Photogrammetry: a useful tool for three-dimensional morphometric analysis of small mammals

Abstract

Recently, the improvement of methods for shape analysis has revolutionized the field of morphometrics. While three-dimensional (3D) imaging technology is increasingly available, many studies of 3D structures still use two-dimensional (2D) data, even when this may result in the loss of important information. This is particularly conspicuous in the study of small mammals, as devices precise enough for 3D digitization of small objects are the most expensive. Thus, the development of low-cost methods aimed to recover 3D shape from small mammals would be of wide interest. Photogrammetry allows for obtaining 3D data with a lower cost than other 3D techniques, but it has not been previously applied to the study of small mammals. Accordingly, here we test the suitability of photogrammetric techniques to obtain 3D landmarks on mouse skulls as a model for small mammals. Shape and size of 3D models obtained with photogrammetric techniques were consistent among replicates, even when different sets of photographs were used. The linear measurements obtained from 3D models produced here were highly correlated with measurements obtained with callipers on actual crania, and differences among both sets of measures were smaller than those among individuals in most of the tested measures. These results show for the first time that photogrammetry is a precise technique for 3D shape analysis of small mammals. Photogrammetry also proved to be accurate for obtaining linear measurements between 3D landmarks; however, further studies are needed to demonstrate that this technique is also accurate to recreate 3D shapes.