Perspective: Hormones and the ‘language of love’

How do you show love to someone? Maybe you give them a hug or a kiss, or perhaps you buy them a special gift? Maybe you listen to them when they have something they want to say, or perhaps you affirm their feelings and perceptions?

There are so many complex ways that we can show others that we love and care for them. But humans are not the only animals that are adept at expressing their “love.” You need only to turn on a nature documentary to discover the many complex ways that animals might say “I think I love you!” to another. In fact, you will likely find that some of the modes by which certain animals communicate these feelings are nothing less than downright bizarre, with animals behaving in ways that seem utterly inexplicable to us.

One of my favorite examples is the western grebe, a species of water birds. Males and females decide whether they are going to breed by performing a dance called “rushing.” Basically, the pair runs in synch across the surface of a lake, literally running on water!



This spectacular display is not only incredible to watch because of its sheer beauty, but it is also an incredible mechanical feat (a grebe weighs about as much as a bottle of ketchup — can you imagine something that size moving so fast that it can stand atop a body of water?).

Another great example is the Japanese pufferfish. When males look to woo females, they spend their time creating large, ornate circles in the sand. These structures are sometimes called crop-circles.



Keep in mind that circles are roughly 20 times larger than the fish who makes them, and they can take up to six weeks to build on the sea floor. Females fastidiously inspect these circles, as they prepare to spawn. What a curious way to ask: “do you love me?”

My research seeks to understand evolutionary origins of this unusual “love language.” However, I approach the topic somewhat differently than most other scientists, because I’m interested in more than why these peculiar behaviors evolve, but rather how they arise. Think about it this way: you might stand to impress your love interest by running across the surface of water, but you have no way of actually doing it. Therefore, if this behavior is going to evolve, then our body must similarly undergo evolution supportive mechanisms. My work addresses what this process looks like, and how the brain and body can change through the long course of time to support crazy mating displays.

One species I work with is called the golden-collared manakin, a small bird that lives in the rainforests of Panama and Colombia. Males court females by creating a small arena on the forest floor and then jumping among twigs that line the edges of this court. The birds move so fast that our own eyes see little more than a flash of gold and black feathers rush by. Even more intriguing is that during each of these jumps, the male bird will take his wings and rapidly hit them together in mid-air above his back. This gesture produces a loud firecracker-like snap that echoes throughout the entire forest. Here’s a video of a male preforming a half-hearted dance:



Note that the action begins to pick up about 50 seconds into the video, but listen to all the loud snapping sounds in the time leading up to this moment — those are other manakins in the vicinity that are hitting their wings together, sometimes in rapid succession!

My work shows that the skeletal muscles that control these fast, snapping movements of the wings have evolved to work at “superfast” speeds. In fact, the main muscle that governs this behavior is one of the fastest — if not the fastest — vertebrate limb muscles on record. At the same time, my work also points to one critical thing that dials up this bird’s ability to generate his rapid love language: the hormone testosterone. The fast-wing muscle that help males produce the snaps that females want to hear is teaming with receptors, which are proteins that grab onto testosterone in the blood. When this happens, the muscle goes from being normal to incredible! The reason is that testosterone influences how different genes are expressed within the cells that make up the entire muscle. This changes how muscle works by redefining what it can do, and what it cannot do. One of the major mysteries is how testosterone can make a muscle change as much as it seems to in golden-collared manakin — going from “normal” to “superfast” is a big step.

You might be asking yourself, “is it common for testosterone to change how an animal’s muscles control what it does (or how it might dance)?” I think the answer is “yes.” I have studied the connection between testosterone and motor control in several species, and whenever a new unusual display emerges to help individuals communicate in the context of “romance,” testosterone and its ability to act through muscle seems to be involved. So, I think that when evolution favors such a display, it exploits the ability of testosterone to redefine an animal’s underlying physiology. In doing so, evolution can slowly “create” mechanisms that help birds get faster and faster (remember, males that display more rapidly are more likely to mate and pass on their genes).

So, while we might show our love by physical touch or buying a gift, other animals like the golden-collared manakins do so through extraordinary dance. And testosterone appears to be the key in this incredible process.

Matthew Fuxjager is an associate professor of ecology and evolutionary biology at Brown University who is affiliated with the Carney Institute for Brain Science. He is also the co-director of graduate studies for the Department of Ecology and Evolutionary Biology. Fuxjager’s work focuses on the physiological and evolutionary basis of animal behavior.