Forget the whole argument over whether or not fish can “feel.” Researchers have recently shown that fish have personalities.

That’s right—just like humans, when faced with a new situation, some fish are reticent and fearful while others are confident, we learned from a recent article in Nature. Lead scientist Lynne Sneddon and her team pitted shy and bold rainbow trout against each other in the lab.

Bold fish that won their fights tended to be even more bold when later presented with a novel food item; losing their fight caused them to be much more cautious.

The idea of animal personalities — known to researchers as 'behavioural syndromes' — has been around for a while. The idea aims to explain why some animals' behaviour is not always ideally suited to their circumstances. A male with a naturally aggressive temperament, for example, might be great at fighting off rivals, but might never get to mate because his heavy-handed seduction tactics scare off the ladies.

The new research suggests that these traits are not set in stone, and that animals can gradually adapt their personalities. "Traditionally they were thought to be consistent," says Sneddon. "But actually no one had looked to see."

Sneddon suspects that shyness and boldness are linked to physiological factors such as levels of stress hormones. Losing a fight might boost levels of stress-related chemicals such as cortisol, which might make a fish much more wary in future.

All of this makes us wonder about fish in the wild. If losing a fight for food can affect a fish’s personality, what about even more stressful situations? Say, for instance, a male fish growing female reproductive organs. This is no joke: Thanks to endocrine disruptor chemicals (which mimic natural hormones and are found in products ranging from birth control pills to some deodorants and toothpastes) being released into waterways, these so-called intersex fish have been popping up around the globe, from the Japan to our very own Potomac River.

If all of these changes are causing anxiety levels to rise among the aquatic community, we could always just pipe a little Xanax into the water.

Story by Alisa Opar. This article originally appeared in Plenty in November 2006. The story was added to in July 2009.

Copyright Environ Press 2006.