Ed Yong tells you more than you ever wanted to know:
Rather than the sleek, smooth tadpoles that other mammals have, the mole rat’s sperm had all manner of abnormally shaped heads. Some are squashed, others long. Some have bulbous lobes all over them. There are sperm with shrunken heads, two heads, conical heads. The DNA inside them isn’t packaged properly. Most are asymmetrical. And all the males – whether breeding or not – had the same misshapen sperm.
The theory is that because only two mole rats (the queen and her male consort, usually) will reproduce in any single colony, there's no evolutionary draw to have competitive sperm. Or as Yong put it, "The rats won’t be lovers, so there’s no point for their sperm to be fighters."
(Photo by Smithsonian's National Zoo)
The Dish 