Comparison of the endocast growth of chimpanzees and bonobos via temporal regression and spatiotemporal registration

In this paper, we aim at characterizing and quantifying the differences between the growth of bonobos (\emph{Pan paniscus}) and chimpanzees (\emph{Pan troglodytes}). We use a collection of endocasts of wild-shot animals of both species. Each sample has been associated with a dental age, as a common temporal marker. To compare the endocasts, we used the current-based metric which allows us to quantify the shape differences \emph{without} the need to find homologous landmarks on the surfaces. First, we perform a temporal shape regression, which estimates a typical growth scenario of the endocast for the bonobos and the chimpanzees. Then, a spatiotemporal registration scheme is used to quantify the differences between these two growth scenarios. The variations are decomposed into one morphological deformation and one time warp. The morphological deformation accounts for the anatomical differences \emph{independently} of the age. The time warp accounts for the change of the dynamics of growth. It shows that the growth speed of the bonobos at juvenility is more than twice less than the one of the chimpanzees. This estimation gives more insights into the developmental delay observed in the bonobos growth.