- [Rossie] Humans and cockroaches have been locked in an arms race for decades.

No matter how hard we try to eradicate them from our homes, they always evolve to evade us.

- The cockroach spends all of its time inside the house, constantly exposed to insecticides, with something like eight generations a year or more.

And so mutation can arise and be selected upon much faster.

- [Rossie] Because of that rapid mutation time, cockroaches develop resistances to insecticide within three to five years.

They've even evolved to dislike the sugar in the poison's bait.

And now, scientists are finding that pressure from humans has changed one of their most basic evolutionary functions, how they mate.

- We need to keep in mind that there are about 5000 species of cockroaches worldwide, and a lot of these species are really important ecologically, only really a handful of species are pests in our homes.

- [Rossie] We're in scientist Coby Schal's lab, where he and his colleague, Ayako Wada-Katsumata, study cockroach evolution.

And of course, he convinced me to hold a Madagascar hissing cockroach.

[cockroach hisses] Ah!

[Rossie laughing] Which is not considered a pest.

People actually keep them as pets.

It's pretty cute.

I admit.

These are cute.

But these cockroaches, the German cockroaches, are not cute.

They're the ones that have adapted to live entirely alongside humans.

- Globally, we don't know of any natural populations of the German cockroach.

So to put anthropomorphically, the German cockroach has kind of abandoned its natural life, and has joined humans wherever humans are.

- [Rossie] In the last few decades, insecticide companies noticed that some cockroaches were no longer attracted to the glucose or the sugars used as baits.

Ayako discovered the reason why.

The neurons inside the tiny sensing hairs on cockroaches' heads, called sensilla, changed the sweet receptors to bitter receptors.

So when a mutant cockroach encounters something sweet, it closes its mouth and runs away as if it was bitter.

We don't know if that mutation was already present in cockroaches at very low levels, but Coby and Ayako do think that these glucose-averse cockroaches survived and passed the trait down to their children, amplifying it in the gene pool.

- As Charles Darwin very well put it, natural selection drives adaptation.

- [Rossie] But that adaptation has implications for a cockroach's mating behavior, and to explain that, we need a PG-13 primer on cockroach sex.

When a male cockroach loves a female cockroach very much, it, okay, just kidding.

Male cockroaches entice females with what scientists call a nuptial gift.

It's a sticky sweet mix of compounds on the male's back.

While the female eats it, the male shimmies down her body to assume the mating position.

He locks her into place, and they proceed to mate for an average of 90 minutes.

But glucose-averse females reject the nuptial gift.

[all chuckling] She's like, hell no!

You might be saying here okay, wait.

That's a good thing, right?

Less mating, less cockroaches.

Well, no.

- We found the glucose-averse male behaved differently than the normal male.

And also, he provide the more sweet secretion to glucose-averse females.

- [Rossie] Basically, male cockroaches have changed the chemistry of their nuptial gift, making it even sweeter so that it takes longer for females to break it down into glucose in their mouth.

The mutant cockroaches also act faster than normal cockroaches, shaving off 50% of the time it takes to start mating.

- The glucose-averse male is a lot quicker to draw.

- [Rossie] And so, once again, cockroach evolution has robbed us of a win.

- Some of them always survive, and develop totally new populations.

- [Rossie] In the future, insecticide companies will have to keep changing their baits to stay one step ahead of cockroaches.

- We're really suggesting that the formulation of bait should continue with some sugar, but reduce the amount of sugar, and include more savory stuff.

Our taste and the cockroach taste are actually quite similar, and we've evolved together in the same environment.