Some birds trick their way into mating by removing testosterone from their bodies

Picture this: You’re a male bird trying to get lucky. Traditional wisdom says pump up those testosterone levels, fight off the competition, and strut your stuff. But some clever birds have figured out a totally different game plan – one that involves producing more testosterone than their macho counterparts, only to destroy it moments later.

Researchers from the Max Planck Institute for Biological Intelligence, in collaboration with international colleagues, have discovered that these birds produce a super enzyme that rapidly breaks down testosterone.

The findings reveal that a single gene drives this process and that it happens in the blood, a tissue previously overlooked in hormone regulation.

Role of testosterone in male behavior

Testosterone is a key hormone in male physiology. It plays an essential role in sexual development, aggression, and reproductive behavior.

Most testosterone is produced in the testes, where it supports sperm production and influences secondary sexual characteristics. The blood then carries the hormone to other parts of the body, including the brain, where it affects aggression and courtship behavior.

Traditionally, scientists have viewed high testosterone levels as advantageous for male animals, helping them compete for mates and establish dominance. However, recent research led by Clemens Küpper challenges this notion in ruffs.

The researchers found that some males actively eliminate testosterone from their bloodstream, raising an important question: why would a sexually competitive bird species reduce a hormone typically linked to mating success?

Three morphs with distinct mating strategies

Male ruffs are unique because they exist in three different morphs, each with a distinct appearance and reproductive strategy. The majority of males belong to the “Independent” morph, characterized by darker plumage and territorial behavior.

These males compete aggressively in mating arenas known as leks, where they defend small territories and display for females. Their high testosterone levels support this assertive and dominant approach.

The second morph, known as “Satellites,” is slightly smaller and has lighter-colored plumage. Unlike Independents, Satellites do not defend territories. Instead, they form alliances with Independents, peacefully positioning themselves within the lek to increase their chances of attracting a mate.

Their mating success does not depend on aggression, making high testosterone levels unnecessary.

The rarest morph, “Faeders,” takes an entirely different approach. These males mimic females in both size and appearance, allowing them to slip into the lek unnoticed.

Disguised as females, they avoid direct competition while still managing to mate. For both Satellites and Faeders, excess testosterone could be a disadvantage, as it promotes aggressive behaviors that conflict with their alternative strategies.

Basis of testosterone regulation in birds

Until recently, little was known about the genetic mechanisms that regulate testosterone in animals.

The study of ruffs has provided key insights, as the three male morphs differ due to a single “supergene” – a section of DNA containing approximately 100 genes.

This genetic variation emerged around four million years ago when a chromosome fragment broke off and reinserted itself in the opposite direction, leading to the evolution of distinct morphs.

As researchers analyzed gene expression, they identified a crucial gene within the supergene that codes for an enzyme responsible for breaking down testosterone. This enzyme was produced in high quantities in Satellites and Faeders but was absent in Independents.

Surprisingly, when the team measured testosterone levels in the testes, they found that Satellites and Faeders actually produced more testosterone there than Independents.

This finding raised another question: if these males produce more testosterone in their testes, how do they maintain significantly lower levels of the hormone in their blood?

Super enzyme in the blood

The answer lies in an exceptionally powerful enzyme – what researchers call a “super enzyme” – that rapidly degrades testosterone.

This enzyme is present in large amounts in the blood of Satellites and Faeders, breaking down the hormone before it can circulate through the body.

In contrast, Independents completely lack this enzyme in their bloodstream, allowing them to maintain high testosterone levels that fuel their aggressive territorial behavior.

Birds breaking down testosterone

Before this discovery, scientists assumed that testosterone breakdown occurred primarily in the liver and other organs, with little to no regulation occurring directly in the blood.

“Based on previous research in other animals, we had assumed that this enzyme plays no role in the blood,” explained Alex Zemella, one of the first authors of the study from Max Planck Institute for Biological Intelligence.

“But our results revealed a previously unknown site there that the enzyme acts on. This means that the amount of circulating testosterone can be regulated directly in the blood – something that had previously been overlooked.”

A single gene shapes complex behavior

This discovery highlights the sophisticated strategies male ruffs have evolved to maximize their reproductive success. Instead of relying solely on aggression, Satellites and Faeders have adapted a biochemical mechanism to regulate their behavior in a way that benefits their unique mating tactics.

“All males need testosterone in their testes for producing sperm,” noted Jasmine Loveland, a lead author of the study.

“But the hormone also has other effects, for example in the brain, where it can promote aggressive and courtship behavior. It is interesting that the non-aggressive ruffs seem to have increased levels of the super enzyme throughout the brain too, especially in the hypothalamus.”

“These results, along with the low testosterone levels in the blood, likely play an important role in shaping their mating strategies.”

The ability to control testosterone levels so precisely allows Satellites and Faeders to optimize their reproductive success without the drawbacks of excessive aggression.

By modifying the way testosterone circulates in the body, they have developed a more subtle but equally effective way to compete for mates.

Future research on birds and testosterone

The finding that a single gene can significantly influence testosterone regulation opens up new possibilities for research. Understanding how hormone levels shape complex social behaviors in ruffs may provide broader insights into other species, including mammals.

Going forward, the research team aims to explore how these genetic mechanisms affect brain function and social interactions in ruffs. They also plan to investigate how similar processes might influence reproductive strategies in other animals.

By continuing to study the genetic basis of behavioral diversity, scientists hope to uncover new details about the intricate relationship between hormones, genes, and mating success.

This study challenges conventional wisdom about testosterone and aggression, showing that high testosterone is not always an advantage.

In ruffs, a well-adapted enzyme allows some males to take a completely different route to reproductive success – proving that in evolution, strategy matters just as much as strength.

The study is published in the journal Science.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–