Analysis of upper and lower molars
Analysis of the morphological characteristics of upper molars using principal components revealed that the characteristic values of the first, second and third principal components accounted for 84.95% of all variables (Table 1). Using the first and second principal components as scatter plots, the results were shown in Fig 2. Hainan population was mostly distributed on the left side, while Daxin population was mostly distributed above the image, samples from different regions overlap with each other. TPS analysis results were shown in Fig 3. The deformation of the upper molars of
T. belangeri in different regions mainly occurs at the relative positions of the alveolar and dental margins. Analysis of the morphological characteristics of lower molars using principal components revealed that the characteristic values of the first, second and third principal components accounted for 88.42% of all variables (Table 2). Using the first and second principal components as scatter plots, the results were shown in Fig 4. Hainan population was mostly distributed in the upper right corner, Daxin population was mostly distributed in the lower right corner and Pianma and Tengchong populations were mostly distributed in the left corner. Samples from different regions overlap with each other. TPS analysis results were shown in Fig 5. The deformation of the lower molars of
T. belangeri in different regions mainly occurs in the relative positions of the alveolar, dental margin and occlusal surface.
Multidimensional scale analysis of molar tooth morphology
Integrating the upper and lower molar tooth data of 12 samples of
T. belangeri from different regions and conducting multidimensional scale analysis, it was found that the molar tooth morphology of
T. belangeri varies among different groups, with the populations of Kinma and Tengchong clustered together; Leye, Xingyi, Hekou, Kunming, Xichang, Dali, Mengla and F1 generation populations gather together; Hainan and Daxin populations were separated separately and have significant differences from other populations (Fig 6). The individual dimension weights showed that the upper molar score (0.441, 0.480) has a significant contribution to the second dimension; the lower molar scores (0.490, 0.436) contribute significantly to the first dimension (Fig 7).
In recent years, there had been many reports on the study of molar morphology, involving different fields and levels, including research on functional dynamics and environmental adaptation, as well as exploration of research methods (
Unger, 1998;
Teaford, 2000;
Zhang et al., 2019; Ren et al., 2019a). For example, in the process of studying the function of teeth, it was found that the microwear method was more suitable for studying the feeding behavior of animals
(Ungar et al., 2003; Godfrey et al., 2004) and when searching for factors that determine the function of teeth (especially for molars), it is more suitable for qualitative methods
(Ungar et al., 2003; 2004). Moreover, the research on tooth function mainly focuses on exploring the sources of chewing power and chewing patterns (
Evans and Sanson, 2003;
Evans, 2005). To explore the effects of genetic effects and island climate on island populations, the molars and mandibles of
Apodemus sylvaticus were measured, the results showed that different evolutionary patterns appeared in the molars and mandibles; therefore, they speculated that this phenomenon was related to the distance, size and level of competition of the islands (
Renaud and Michaux, 2007).
Geometric morphology measurement technology can not only quantitatively analyze and describe the changes in the morphology of biological samples, but also identify the causes of morphological variations
(Renaud et al., 2007). Moreover, when using this method to study biological morphological features, it is not necessary to consider the actual size of the sample, but only the morphological changes of the sample itself
(Zhang et al., 2019). This technology has become one of the important methods for studying intra species and inters species morphological differentiation and is widely used in the study of rodents
(Zhu et al., 2014; Gao et al., 2016; Renaud et al., 2007). The present study used geometric morphology measurement technology to study the morphology of the upper and lower molars of
T. belangeri in 12 populations. TPS analysis showed that the deformation of the upper molars of
T. belangeri in different regions was mainly concentrated in the relative positions of the teeth grooves and edges; the deformation of the lower molars is mainly concentrated in the relative positions of the alveolar, marginal and occlusal surfaces. The morphology of the upper and lower molars of
T. belangeri varies to some extent in different regions, with significant variation in the lower molars. A study on
Marmota found that there was a close relationship between molars and food, but the variation in molar morphology is less affected by food (
Avise and Saunders, 1984). There were two main reasons: (1) before the teeth grow out of the gums, the surface enamel of the teeth cannot grow or change anymore; (2) the impact of the environment on molars is relatively small, especially when there is little difference in food. However, in the current study there was a significant variation in the lower molar teeth of
T. belangeri in different regions, which may be related to the food of
T. belangeri. It mainly lived in the mountains, forests, or shrubs near villages and farmland, feeding on insects, fruits and seeds of grains. The environmental differences in its distribution areas were significant, including high mountain, plateau and island environments. The crops in Hainan Island were mainly coconuts and mangoes, the crops in Daxin were mainly sugarcane and longans, the crops in the Yunnan-Guizhou Plateau were mainly corn and rice and the crops in Pianma and Tengchong were mainly melons and fruits. It can be seen that the differences in environment and food may be the main reason for the great variation of the lower molar morphology of
T. belangeri. Moreover, the mandible attached to the lower molars was closely related to feeding. In the study of the mandible of
T. belangeri, it was also found that there was significant morphological variations among different populations
(Li et al., 2020), which was similar to our results.
The results of PCA in our study showed that
T. belangeri from 12 regions were clustered into 4 branches: Hainan was clustered into one branch, Daxin was clustered into one branch, Pianma and Tengchong were clustered into one branch and Leye, Xingyi, Hekou, Kunming, Xichang, Mengla, Dali and laboratory bred F1 individuals were clustered into one branch. This is related to the natural environment in which the
T. belangeri lived. Hainan Island had a tropical island climate, the Yunnan-Guizhou Plateau had a northern subtropical plateau climate and Hengduan Mountains had a subtropical alpine climate. The dual effects of temperature and humidity may lead to morphological changes in the molars of the
T. belangeri. The difference in vegetation in the habitat of
T. belangeri may also be one of the important reasons for the variation of its molar morphology. Moreover, the results of multidimensional scale analysis also showed that there were certain differences in the morphology of the molars of
T. belangeri in 12 regions. Among them, there were significant variations in the Hainan, Daxin, Pianma and Tengchong regions. The spatial distribution map of individual weights showed that the upper molars contributed more to the second dimension, while the lower molars contributed more to the first dimension, which may also be related to the climate and geographical environment (temperature, humidity, food,
etc.) in which
T. belangeri habitat.
