(Inside Science) -- Sometimes the best laid plans -- and eggs -- of fish go awry. In one species, females favor larger males, choosing more frequently to spawn with them. But female attention also unintentionally draws smaller males, which try to prey on a bigger fish's allure. That increased risk of rivals, scientists think, makes large males have unexpectedly speedy sperm.
Among the plainfin midshipman -- a marine fish that can breathe air -- males have historically been divided into two classes of appeal: bigger ones that attract females and make up around 90 percent of the male population, and smaller ones that females ignore. While scientists don't completely understand the inheritance of size in the fish, they know both genes and environment play a role.
A bigger male entices a female -- or more than one -- to his nest with a hum: a sustained, droning bass note that much smaller males can't produce. Once she lays her eggs, he'll guard them until they hatch. Unlike these large "guarder" males, the smallest males must ambush matings between spawning couples to sneak their genes into the next generation.
Because these "sneaker" males can only reproduce in the presence of other males, their sperm has to be competitive. Indeed, a team of researchers found that sneaker males -- which can be one-eighth the size of guarders -- produce faster sperm than many of their more attractive counterparts. Speed is a trait linked to fertilization success.
In addition, the team discovered that the largest, most desirable guarder males also make unusually quick sperm. The researchers think that it's because females flock to these males, inadvertently attracting "sneakers" intent on furtively fertilizing eggs that sperm competition escalates.
Male strutting and fighting to win females has been "an all-encompassing obsession in the field. But the females are actually important in driving some of these competitive dynamics," said John L. Fitzpatrick, evolutionary biologist at the University of Stockholm, who led a new study, published in Behavioral Ecology.
To determine how competition might speed up sperm, Fitzpatrick and his team brought males back to the lab, and measured under a microscope how fast their sperm swam.
While sperm were consistently fast in sneaker males, in guarders, the speed varied.
The bigger the guarder the faster the sperm. Sperm from the largest fish were eight percent faster than sperm from smaller guarders. But against sneaker sperm, the race was neck-and-neck. The largest and smallest fish make sperm that appear matched for competition.
The finding brings nuance to the theory used to describe dynamics between guarders and sneakers and predict their reproductive success, according to Matt Dean, an evolutionary biologist at the University of Southern California in Los Angeles. The theory, developed by Geoff Parker, an emeritus professor at the University of Liverpool, relies on the quantified risk that a male's sperm might compete with another male's.
"When a sneaker male spawns, his risk of sperm competition is 100 percent, whereas a dominant male's risk is somewhere around 15 percent," said Dean. But Fitzpatrick and colleagues' results show that "that 15 percent isn't a static number." It changes based on who females choose to mate with.
"That's a really cool idea, and it's novel. Sexier males have a higher risk," said Dean, who wasn't involved in the study. He points out that before this paper, the field was only thinking in terms of two alternative strategies: sneaker and guarder. But in light of the new study, that classification seems to splinter.
Fitzpatrick agrees that the traditional dichotomy between sneaker and guarder males may not be as clear-cut as once thought.
"The real world maybe has more shades of grey," he said.
How large males can maintain fast sperm remains an evolutionary quandary.
"It's unusual that we get a case where you can invest in both being really, really big, and having great quality sperm," said Fitzpatrick. "Because usually animals can't invest maximally in everything. In evolution, you usually can't get something for nothing."
Both Dean and Fitzpatrick think paternity tests are still needed to tally the reproductive success of males with different strategies. Only then will scientists know if making faster sperm pays off in a competition where more offspring are the ultimate prize.
Alison F. Takemura is a science writer based in Santa Cruz, CA. She tweets @AlisonTakemura.