Chalk up one more for the roaches.

Not only are they nigh indestructible, but they also appear to have an enviable skill: growing bigger testicles. A new study reports that Madagascar hissing cockroaches can specialize as either “lovers” with big testes or “fighters” with large horns.

And true to their name, these roaches do hiss—whether they’re loving or fighting. “They have a short hiss when they’re startled,” says University of Nottingham biologist Kate Durrant. There’s a louder, more aggressive hiss when they fight. “Then they’ve got a third hiss when they court a female, and it’s this long, slow ‘ssssssssss,’” she says.

As sexy as that sounds, the males often have to battle before they get the chance to woo a mate. They thrash one another’s rear ends, and use hornlike bumps behind their heads to shove at each other.

Cockroach vs. Cockroach: Watch How These Insects Fight For Love

“After a bit of scrabbling and butting and trying to flip their opponent over, one or the other will back down, and then the female will usually mate with the winner,” says Durrant. “And he’ll switch to that more seductive hiss.”

Durrant and her colleague Sophie Mowles noticed that one species of hissing cockroach had much larger horns than another one, and wondered if the two species might use different mating strategies.

“We measured the aggression of both species, and the big-horned guys were indeed more aggressive,” Durrant says. “But when we looked at the little-horned guys, their testes were absolutely bigger than the other ones.”

Not only was this true when comparing the two species, but the pattern also held up for males within a species. And that’s even more surprising—it means that individual males make a “choice” of sorts, though not a conscious one, to invest in either horns or testicles.

Most likely, males that are lacking in the weaponry department invest in bigger testes so they can make more sperm, Durrant says. But growing either big horns or big testes requires a substantial investment of energy, so it’s a trade-off. A roach can invest in bigger horns and increase its odds of winning battles, or put those resources into big testes that make lots of sperm, so that when it does get a chance to mate, it has good odds of fertilizing the female. But it can’t do both.

So how does an individual male “know” to grow bigger testes to compensate for small horns? Some insects can change the size of body parts during their final molting phase, when they become adults, Durrant says. So depending on how things have gone for the insect up to that point, bodily resources might be diverted into growing different adult body parts. Dung beetles, for instance, trade off bigger horns for smaller eyes, and youngsters that lose their horns before becoming adults grow bigger eyes during the final molt.

View Images Large testicles are often found in species where females mate with multiple males. Making more sperm buys a male more tickets in the fertilization lottery, and in some cases may allow a male to mate with more females. Photograph by Steve Hamblin, Alamy

But while hissing cockroaches’ testicles are flexible in size, they aren’t particularly huge when compared to other species. The ocean-dwelling brittle star called Ophiocoma alexandri appears to have the world’s biggest testicles relative to body size—the testes can be up to 40 percent of a male’s body mass, says Karim Vahed of the University of Derby. These starfish relatives need to make lots of sperm since they’re shooting it into the sea where it has to waft to a female.

Previously a bushcricket that Vahed had studied held the record at 11 percent of its body mass. Perhaps we can still award it the land-based record for testicle size. (See “Cricket Has World’s Biggest Testicles (But Puny Output).")

Large testicles are often found in species where females mate with multiple males, Vahed says. Making more sperm essentially buys a male more tickets in the fertilization lottery, and in some cases may allow a male to mate with more females.

Vahed says that roaches aren’t alone in trading off weapons and testicle size. In some dung beetle species, for example, large individuals with big horns have smaller testes. “The smaller ‘minor’ males with the relatively larger testes seem to adopt an alternative mating strategy: Instead of aggressively guarding burrows and females, they sneak their way into burrows to usurp paternity from the horned males,” Vahed says.

But not all animals are able to make these trade-offs. A study of cloven-hooved mammals including deer, cattle, and antelopes found no association between weapon size (horns or antlers) and testes mass, Vahed notes.