The Male Fruit Fly's Mating Song May Lead To An Understanding Of Quick Decision Making In Humans

By Shweta Iyer on March 22, 2014 5:33 PM EDT

fruit fly
Scientists are hoping that the fruit fly's basic nervous system, and its mating song, can help us understand how we are able to make quick decisions. (Photo: John Tann, CC BY 2.0)

A male fruit fly's love song may not top the music charts anytime soon, but it surely helps him attract a female's attention. It also helps scientists understand how humans react to sudden changes, according to a new study. Researchers at Princeton University discovered that the mating song of a male fruit fly is based on changes in its environment. Analyzing the song's changing pitch could help researchers understand how humans and other, more evolved beings make quick decisions.

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The mating song of the fruit fly differs from birds, and even whales, because fruit flies do not use a set pattern or predictable sounds. Instead the tempo and tone of their song changes based on the movement of their desired female. These songs do not come out of their vocal chords, and instead, are a disordered series of loud purrs and soft drones produced from their wings' vibrations, which the fruit fly adjusts based on its distance from the female. The faster and further the female flies away from the male, the louder the song gets.

The researchers studied the fruit flies because their central nervous system contains all of the basic components of larger animals, including humans. They studied fruit fly behavior with regard to their songs in order to understand the neural pathways that are involved with animal behavior - specifically with responsive behaviors to sudden changes in the environment, such as a deer trying to escape a lion or a commuter navigating stop-and-go traffic.

Mala Murthy, an assistant professor of molecular biology at the Princeton Neuroscience Institute, and her colleagues, created a model that could predict a fly's choice of song in response to its changing environment, and identified the neural pathways involved in these decisions. "Here we have natural courtship behavior and we have this discovery that males are using information about their sensory environment in real time to shape their song. That makes the fly system a unique model to study decision-making in a natural context. You can imagine that if a fly can integrate visual information quickly to modulate his song, the way in which it does that is probably a very basic equivalent of how a more complicated animal solves a similar problem," she said in a statement. "To figure out at the level of individual neurons how flies perform sensory-motor integration will give us insight into how a mammalian brain does it and, ultimately, maybe how a human brain does it."

These findings corroborate a widely held belief that seemingly random behavior is not so random, and that there is a hidden pattern and predictability to it. For example, the researchers cited a 2010 experiment in which scientists found that a person's movements were highly predictable. For three months, they followed the movements of 50,000 people using cellphone data on where and how a person traveled (with the help of cell tower locations). They found that routes and destinations could be predicted 93 percent of the time.

Like these people's movements, the song of the fruit fly was also considered to be random, and disruptions, or "noise," in the brain's processes. But Murthy and her team show that these songs are deliberate and predictable reactions to the environment. "No one expected that male fruit flies might be fine-tuning their courtship signals based on what the female is doing in real time," Murthy said in the statement. "We were able to test that for the first time and discover that actually there's a very small number of sensory cues the male is using to shape his song structure. That overturns the canonical view that animal songs are variable simply because the nervous system is noisy."

To capture these mating sounds, the researchers constructed an octagonal chamber, covered in copper mesh and fitted with nine high-fidelity microphones. They recorded more than 100,000 song "bouts," or the seconds-long strings of vibrations males produce while trying to attract a female. They then placed sexually mature male and females in the chamber to record the sounds. The researchers genetically altered certain groups of flies so that some of their senses were inhibited. Some males could not see while some were mute. Some females were deaf, and nearly all females were blind and unaffected by pheromones. This was done to make them more responsive to a male's song.

They found that males produced louder sounds, called pulses, which sounded like purrs, while females were further away or moving quickly. When they were closer to the females, they reduced the pitch of their song to quieter buzzing sounds, called sines. But there are other factors involved too. The male changes its song when it can see the female. Blind males did not change their songs in response to female activity. Receptive females slowed down in response to the song while unreceptive females flew faster. Meanwhile, the response of both successful and unsuccessful males was the same - they changed the song in response to the female's movement.

Also, the researchers found a connection between movement of the fruit fly and his song, suggesting that the neural circuits guiding the male's movement also determine the pattern and intensity of their pulses or sines. One might assume that the male's visual neural circuits are at play because his song is a response to her movement, Murthy said. But when a male was recorded without a female present, they found that his song matched his movement. So, instead of the fly's many sensory circuits, such as vision, connecting to the circuits responsible for movement and singing, the singing was determined solely by the fly's movement. That means the fly's dance determines his song.

"These fly songs have a lot of variability. Each time the male produces a bout of song to a female, it's slightly different from the one he produced before," Murthy said. "He measures his distance to the female fly and he uses information about her speed, which translates into his speed because he's following her and chasing her. He's constantly integrating those two pieces of information to determine exactly how to pattern his song. That kind of variability makes flies an attractive model to try to understand how the sensory environment influences behavior."

Aravi Samuel , a Harvard University professor of neuroscience, commended the fly study, saying: "Neuroscience isn't just making electrical recordings of circuits or finding molecules that affect circuit properties. It also is about understanding the behavior itself, from sensory input to motor output. Understanding the computation the animal is making by studying the animal is the proper framework for future mechanistic studies."

These findings should be used to change our outlook on how organisms behave, Samuel said. In humans, the environment dictates our actions to a great extent and so we are not conscious about it. Samuel thinks that human behavior may be much more predictable than previously believed and that more observations are needed to prove this.

"These researchers were able to show that in fact what is unpredictable at first glance can really be thought of as almost a deterministic calculation, just one with several inputs," Samuel said in the statement. "We like to think of ourselves as something other than machines, and that our thoughts and actions aren't entirely prescribed by quantifiable inputs. But it's not impossible that we are unconscious of most of the inputs that we are processing. And if we are, we might be much more predictable and less capable of whimsy than we give ourselves credit for. If we could stand outside ourselves, and plot all the input variables that go into our behaviors, we might find ourselves more like the predictable fruit fly. This would be paradigm shifting."

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