Stefan Lüpold is a sperm guy. The Swiss evolutionary biologist did his masters work on sexual selection and bat genitalia, his PhD on the evolution of bird sperm, and a postdoctorate on how fruit fly sperm compete. "I’m fascinated by the almost unlimited diversity in both size and shape of sperm," he writes me in an email from Zurich, describing his chosen sub-sub-discipline.

I've emailed him because he's recently found evidence to answer a perplexing question: Why are sperm so weird?

Mice, for instance, have sperm that's twice as long as elephants'. The world's longest sperm belongs to a fruit fly. And across the animal kingdom, sperm take on extremely odd and varied shapes and sizes. The "tadpole" shape we most associate with sperm is not at all common outside of mammals. Rat and mice sperm can have hook-like attachments on their heads. "In some species they seem to allow sperm to connect by their heads and form so-called sperm trains," Lüpold says. "These groups of sperm seem to swim faster than individual sperm." Fascinating!

From an evolutionary perspective this raises an intriguing question: Why are sperm so varied among different species when they all have the exact same purpose — fertilizing eggs?

Lüpold has a theory. Analyzing the sperm of 100 species of mammals, he and a co-author found a pattern amid the chaos: the larger the species, the smaller the sperm. The results were recently published in Proceedings B, a journal of the Royal Society of London.

Why would evolution favor such a pattern? Lüpold explains that longer sperm has some advantages — they are better at "elbowing" aside the competition. But it also takes a lot of energy to make long sperm, which larger animals can't afford. So it's a trade-off:

If there were no constraints on sperm production and assuming that longer sperm are advantageous, each male would probably produce lots of impressively big sperm. But in nature there are always constraints because resources and energy are not unlimited. For a testis of a given size, producing bigger sperm thus means it cannot produce as many of them (producing big sperm takes more resources, energy and time). So, whether investing more in sperm size or in sperm number to maximize sperm competitiveness really depends on the circumstances, for example the size of the female reproductive tract. In large species, the female reproductive tract is massive compared to the tiny sperm, so sperm can easily be lost or diluted in it. Males have to compensate by transferring more sperm. Simply making longer sperm really wouldn’t make a difference in an elephant. They would have to be incredibly large. So males are better off making lots of tiny sperm.

This inverse correlation between animal size and sperm size might be a consistent pattern across the animal kingdom. Almost all animals with sperm longer than a 10th of a millimeter, he explains, weigh less than one or two pounds. "Our results certainly suggest a unifying pattern that is likely to explain much of the diversity in mammalian sperm size and possibly beyond," he says, while noting more research is still needed.

The mammal with the longest sperm? It's not the human. That distinction belongs to the honey possum, a very small (they grow to 3.5 inches long ) marsupial that lives in western Australia. They are adorable. Their sperm is 350 micrometers (.014 inches) long.