The jury is still out. Other odd nipple facts:
Pinnipeds (including seals, sea lions, and walruses) have retractable nipples that remain tucked when pups aren’t feeding on them. Adult male rats don’t have nipples, and male horses don’t have teats. The nipples of a female dog shrivel after they get spayed. The platypus and the echidna don’t have nipples at all, but rather secrete milk through the pores in their skin.
The Dish 