Natural selection does not act directly on any particular property of bone, rather it acts on the reproductive success of an animal. Therefore all the properties of a bone are selected at the same time. When different properties conflict this produces a compromise;​ where one property is of paramount importance the mechanical properties of a bone may become extreme. The important requirements of bone change during ontogeny and the properties of the bone change accordingly. The safety factors that bones may have will vary greatly according to various factors that can, to some extent, be quantified. Methodologically it is important to consider what the functions of a particular bone are;​ natural selection has done a good job and it is up to us to find out what that job is.