A cockroach seeks shelter from bright lights.

A cockroach seeks shelter from bright lights.

Isaac Planas-Sitjà

Even cockroaches have personalities

Filthy, smelly, repulsive. There are a lot of ways to describe cockroaches, but “full of personality” usually isn’t one of them. Yet a team of scientists has not only found evidence that the scuttling insects have personalities, but also discovered that when cockroaches get together, they create a group personality. The group personalities of cockroaches vary, too.

“A lot of studies show personality in other invertebrates,” says Isaac Planas-Sitjà, a behavioral ecologist at the Free University of Brussels and the lead author of the study. “But no one had looked at the American cockroach.”

Over the last 2 decades, scientists have documented personalities—that is, consistent behaviors, such as boldness, shyness, sociability, or aggressiveness—in a range of invertebrate species, from octopuses to water striders to social spiders. Planas-Sitjà was drawn to cockroaches not out of fondness, but because they don’t live in societies with leaders and followers—social castes that can make it difficult to spot an individual’s personality. “They are all independent, even though they are gregarious,” he says.

To find out if the cockroaches had personalities, Planas-Sitjà and his colleagues glued tiny radio frequency identification chips to the thoraxes of 304 roaches so that they could track each insect after it was placed in a new environment. The scientists divided the animals into 19 groups of 16 individuals (all males about 4 months of age, because an animal’s age and gender can affect its behavior, making it more difficult to tease out its personality type). Three times a week, the team placed each group in the middle of a brightly lit, plastic circular arena that was surrounded by an electric fence so that the roaches could not escape. Two identical Plexiglas disks covered with red filters hovered just above the arena, creating red circles that the light-phobic insects perceived as shelters. Each shelter was large enough for all 16 cockroaches to gather beneath.

Over a 3-hour period, the scientists measured the amount of time individual cockroaches spent inside a shelter and how much time each took to pay its first visit. To see if the insects reached a consensus about where to gather (an indicator of group personality), they tallied how many insects were beneath each disk at the end of the experiment. Their analysis showed that like other species, from spiders to lions, these cockroaches had shy and bold individuals. The shy roaches ran for cover as soon as they entered the arena, whereas bold individuals spent more time exploring, the team reports online today in the Proceedings of the Royal Society B. And the roaches consistently behaved in these same ways in each test.

Despite these individual personality differences, by the end of each experiment the groups always ended up crowded together beneath the same shelter. “There is a collective dynamic—a social influence—that dilutes the individual personality differences,” Planas-Sitjà says. “So in the group, you end up with a similar behavior in everyone.” This conformity happens even though the researchers know, based on previous experiments, that some cockroaches when left alone in the arena never dash to a shelter, whereas others spend only a short amount of time beneath one. Yet they change their behaviors as soon as they’re in a group. “Then they all run to the shelter,” says Planas-Sitjà, who hopes to tease out why and how this happens with further experiments.

The team’s discovery that “the collective outcome [the group personality] is different from the sum of the personalities is very cool,” says Noa Pinter-Wollman, an animal behaviorist at the University of California, San Diego, who was not involved in the study. “It implies that the whole is greater than the sum of its parts.” Social spiders, bees, and ants are also known to have group or colony personalities.

“To be able to show group personality as they have done is very exciting and intriguing,” adds Odile Petit, an ethologist at the French national research agency CNRS in Strasbourg. “And they’ve shown that individuals and their personalities matter even in simple animals.” Yes, even in cockroaches.