When you think of two individuals coupling up to raise and protect a family together, you might think of people or birds. But probably not cockroaches.
And yet, in a paper published in Royal Society Open Science, a trio of researchers presents evidence that suggests that Salganea taiwanensis, a kind of wood-feeding cockroach, may engage in what's known as pair bonding.
That "just means that two individual organisms will spend an extended period of time with each other and will exclude other individuals from the bond," says Nate Lo, an evolutionary biologist at the University of Sydney and an author of the new study. "The two individuals know that the other member has their back."
Pair bonding comes with numerous benefits, including grooming, the joint defense of nest and offspring, and the sharing of food. It requires enough brain power to learn to recognize and remember one's partner. The behavior shows up frequently in birds, mammals, and even fish.
"But we very rarely see it in invertebrates, so things like insects or crustaceans or other creepy crawlies," says Lo.
Now, Lo and his colleagues believe they've found some of the first indications of pair bonding in an invertebrate. This means that these roaches, and perhaps other insects, may have more sophisticated cognition and social behavior than researchers once thought.
Love at first bite
The wood-feeding cockroach that Lo studied lives in the forests of Okinawa, Japan, among other places.
"The male and the female will burrow into the rotting wood and form a little gallery," he says.
And then they do something… unique. Over a period of hours, the two roaches chew off each other's wings — and eat them.
The "female eat[s] the male's wings and the male eats [the] female's wings," says Haruka Osaki, a behavioral ecologist at the Museum of Nature and Human Activities in Hyogo, Japan. And when this one-time meal is complete, "it means they formed a pair."
This is when the couple starts making a nest out of their little patch of rotting paradise where they then mate and care for their young. "The wings, they're a protein source," explains Lo, "and this seems to set them up for some kind of romance into the future."
That is, the wing-eating appears to be a kind of consummation.
But pairing doesn't necessarily mean a pair bond. And the researchers wanted to know just how exclusive these insects are and how strong their bond actually is.
To figure this out, Osaki collected a bunch of roaches from Okinawa. "Just go to forest and find a log on the ground and chop it with my hatchet," she says. "So I destroy their house."
A pact resembling a pair bond
The experiment was simple. The team put pairs of roaches in artificial nest boxes. Some of those pairs had eaten each other's wings and some had not. The researchers then introduced a single intruder to the pairs.
Lo calls up a video to show what happened. A pair of roaches that both still have their wings appears on the screen. When the intruder enters the nest, there's no aggression or fuss. The interloper — whether male or female — is allowed to stay.
"You see, they're just very relaxed about it," says Lo.
But when he calls up a roach pair that had eaten one another's wings, "both the male and the female attack," he observes, by ramming the intruder. "They also wiggle their butts and hit them with their butts. They're quite aggressive little creatures."
"You can see the intruder's very worried," adds Lo. "It's trying to escape because it knows that it's in trouble. So that suggests they don't want to have a third wheel. It's like they've got this pact."
It's a pact that the researchers say looks a lot like a pair bond, in which the two roaches, upon eating each other's wings, become highly aggressive towards outsiders and only tolerant of their partner. This ability to distinguish between individuals can provide benefits in terms of survival and reproduction.
There's another takeaway. "Invertebrates probably are more complex and have some form of cognition, more than we might expect," says Lo. "Even though they've got tiny brains, they can develop quite human-like characteristics." Or as Osaki puts it, humans may not be as special as we like to think.
Lo does point out that the two seemingly pair-bonded roaches spent an extra 24 hours together before the experiment (during which they ate each other's wings) compared to the roach pairs that hadn't bonded. "We're not sure about how important that is," he says, "so we're planning more experiments."
Jessica Ware is the curator and chair of invertebrate zoology at the American Museum of Natural History. She wasn't involved in the research, but agrees that it offers strong evidence for pair bonding in this type of cockroach.
Ware says the findings also open up lots more questions. Might there be other insects that form pair bonds? How did this behavior evolve in these wood-feeding cockroaches? And how exactly is it that these roaches recognize one another?
"Maybe that's part of why they might consume the wing, is to get some chemical information about the mate," suggests Ware. They might then use this information to remember and recognize their partner over time.
It's pretty remarkable, she says. "We often tend to think of things that we don't like — like cockroaches — as being less interesting or that they don't have beautiful, interesting stories. And they do."
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