The baculum (penis bone) is one of the most variable bones in the mammalian skeleton – you can even ID an animal by the shape and size of that one bone. Just like with insect penises, the shapes of these reproductive structures can change quickly in evolutionary terms (Carl Zimmer writes a wonderful post looking at these shifts)
What’s its function?
Well, we’re not 100% sure yet. It may allow males to copulate before being fully, ahem, at attention. Or it may stimulate the female to ovulate. Some bacula grow spiny projections which may, like some insect penises, clear out other sperm before injecting one’s own.
Let’s not leave out the girls!
The penis and clitoris develop from the same structure. So, if the penis has a bone… does the clitoris? You bet! It’s called the os clitoridis (or baubellum).