From "It pays to be well hung, if you're a rat"
[
www.newscientist.com]
Rodents with relatively large testes also tended to have relatively long penises, Ramm found (The American Naturalist, vol 169, p 360). The advantage this confers on rodents is unknown, but a generously proportioned organ may deposit a male's sperm further up the female reproductive tract, giving them a head start in the race to the egg, Ramm speculates.
cheers
scott