JUANA SUMMERS, HOST:
And now we turn to cockroaches, specifically the German cockroach, which has evolved to live only in human environments. So it has had to adapt time and again to every weapon we've thrown its way, even to the point of changing its mating rituals. Science reporter Ari Daniel has the latest twist in its scurrying journey.
ARI DANIEL, BYLINE: Let me paint you a picture of roach romance.
COBY SCHAL: Actually, I have some roaches for you. If you're interested, I can try to put them together and demonstrate courtship.
DANIEL: Yeah, please.
Over Zoom, North Carolina State University entomologist Coby Schal points his webcam at a female German cockroach.
SCHAL: There we go. I just introduced a male.
DANIEL: Each roach is about the size of a lima bean.
SCHAL: See, he's starting to follow the female.
DANIEL: Once the two make contact, he raises his wings.
SCHAL: That exposes a gland on his back.
DANIEL: From which he secretes a nuptial gift, a sweet chemical slurry the female consumes. But to lap it up, she's got to mount the male.
SCHAL: That places the female in a perfect position. While she feeds, the male has this telescoping penis that has a hook on it.
DANIEL: Which he uses to lock onto the female's genitalia. Then, for the next 90 minutes, he produces a sperm package.
SCHAL: Which inseminates the female.
DANIEL: OK, so now you've got a mental picture of how two roaches make baby roaches. But back to that sweet nuptial gift, often made with a good helping of glucose, a sugar that pest controllers added to their lethal baits to lure roaches in starting in the '80s. The results were immediate. German cockroaches dropped like flies. But then a few years passed, and a pest control company noticed something unsettling.
SCHAL: A population of cockroaches in Florida in a Florida apartment just could not be controlled.
DANIEL: Turns out some of the roaches had evolved a glucose aversion. Glucose no longer tasted sweet to them. It was bitter.
SCHAL: And therefore, they refused to eat the bait.
DANIEL: But that was a problem for male roaches offering a glucose gift to potential mates because now some females found it repulsive.
SCHAL: So now this female, she immediately interrupts feeding and dismounts the male. So this poor male has just lost his opportunity to copulate with the female. Suddenly, this adaptive trait becomes maladaptive in a context of sexual interaction.
DANIEL: But if there's ever been something we can count on, it's that roaches beget more roaches. So when facing an existential crisis, like the inability to woo a partner, they always seem to find a workaround. In the case of Schal's German cockroaches, he's pinpointed two genetic changes that give males a shot with glucose-averse females.
SCHAL: The first one is the males change the chemistry of the nuptial gift that he offers the female.
DANIEL: He's tinkered with the recipe, slashing the amount of glucose and increasing the amount of a different sugar, so the gift stays sweet. Secondly, the three to four seconds it usually takes for the male to lock on to the female's genitalia, he's managed to shave off more than a second.
SCHAL: Before the female senses the presence of glucose in her mouth.
DANIEL: And can decide to walk away. Schal and his colleagues published their results in the journal Proceedings of the Royal Society B.
JESSICA WARE: Cockroaches are more than just pests. This is a beautiful evolutionary ecology example.
DANIEL: Jessica Ware, an entomologist at the American Museum of Natural History, wasn't involved in the research.
WARE: Because it shows how this elaborate behavior, which evolved presumably over hundreds of millions of years, this nuptial gift, has, in just a short period of time, been altered dramatically by humans.
DANIEL: So perhaps now's the time to reformulate the bait, though even if we succeed in tricking the roaches once more, it probably won't be long before their numbers begin to crawl upwards again. For NPR News, I'm Ari Daniel. Transcript provided by NPR, Copyright NPR.
