Why the tiny fruit fly has giant sperm: Size matters to her

WASHINGTON (AP) — For a long time, the debate has gone on: Does size matter to females? Biologists now say, definitively, that it does.

Among fruit flies.

At issue is the fruit fly sperm, which is gargantuan in the tiny world of that speck-sized insect. How big? A fruit fly's sperm is 2.3 inches long, or about 23 times longer than its body. It's more than 1,100 times longer than human sperm. If a man had sperm the same proportion to its body as a fruit fly, it would be nearly 140 feet long.

This has been long known to scientists; they've also known that the sperm fits into the bug because it's coiled tightly, like a ball of yarn.

But they wondered was why such a little bug has such big sperm.

In other animals, it's like a lottery. The more sperm males produce, the better chance of winning, or in this case reproducing. Males do better with lots of tickets. And the theory had been that longer sperm aren't as healthy, so bigger isn't better for males.

But in the case of fruit flies, it isn't about the males. It's about what the females want. It's about how the females genetically evolve their bodies to get the biggest and best sperm possible out of males, according to a new study released Wednesday by the journal Nature.

"Males are all suckers evolutionarily," said study author Scot Pitnick, a biology professor at Syracuse University. "We're just trying to hold our own. Females are driving all of it."

Over time, female fruit flies have encouraged the production of bigger sperm by themselves developing larger organs or receptacles to store the sperm to later fertilize eggs. Smaller sperm get pushed out as the female mates voraciously with lots of males. For the males, longer sperm means fewer sperm, but then "only the highest quality males can manufacture enough sperm to capitalize," said Pitnick, who has a 50-inch sperm tattoo wrapped around his right arm.

Pitnick's work found a genetic correlation between females evolving larger storage receptacles and males making larger sperm.

Geoff Parker, a biologist at the University of Liverpool who wasn't part of the research, said Pitnick makes "the most plausible explanation to date."

In the world of animals, there's a lot of weird features that evolve in sexually selected traits — elaborate peacock feather tails, lizard horns, deer antlers — but the fruit fly giant sperm "are possibly the most extreme ornaments in all of nature," the study said.

Pitnick ranked those ornaments in relation to body size. Fruit fly sperm are off the chart, much higher than any of its nearest competitors.

"It's the females that are driving the evolution of these absolutely absurd, ridiculous traits in males," Pitnick said.