Some Fish Are Braver Than Others: Guppies Have Complex Personalities Under Stress

The guppy studied is the tiny fish in the bottom-left corner. Also shown are the two predators researchers used to stress the guppy out: a heron and a blue acara cichlid. University of Exeter

How people respond to stressful situations can say a lot about their personalities—and it turns out the same is true of a little fish called the Trinidadian guppy. That's according to a study published this week in the journal Functional Ecology, for which a team of scientists stressed out a bunch of guppies and watched how they responded.

Trinidadian guppies don't have bills to pay, bosses to please or relationships to manage, but they do know they make a tasty snack for a range of animals, and they like being able to hang out in groups, called shoals.

That means they find being in a tank on their own a little bit stressful, and catching sight of something that might eat them particularly stressful—there's nothing fishy about that. A team of scientists led by Tom Houslay at the University of Exeter decided to use those characteristics to see whether all guppies responded to stressful situations in the same way.

The scientists caught each fish from the main tank and put it in a private suite, which was filmed throughout the experiment and included a small area where the fish could hide. That technique has a long history in behavioral studies, including of guppies, and is considered a reliable indicator of their behavior. The scientists gave the fish a fright: either dipping a model of a heron head into the tank or showing a large predator fish called a cichlid in the tank next door, to mimic two types of predators these guppies face in the wild.

The entire process took about five minutes, and each fish went through each of those ordeals once a week for a month, letting scientists get a handle on the consistency of their individual responses. For each recording, the scientists analyzed how much of the tank the fish swam through, how much time it spent in the sheltered area, how many times the fish stopped swimming and how much space the fish covered.

Taken together, those factors gave a picture of the individual's behavioral coping style, a consistent response to stress. According to lead author Houslay, the scientists noticed patterns in each fish, known as "repeatable among-individual behavioral (co)variation"—a mouthful that you're actually quite familiar with under another name: personality.

"It was really interesting to see that these personality differences were quite complex," Houslay wrote in an email. "Some guppies reacted to a new, unfamiliar environment by hiding, others were more 'panicky' and tried to escape, and others were bolder, and explored the new area they were in."

Previous studies have looked at guppies under stress, including one that found that among fish left vulnerable to a predator, bolder guppies survived longer; and one that attempted to connect perceived personalities to metabolic differences among fish. Houslay says this project was the first with the statistical depth to grasp the complexity of their responses.

He's continuing to work with guppy personalities, looking at whether their responses are inherited and how their behavior relates to their hormone levels, as changes in the body are also a key element of the stress response.

So the next time you're in the market for a pet guppy, remember to consider personality as well as appearance.