This paper discusses intraspecific variations and interspecific differences in the “hibernation zone” and the mechanism of its formation on the incisor surface in three groups of rodents with different hibernation patterns: marmots, ground squirrels, and hamsters. The incisors of these animals continue growing during the hibernation period. Marmots and ground squirrels do not feed during hibernation; accordingly, the mechanical stress on their incisors differs significantly prior to, during, and after the hibernation period. During hibernation, the incisor growth rate is minimum, and a “hibernation zone” is formed on the incisor surface. In marmots and ground squirrels, this zone has two common features: very narrow increments and changes in the enamel–dentin junction. The interspecific differences between marmots and ground squirrels in the “hibernation zone” patterns can be attributed to regional differences in the climate. Hamsters can feed during hibernation. Their incisors grow more significantly, and the enamel–dentin junction on the incisor surface does not change. Thus, the “hibernation zone” is distinguished only by narrower and more distinct increments in comparison with daily ones. The interspecific differences between hamsters in hibernation records can be significant, up to the complete absence of such records. Apparently, these differences are determined not as much by climate as by differences in the hibernation patterns.

As was shown earlier, in some hibernating rodents the patterns of hibernation are recorded by tooth dentine and can be revealed in stained or ground sections of the incisors, as well as on the incisor surface. Hibernation in the common hamster, Cricetus cricetus, can be described as facultative with great individual variability of the body temperature dynamics during the winter. To investigate the records of hibernation on the incisor surface of this species, we studied the lower incisors of hamsters that had wintered with implanted temperature data loggers under natural conditions in a city park of Simferopol, Crimea (two animals); in semi-natural conditions in the village of Khunzakh, Dagestan (two animals); and using 19 hamsters from a natural population found dead in Simpferopol or its environs during late winter or early spring. Previous studies of eight hamsters that were kept under laboratory conditions and that had received tetracycline injections confirmed the daily formation of the increments on the incisor surface. This data allowed us to find the part of the incisor that could have been formed during hibernation in the hamsters with implanted loggers. In three of four of them, the typical pattern of daily increments was disrupted in the parts presumably formed during hibernation. The patterns of disruption varied in different animals, and it was impossible to interpret them as a record of hibernation. Those hamsters revealed no sequences of narrow and clearer increments characteristic of the records of hibernation on incisors. Such a sequence was seen on incisors of only one of 19 hamsters from the natural population. Consequently, no unambiguous record of the periods of hypothermia was found on the surface of incisors in the common hamster. According to the data published earlier, hibernation records on incisors were found in other hamsters of the subfamily Cricetinae (the genera Allocricetulus and Mesocricetus, in particular). A comparison of the dynamics of body temperature during hibernation in these hamsters and in the common hamster allow us to surmise that the formation of narrow increments can be determined in part by the depth and duration of hypothermia, but it mainly depends on the duration of normothermia and the feeding habits of animals during hibernation.

Intra- and interspecies variations in, and the mechanism of the formation of, a "hibernation zone" on the incisor surface are described in three groups of rodents differing in the mode of hibernation, i.e. marmots, ground squirrels, and hamsters. The growth of their incisors proceeds during hibernation. In marmots and ground squirrels that do not feed during hibernation, mechanical stress on incisors before, during, and after hibernation changes significantly. During hibernation, their incisors show the minimal growth and form a "hibernation zone" on the surface. There are two characters of the "hibernation zone" that are common for marmots and ground squirrels: very narrow increments and changes in the enamel-dentin junction. Interspecies variations in the patterns of "hibernation zone" may be attributed to differences in climate. In hamsters that can feed during hibernation, mechanical stress on the incisors during hibernation changes less significantly, the growth of their incisors is more notable, the enamel-dentin junction on the incisor surface does not change, and the "hibernation zone" can be noted only because the increments are more narrow and distinct than daily ones. Interspecies variations in the record of hibernation in hamsters can be significant, up to absent. Perhaps these differences are related not so much to climate as to the patterns of hibernation.

© 2020 г. Г. А. Клевезальa, *, Е. А. Зайцеваb, **, Д. В. Щепоткинa, *** , Н. Ю. Феоктистоваb, ****, М. М. Чунковc, *****, А. В. Суровb, ****** aИнститут биологии развития им. Н.К. Кольцова РАН, Москва 119991, Россия bИнститут проблем экологии и эволюции им. А.Н. Северцова РАН, Москва 119071, Россия cПрикаспийский институт биологических ресурсов ДНЦ РАН, Республика Дагестан, Махачкала 367000, Россия *e-mail: klevezal@bk.ru ** e-mail: zaycevaolena@gmail.com ***e-mail: dm_shchepotkin@mail.ru ****e-mail: feoktistovanyu@gmail.com *****e-mail: chunkov@mail.ru ******e-mail: surov@sevin.ru Поступила в редакцию 14.02.2019 г. После доработки 20.02.2019 г. Принята к публикации 28.03.2019 г.

Abstract—The lower incisors of ten hamsters from the Khunzakh region of Dagestan were studied. One hamster implanted with a temperature data logger successfully survived one winter in natural conditions, and its body temperature curve revealed 25 episodes of hypothermia with the body temperature dropping to 1–9°C, i.e., the period of hibernation. On the surface of the incisors, against a background of very indistinct increments, a zone with more distinct increments was observed. From the apical part of the zone to its basal part, the width of these increments decreased to very narrow, almost indistinguishable increments, and then increased. A comparison of the number of increments formed after this zone with the number of days the hamster survived after arousal from hibernation indicated that the zone was formed during hibernation. The pattern of variation in the increment width in the “zone of hibernation” corresponded to that observed in the episodes of normothermia during hibernation. A similar “zone of hibernation” was seen on the incisor surface in six of nine specimens caught in the field in June to early July. The date of their arousal was calculated from the number of increments corresponded to the date of arousal of the hamsters in this region, according to field observations. The pattern of the hibernation record on the incisors of these hamsters was mainly similar to that in the hamster with the temperature data logger and also demonstrated individual variations. Due to the poor contrast of increments on the incisor surface of these hamsters, it is difficult to use this record of hibernation for precise calculation of the data of arousal or for estimation of the duration of every episode of normothermia during hibernation. However, it is possible among the animals caught in spring to early summer to distinguish specimens aroused earlier or later using the position of the “zone of hibernation” on the incisor. It is also possible to estimate the relative duration of periods with short or long episodes of normothermia using the ratio of fragments with different widths of the increment in the “zone of hibernation.”

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.

This text focuses on the principles and methods of using growth layers formed in teeth and bones of mammals to make a judgement on essential traits of the animal's life history. In nearly all mammalian species, including man, the age of individuals can be determined from the number of growth layers and, at least in some of them, it is possible to estimate the season of an animal's birth and death, age of sexual maturation, periodicity of reproduction, certain feeding habits and other aspects of the individual's biology. It is also possible, from tooth-enamel analysis, to assess doses of radiation accumulated by animals and human beings during their lifetime. This book is intended for zoologists, wild-game biologists and zoo archaeologists, but some of the sections could also be of interest for anthropologists, radioecologists and conservation biologists.

Investigations of growth layers in bone tissue of amphibians, reptiles, birds and mammals, in tooth tissues of reptiles and mammals and in some horn epidermal structures of reptiles and mammals are reviewed. The history and the current state of studies of the layers (from the annual up to the daily ones) for the determination of age, growth and other parameters of life history (as well as using stable isotopes and tetracyclines) are described. To attract special attention to papers published in the Zoological Journal, references to such publications are given in bold.

To evaluate how the course of irregular winter hibernation is recorded in the incisor teeth of Allocricetulus hamsters, an analysis was performed of the incisor surface in seven A. curtatus and two A. eversmanni hamsters and stained section of the incisors in one A. curtatus hamster. The animals were kept in cages at natural temperature and photoperiod. Five hamsters were implanted with a temperature data logger, and another five received tetracycline injections (5–20 days before death). In addition, the incisor surface was examined in eight A. curtatus and eight A. eversmanni hamsters trapped in the field. In hibernating hamsters, the duration of alternating periods of hypothermia (torpor bouts) and normothermia (arousals) and their ratio varied irregularly and randomly within and between individuals. Increments on the incisor surface were of two types: wide and poorly defined or narrow and distinct. The increments in hamsters trapped in the field were slightly more distinct than in experimental captive animals, and those in two hamsters that failed to hibernate were similarly wide and indistinct. In hibernating hamsters, wide increments proved to correspond to long periods of normothermia, while narrow increments were formed when torpor bouts alternated with short arousals. More distinct narrow increments formed when torpor bouts followed by short (no more than 1.5-days) arousals occurred in a series rather than singly. Although the number of narrow increments was approximately similar to that of short arousals, in was impossible to localize the record of each particular hypo/normothermia episode on the incisor surface. Dentin layers in stained preparations had no definite pattern and corresponded to these episodes no better than the increments on the incisor surface. In general, the narrow and distinct increments in a given Allocricetulus hamster is evidence that the animal did hibernate, and the number of such increments indicates the approximate minimum number of torpor bouts followed by arousals. The record of winter hibernation on the incisors of Allocricetulus hamsters has a pattern similar to that in Mesocricetus brandti (Batavia et al., 2013) but significantly differs from the pattern described in M. raddei (Klevezal et al., 2012).

The surface of incisors in nine specimens of Allocricetulus hamsters and stained sections of incisors in one of them were investigated to assess the possibility of recording the irregular hibernation on the incisors' surface. The hamsters were kept outdoors in captivity, termoregisters were implanted to five of them, and tetracycline was administrated to five of them 5-20 days before the death. Besides, incisors of 16 specimens from nature were studied. In the hibernated hamsters, the lengths of hypothermia periods and normothernia periods following them, as well as the relation of adjacent periods varied irregularly within and between specimens. Two kinds of increments were visible on the incisors' surface: wide indistinct increments and narrower distinct ones. In the specimens from nature, the increments were slightly clearer than those in the experimental ones. All the increments were wide and indistinct in two specimens that did not hibernate. In the hibernated hamsters, wide indistinct increments formed during normothernia and narrow ones formed when periods of hypothermia were followed by relatively short (not more than 1.5 days) periods of normothermia. The more distinct narrow increments were seen when relatively long series of such short normothermia periods, not separate ones, took place. The number of narrow increments roughly corresponded to the number of short periods of normothermia, but there was no possibility to locate a record of each concrete hypo/normothermia episode on the incisor surface. The dentin layers in the stained incisor sections corresponded to hypo/normothermia episodes not better than the increments on the surface. As a whole, the narrower and more distinct increments on the surface of incisors in the specimen of Allocricetulus are an evidence of its hibernation, and the number of such increments indicates the minimal number of hypo/normothermia episodes. The record of hibernation in incisors of Allocricetulus is similar to the record described in incisors of Mesocricetus brandti (Batavia et al., 2013) and quite different from the record in incisors of Mesocricetus raddei (Klevezal et al., 2012).

Patterns of dentin increments on the incisor surface and in stained cross sections of incisors were studied in marmots Marmota baibacina centralis (sample 1, lower incisors of 65 ind. from one population; sample 2, upper and lower incisors of 5 ind. from another population), M. camtschatica camtschatica (upper and lower incisors of 4 ind.), and M. camtschatica bungei (upper and lower incisors of an animal whose hibernation had been monitored with a temperature data logger implanted intraperitoneally). All the animals had overwintered at least once. Their age was estimated from annual layers in cheek teeth. Intrapopulation, interpopulation and interspecific differences were revealed either in the daily increments or in the “hibernation zone” (the record of hibernation on the incisor surface). In some cases, this zone was hardly visible in the lower incisor while being well detectable in the upper incisor and in stained sections of both incisors of the same animal. The distinctness of daily increments decreased in the series M. baibacina centralis sample 1-M. baibacina centralis sample 2-M. camtschatica camtschatica-M. camtschatica bungei. The incisors of all M. baibacina marmots contained one hibernation zone in the position corresponding to the time of animal death, with traces of previous hibernation at the apex of the incisor being detected in the oldest animal. In three out of four M. camtschatica camtschatica marmots, two hibernation zones were detected, one in the basal and one in the apical part of the incisor, but it was only in one animal that the number of daily increments between them corresponded to the period of activity between hibernations known for the literature. The hibernation zone in young M. baibacina marmots was significantly less distinct than in adult animals from the same sample. Differences in this parameter between adult M. baibacina marmots from the two samples lacked statistical significance, while those between adult M. baibacina and M. camtschatica camtschatica were statistically significant. In the M. camtschatica bungei marmot that hibernated alone during the experiment, the hibernation zone could hardly be detected on the incisor surface, being also indistinct in stained cross sections, which did not allow the record of hibernation in the incisors to be compared with the data recorded by the temperature logger. According to published data, energy expenditures for hibernation may differ between species; moreover, they are higher in young than in adult marmots and, in experiment, in animals hibernating alone than in those hibernating in groups. Therefore, a probable explanation to the observed differences in the pattern of hibernation zone is that the energy cost of hibernation has an effect on the quality of its record on the incisor surface.

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.

The growth of incisors was studied in two and two 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 and one 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.

The surface of the lower incisor teeth was studied in an adult male 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.