One of the riddles of mammal evolution explained: the strong conservation of the number of trunk vertebrae. Researchers of the Naturalis Biodiversity Center and the University of Utah show that this conservation is probably due to the essential role of speed and agility in survival of fast running mammals. They measured variation in vertebrae of 774 individual mammal skeletons of both fast and slow running species. The researchers found that a combination of developmental and biomechanical problems prevents evolutionary change in the number of trunk vertebrae in fast running and agile mammals. In contrast, these problems barely affect slow and sturdy mammals. The study will appear on 14 July 2014 in PNAS.