of awake hamsters. This increase in activity occurred prior to the onset of hibernation while the animal was still homeothermic. Further, this acclimation seemed not to occur in those hamsters that failed to hibernate within a short time. The apparent affinity for K is increased at low temperature in Na-K-ATPase of hamsters. In the enzyme from hibernating hamsters high K concentration produces further stimulation. The possibility is discussed that these changes result from alteration of the lipids that are necessary for enzymatic activity.     

The effects of cold-stress,hibernation, and prolonged inactivity on bone dynamics in the golden hamster,Mesocricetus auratus

The effects of cold-stress and hibernation on bone dynamics in the femurs of hamsters were investigated using histometric analyses. Control animals were maintained at 27° C for 90 days; experimental animals were kept at 5° C and hibernated for 7, 15, 21, 50, or 90 days. Histometric analyses of cross sections indicated that bone diameter and cortical thickness at the femoral midshaft increased after 83 days of extreme cold and 7 days of hibernation but decreased significantly after 69 days of cold stress and 21 days of hibernation. Osteoporosis was evident although the number of osteons per unit area of bone increased during hibernation. An initial decrease in the number of non-Haversian longitudinal vessels per unit area of bone was seen in experimental animals which was apparently related to a corresponding reduction in cortical thickness. Lacunar area increased in these animals, suggesting that osteocytic osteolysis may be a significant mechanism for calcium regulation during hibernation.     

Effect of suprachiasmatic nucleus lesion on circadian dentin increment in rats

Mammalian dentin universally shows circadian increments. However, little is known about the mechanism of this phenomenon. The purpose of the present study was to investigate the role of the suprachiasmatic nucleus (SCN) in the generation of circadian rhythm in dentin increment. Rats underwent lesion of the SCN by electrodes and were maintained under constant light to examine whether the circadian increment free runs. The rats were injected with nitrilotriacetato lead to chronologically label the growing dentin. Two weeks after the operation, maxillary incisors and the locations of lesions in the brain were examined histologically. A harmonic (Fourier) analysis was performed to examine the densitometric pattern of the dentin increments to determine their periodicity. In rats with a completely lesioned SCN, ultradian increments, but no circadian increments, were observed in the dentin. Alternatively, in rats with an intact or only partially lesioned SCN, circadian increments persisted or were only temporarily disturbed. These results suggest that the SCN plays an important role in the generation of the circadian dentin increment in rats.     

A record of hibernation on the surface of incisor teeth in the hamster Mesocricetus raddei (Nehring, 1894)

The surface of the lower incisor teeth was studied in an adult male Mesocricetus raddei hamster with a temperature logger implanted into its peritoneal cavity in June 2010. From July 2010 to March 2011, the hamster lived under natural conditions in an enclosure and overwintered in a self-made burrow. Logger data showed that, beginning from mid-August, the animal body temperature periodically dropped and then returned to the norm, with the duration and depth of hypothermia (torpor bouts) increasing and those of nor-mothermia (arousals) decreasing with time. Growth increments on the incisor surface (presumably, daily increments) were unclear in the apical and middle tooth parts but very distinct in the basal part. The number and width of basal increments generally corresponded to the number and duration of arousals. The growth of the incisors apparently continued throughout the winter season, with every arousal after a torpor bout being marked by the formation of a distinct increment on the incisor surface. Thus, it appears that the pattern of increments on the incisor surface can be used to evaluate the course of wintering in hibernating hamsters. Experiments with vital markers are needed to confirm this assumption.     

Reconstruction of individual life histories of rodents from their teeth and bone

Age changes of bone tissue and molar cementum, and formation of daily layers in dentin of incisors enable us to estimate age and maximum lifespan, season of birth, season of death, sexual maturation, sex in adults, and in combination with tetracycline prebaiting, movements and space distribution of rodents. In hibernating rodents, the hibernation zone formed in incisor dentin can be used to assess inter- and intrapopulation variations in seasonal activity and growth. This approach is illustrated by examples from the study of rodents of the generaApodemus andSicista.     

The effects of hibernation on the myenteric plexus of the golden hamster small and large intestine

We have examined the effects of hibernation on the neurochemical composition of myenteric neurones in the small and large intestine of the golden hamster using immunohistochemical and histochemical techniques. Hibernation was induced in golden hamsters by altering the photoperiod and external ambient temperature. Age-matched hamsters kept at room temperature and those kept at 5 degrees C but which failed to hibernate were used as controls. Cell counts were carried out to examine possible changes in the numbers of cell bodies immunoreactive to all of the markers examined. The results demonstrated a significant increase during hibernation in the number of neurones immunoreactive to vasoactive intestinal polypeptide, substance P and calcitonin gene-related peptide; cell bodies positive for tyrosine hydroxylase, which were largely absent in the control animals, were prominent in the hibernating animals. There was a significant decrease in the number of neurones immunoreactive to 5-hydroxytryptamine, and no significant changes in the numbers of neurones immunoreactive to protein gene-product and nitric oxide synthase. It is suggested that selective upregulation and downregulation of myenteric neurones containing certain neurotransmitters may occur as a protective mechanism during hibernation to maintain the integrity of the muscular and mucosal layers of the intestine in the absence of luminal contents.     

Timing of torpor bouts during hibernation in European hamsters (Cricetus cricetus L.)

Body temperature (T _b) of seven European hamsters maintained at constant ambient temperature (T _a = 8 °C) and constant photoperiod (LD 8:16) was recorded throughout the hibernating season using intraperitoneal temperature-sensitive HF transmitters. The animals spent about 30% of the hibernation season in hypothermia and 70% in inter-bout normothermy. Three types of hypothermia, namely deep hibernation bouts (DHBs), short hibernation bouts (SHBs), and short and shallow hibernation bouts (SSHBs), were distinguished by differences in bout duration and minimal body temperature (T _m). A gradual development of SSHBs from the diel minimum of T _b during normothermy could be seen in individual hamsters, suggesting a stepwise decrease of the homeostatic setpoint of T _b regulation during the early hibernation season. Entry into hibernation followed a 24-h rhythm occurring at preferred times of the day in all three types of hypothermia. DHBs and SHBs were initiated approximately 4 h before SSHBs, indicating a general difference in the physiological initiation of SSHBs on the one hand and DHBs and SHBs on the other. Arousals from SHBs and SSHBs also followed a 24-h rhythm, whereas spontaneous arousals from DHBs were widely scattered across day and night. Statistical analyses of bout length and the interval between arousals revealed evidence for a free-running circadian rhythm underlying the timing of arousals. The results clearly demonstrate that entries into hypothermia are linked to the light/dark-cycle. However, the role of the circadian system in the timing of arousals from DHBs remains unclear. Accepted: 11 December 1996     

Overwinter body temperature patterns in captive jerboas (<Emphasis Type="Italic">Jaculus orientali</Emphasis>s): influence of sex and group

The jerboa (Jaculus orientalis) has been described in the past as a hibernator, but no reliable data exist on the daily and seasonal rhythmicity of body temperature (T _b). In this study, T _b patterns were determined in different groups of jerboas (isolated males and females, castrated males and grouped animals) maintained in captivity during autumn and winter, and submitted to natural variations of light and ambient temperature (T _a). T _b and T _a variations were recorded with surgically implanted iButton temperature loggers at 30-min intervals for two consecutive years. About half (6/13) of isolated female jerboas hibernated with a T _b < 33°C, with hibernation bouts interspersed with short periods of normothermy from November to February. Hibernation bout durations were longer (4–5 days) than those of normothermia phases (1–4 days). During hibernation, the minimum T _b was low (T _bmin ~10.7°C). In contrast, one of the 12 isolated males showed short hibernation bouts of ca. 2 days late in the hibernation season, February–March. The males had T _bmin values of 15.1°C. In contrast to predictions, no castrated males hibernated. When jerboas were grouped, females and males exhibited concomitant torpor bouts. In males, the longest bouts were observed during the late hibernation season. These data suggest complex regulation of hibernation in jerboas.     

Daily Increments on the Incisor Surface in Rodents: Intra-and Interspecific Variations

Investigation of the incisors of 17 rodent species from 11 genera revealed substantial interspecific and intraspecific variations in the pattern of the daily increment on the incisor surface. The morphology of daily increments is described in detail. Interspecific variations in the pattern are correlated with interspecific variations in the daily activity rhythms in rodents. Intraspecific (up to individual) traits can be attributed to intraspecific (up to individual) differences in daily activity. We can conclude that the daily activity of an individual determines the pattern of its incisor growth rhythm, not being a direct cause of daily increment formation. Thus, the pattern of the increment on the incisor surface can be considered as recording the rhythm of the daily activity of a rodent during the period of incisor renewal.     

Hormonal changes and energy substrate availability during the hibernation cycle of Syrian hamsters

Animals have to adapt to seasonal variations in food resources and temperature. Hibernation is one of the most efficient means used by animals to cope with harsh winter conditions, wherein survival is achieved through a significant decrease in energy expenditure. The hibernation period is constituted by a succession of torpor bouts (hypometabolism and decrease in body temperature) and periodic arousals (eumetabolism and euthermia). Some species feed during these periodic arousals, and thus show different metabolic adaptations to fat-storing species that fast throughout the hibernation period. Our study aims to define these metabolic adaptations, including hormone (insulin, glucagon, leptin, adiponectin, GLP-1, GiP) and metabolite (glucose, free fatty acids, triglycerides, urea) profiles together with body composition adjustments. Syrian hamsters were exposed to varied photoperiod and temperature conditions mimicking different phases of the hibernation cycle: a long photoperiod at 20 °C (LP20 group), a short photoperiod at 20 °C (SP20 group), and a short photoperiod at 8 °C (SP8). SP8 animals were sampled either at the beginning of a torpor bout (Torpor group) or at the beginning of a periodic arousal (Arousal group). We show that fat store mobilization in hamsters during torpor bouts is associated with decreased circulating levels of glucagon, insulin, leptin, and an increase in adiponectin. Refeeding during periodic arousals results in a decreased free fatty acid plasma concentration and an increase in glycemia and plasma incretin concentrations. Reduced incretin and increased adiponectin levels are therefore in accordance with the changes in nutrient availability and feeding behavior observed during the hibernation cycle of Syrian hamsters.     

Bite force and body mass of the fossil rodent Telicomys giganteus (Caviomorpha,Dinomyidae)

An exceptionally well-preserved skull of the Pliocene rodent Telicomys giganteus allowed the first estimation of body mass and analysis of the bite mechanics of this species of South American giant rodent. In this study, we reconstructed the main anatomical features of the skull of this Pliocene rodent and related them to the bite force at the incisors. The average of an estimation body mass gives 100 kg. We also estimated the bite force using three different techniques. Two methods suggest that bite forces at the incisors have a range of 500–1000 N. However, the incisors seem to be stronger than expected for this bite force, implying that the bite forces may have been greater than 2000 N. We consider the hypothesis of defense against predators or other agonistic behavior to explain our results.     

Chemical protection of golden hamsters against ionizing radiation

It was shown on Golden hamsters, which are characterized by high radioresistance of the intestine, that S-2-(omega-aminopropylaminoethyl) thiophosphorous acid (gammaphos) exerts a protective action against both X- and neutron-radiation although in the latter case the protection is less pronounced. In conditions of hibernation, the protective effect of gammaphos against X-radiation was not statistically reliable. Hibernation during exposure and at the postirradiation period increases considerably the radioresistance of animals. The influence of hibernation depends upon its duration.     

Temporal organisation of hibernation in wild-type and tau mutant Syrian hamsters

The temporal pattern of hibernation was studied in three genotypes of Syrian hamsters with different circadian periodicity to assess a potential circadian control of alternating torpor and euthermy. We recorded the pattern of hibernation by measuring activity in continuous dim light and constant environmental temperature (6 +/- 1 degrees C). In spite of differences in the endogenous circadian period of three genotypes (tau +/+: approximately equals 24 h, tau +/-: approximately equals 22 h, and tau -/-: approximately equals 20 h) torpor bout duration was statistically indistinguishable (tau +/+: 86.9+/-5.3 h; tau +/-: 94.2+/-3.3 h; tau -/-: 88.8+/-6.2 h). The time between two consecutive arousals from torpor showed unimodal distributions not significantly different between genotypes. The first entry into torpor occurred within the active phase of the circadian cycle in all genotypes whereas the first arousal from torpor appeared to be timed randomly with respect to the prior circadian cycle. The amplitude of the activity rhythm was lower after hibernation compared with the amplitude before hibernation. The results suggest that in the Syrian hamster the circadian system does not control periodicity of torpor and arousal onsets in prolonged hibernation at 6 degrees C.     

Body temperature patterns and use of torpor in an alpine glirid species,woolly dormouse

Woolly dormice, Dryomys laniger Felten and Storch (Senckenbergiana Biol 49(6):429–435, 1968), are a small (20–30 g), omnivorous (mainly insectivorous), nocturnal glirid species endemic to Turkey. Although woolly dormice have been assumed to hibernate during winter, no information exists on body temperature patterns and use of torpor in the species. In the present study, we aimed to determine body temperature patterns and use of torpor in woolly dormice under controlled laboratory conditions. Accordingly, body temperature (Tb) of woolly dormice was recorded using surgically implanted Thermochron iButtons, small and inexpensive temperature-sensitive data loggers. Woolly dormice exhibited robust, unimodal daily Tb rhythmicity during the euthermic stage before the beginning of hibernation. They displayed short torpor before they began hibernation, although the tendency to enter short torpor was different among individuals. Woolly dormice began hibernation within 1–3 days after exposure to cold and darkness, i.e., on October 22–24, and ended hibernation in the first half of April. Hibernation consisted of a sequence of multiday torpor bouts, interrupted by euthermic intervals. Thus, the patterns of hibernation in woolly dormice were similar to those observed in classical hibernating mammals.     

Inhibition of hibernation by exercise is not affected by intergeniculate leaflets lesion in hamsters

The circadian clock of mammals, located in the suprachiasmatic nuclei (SCN) of the hypothalamus, has been demonstrated to integrate day length change from long (LP) to short photoperiod (SP). This photoperiodic change induces in Syrian hamsters a testicular regression through melatonin action, a phenomenon that is inhibited when hamsters have free access to a wheel. The intergeniculate leaflets (IGL), which modulate the integration of photoperiod by the SCN, are a key structure in the circadian system, conveying nonphotic information such as those induced by novelty-induced wheel running activity. We tested in hamsters transferred from LP to a cold SP the effects of wheel running activity on a photoperiod-dependent behavior, hibernation. Lesions of the IGL were done to test the role of this structure in the inhibition induced by exercise of photoperiod integration by the clock. We show that wheel running activity actually inhibits hibernation not only in sham-operated animals, but also in hamsters with a bilateral IGL lesion (IGLX). In contrast, IGL-X hamsters without a wheel integrate slower to the SP but hibernate earlier compared with sham-operated animals. Moreover, some hibernation characteristics are affected by IGL lesion. Throughout the experiment at 7 degrees C, IGL-X hamsters were in hypothermia during 18% of the experiment vs. 32% for sham-operated hamsters. Taken together, these data show that the IGL play a modulatory role in the integration of photoperiodic cues and modulate hibernation, but they are not implicated in the inhibition of hibernation induced by wheel running activity.