Variation in Increments and "Hibernation Zone" on the Incisor Surface in Marmots (Marmota)

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, inter population 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.