Newsroom: Fish of a Feather, School Together—Understanding how group membership affects behavior in stickleback fish

As tropical fish glide lazily through an aquarium, they may seem like little more than moving art; but fish lead complicated lives and have even been found to possess what amounts to personalities. Indeed, individuals all throughout the animal kingdom have behavioral patterns that parallel personality in humans. There is variation between individuals and consistency within an individual. In other words, even in fish some individuals are bold, and some are shy, and if they are bold now, they will likely be bold in the future. 

However, this behavioral consistency can result in suboptimal decisions. Take a highly bold little fish for example. By hanging out in the open, he may be more likely to attract a mate, but he is also more susceptible to being eaten by a predator. Shy fish are better at avoiding predators but may be less likely to be spotted by a female. The “perfect” fish would be bold in the presence of females, but shy in the presence of predators. And yet, these patterns of behavioral consistency remain; so, the question becomes, “why?”

Researcher Marcus Michelangeli prepares to net fish in Beaver Pond, near Davis, CA. 

One potential explanation is the Social Niche Hypothesis. Basically, this is the idea that in social groups, tiny differences between individuals become magnified as other group members come to expect them to behave in a specific way. Imagine a group of friends planning an adventure. The first time all the jobs have to be negotiated. Who will drive? Who will plan the day’s activities? Who will be in charge of the radio? The more time the friends spend together, the easier this becomes, until their roles are streamlined. They know who is always going to be 10 minutes late and needs to be lied to about the meeting time and who will remember to bring enough water and snacks. In groups, behavioral consistency makes coordinating tasks easier. 

While this is a compelling theory, it has been poorly tested. Researchers at the University of California, Davis tested this theory using three-spined stickleback—a common freshwater fish. They first collected wild fish and brought them into the lab. There, they wanted to “ask” the fish how social they were. To do this individual fish were put in a large tank with two jars–one with 2 fish in it and one with 10 fish. They then measured how much time the fish spent close by each jar. Fish that spent more time next to the jar with 10 fish were counted as more social. Each fish was measured twice. Then fish were housed in one of two treatments—some of the fish were housed alone and the other fish were kept in groups of 10. After one month, all fish were measured for sociality twice more. This allowed the researchers to look at changes in social behavior and changes in how consistent an individual was before and after the housing treatment. 

Experimental timeline

While housing treatment did not affect how consistent an individual was, it did affect the amount of variation. There was much more variation between individuals housed in groups of 10 compared to fish housed alone. At first, this may seem to support the Social Niche Hypothesis. However, when they dug deeper, they found that within a tank, fish were actually fairly similar after being housed together. Instead, the change of variation that the researchers observed following the treatment was actually occurring at the group-level; that is groups, rather than individuals, became more different from each other. There was some evidence that these group differences were driven by the most social fish going into the treatment. Fish that were part of a group with one highly social individual to start all became more social over time. This supports the Social Conformity Hypothesis—individuals are conforming or becoming more similar to extreme group mates. More work is needed to fully understand this—does group membership affect all behaviors equally? Are behaviors like activity or exploratory tendency as susceptible to extreme traits in other individuals as social behavior is? But one thing seems true—who fish spend time with, can shape who they are. 

After being housed in a group for one month, fish behaved more similarly to the most social individual in their group. 

For more information:

Munson, A., Michelangeli, M., & Sih, A. (2021). Stable social groups foster conformity and among-group differences. Animal Behaviour, 174, 197-206,

[All images provided by Amelia Munson; Edited by Lindsey Broadus]

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