Trinidadian guppies (Poecilia reticulata), also known as millionfish and rainbow fish, seem to have complex individual personalities. The finding was reported by a team at the University of Exeter, UK, who studied the fish — a favored pet for many Americans — in various situations only to discover many exhibited and retained complex behavior across these scenarios.

Credit: Finding Nemo (2003).

Guppies are one of the best vertebrate species for studying evolution in the wild, particularly on the island of Trinidad in the West Indies where you can find them in freshwater streams. These fish have a very short maturation period of only a couple of months and waterfall barriers in their habitat often lead to significant variations in behavior, life history, physiology, and appearance among populations. One 2004 study found guppies “from the downstream population responded more strongly to the aquatic predator than did fish from the upstream population,” highlighting “the importance of multiple selection pressures acting on an organism.”

Now, British researchers have shown in the journal Functional Ecology that not only do different populations exhibit varying behavior but also individual guppies.

Previously, scientists have tried to explain individual variance in behavior through the idea of a simple spectrum of how risk-averse or risk-prone guppies were. It didn’t take too long for Tom Houslay and colleagues at the Centre for Ecology and Conservation (CEC) to realize a ‘simple spectrum’ just wouldn’t cut it for the guppies. “Our research shows that the reality is much more complex,” Houslay said.

Some are braver than others

The team’s experiment involved introducing guppies to ‘mild stressors’. For instance, a mildly stressing situation for the guppies is individually introducing the fish in unfamiliar aquariums. Introducing models of predators triggered a higher stress.

When the fish were placed in an unfamiliar environment, the guppies employed varying coping strategies. “Many attempt to hide, others try to escape, some explore cautiously, and so on,” Houslay said.

A mock-up image showing a Trinidadian guppy (the small fish), a blue acara cichlid and a model of a heron. Credit: Copyright Tom Houslay, University of Exeter

The presence of the fake predators made all the guppies cautious overall — but even during such a stressing situation, individuals still retained their distinct personalities.

“The differences between them were consistent over time and in different situations. So, while the behaviour of all the guppies changed depending on the situation – for example, all becoming more cautious in more stressful situations – the relative differences between individuals remained intact,” Houslay said in a statement.

In 2014, researchers at the same University of Exeter found “sharks have distinct, individual personalities some being more friendly and open, thus willing to congregate in groups, while others are loners, much like human introverts.”

Next, the research will attempt to identify whether there are any genetic factors that underly these personality traits or to what extent the environment influences these traits.

“The goal is really gaining insight into evolutionary processes, how different behavioural strategies might persist as species evolve,” said Professor Alastair Wilson, also from the CEC at the University of Exeter.

The bottom line is many animals people commonly see as mindless drones — seen one, seen them all — can be very different on an individual basis.