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

Abstract

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.