is an exclusive characteristic of placentals and closely related eutherians, being absent in other mammal clades, and it has been speculated to be derived from the epipubic bones more widely spread across mammals, but notoriously absent in placentals.

Among the primates, marmosets, weighing around , have a baculum measuring around , while the tiny galago has one around long. The great apes, despite their size, tend to have very small