Size Does Matter

Well, at least if you're a walrus. A recent study has found that mammals that live in colder climates tend to have larger baculi (penis bones) because a greater chance of insemination might provide an evolutionary advantage, as opposed to animals who live in more temperate climates and where physical prowess is a more desired trait:

Longer penis bones may ensure that the male's sperm is inserted closer to the egg, says Ferguson. So a well-hung male is more likely to succeed in becoming a father.

And it's also refreshing to see a prominent and (arguably) reputable science journal use the term "well-hung."