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.