Some of the coolest results are those which were discovered by mistake.

Researchers studying the effects of cortisol (a steroid hormone released in response to stressful situations) on fat storage in female mosquiotofish found a rather surprising result: exposing the females to high concentrations of cortisol made them manly!

Females exposed to high levels of this naturally occurring hormone began to look and act like males. Male mosquiotofish have a modified anal fin called a gonopodium that deposits sperm (think of it as a fish penis). When females were treated with high levels of cortisol their anal fins began to change shape, so that they began to look like male gonopodiums! What an unpleasant surprise that must have been to wake up to!

Not only do the females begin to look like males, they also begin to court females as though they were male! In the video below (created by the Knapp Lab that conducted the research), you’ll see how male-like the cortisol-treated females have become:

The authors suggest that this increase in maleness is caused by an enzyme called 11β-HSD that happens to have multiple functions. One function of this enzyme is to decrease inappropriately high levels of cortisol. This is important because prolonged, high levels of cortisol can have detrimental physiological effects. The same hormone that brings down cortisol levels is also associated with the production of 11-ketotestosterone (or 11-KT). 11-KT is an androgen (like testosterone is an androgen) in fish which is important in controlling male sexual traits, including male morphology and behavior.

It’s possible that females whose bodies were attempting to decrease dangerously high cortisol levels inadvertently amped up their 11-KT production, causing the females to look and act like males.

Hopefully this study will get researchers excited about looking more closely at interactions between stress, sex steroids, and sexual traits so we can determine the extent to which this result is applicable to other vertebrates.

A link to the manuscript can be found here.