Fruit Flies Choose Food Over Sex When Deprived of Both

Scientists identified competing neural pathways that control flies' decision making.
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A fruit fly
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Karen Kwon, Contributor

(Inside Science) -- Life is about making one choice after another. Should you have Italian for dinner or Vietnamese? Should you go camping with your friends this weekend or chill at home with your cat? 

Many choices aren't a matter of life or death, but sometimes they are, as was the case for the fruit flies that neuroscientist Carolina Rezaval and her team studied. When the team gave male fruit flies a choice between food and sex after denying them both for 24 hours, the flies chose sugar water over a potential mate. The team also discovered competing neural pathways that direct the flies to either eat or mate, based on how hungry they are.

Animals' decision-making is dictated by complex neural processes, said Rezaval, a researcher at the University of Birmingham in the U.K. To understand how an animal chooses between conflicting options, she thought studying a fruit fly brain would be a good entry point because fruit flies have only about 100,000 neurons while mammals have millions and humans billions, she said.

In addition to finding out that food- and sex-deprived male fruit flies choose eating over mating, the team also learned that this clear preference for food over sex happens more notably after 15 hours of starvation. Once fed, though, these fruit flies go after potential mates. 

And it turned out that the decision between food and sex is determined by two competing neural pathways controlled by a molecule called tyramine. By monitoring the neuron activities using fluorescent tags and microscopes, the team discovered that tyramine is produced when fruit flies are hungry. The molecule then activates the neuron that encourages eating. When the fly is full, its body stops producing tyramine, and then the courtship neuron is activated.

This doesn't mean that the flies won't mate when they're hungry, however. When there is no food in sight, the male fruit flies will mate even though they're hungry. The results were published in the journal Current Biology this month.

Though it is too early to talk about what the results would mean for human behavior studies, tyramine is thought to serve a similar function as the hormone norepinephrine, which controls certain aspects of sexual behaviors in mice, said Saloni Rose, a graduate student in Rezaval's lab and one of the authors of the study. This could mean that mammals may also regulate their eating and mating behaviors using similarly competing neural pathways.

William Ja, a behavioral neurobiologist at the Scripps Research Institute in Florida who was not involved in the study, said the research is "pretty cool" and "solid." And a study on fruit flies could pave the way for future research on more complicated organisms. For example, the gene responsible for circadian rhythm was first identified in fruit flies, and later research confirmed that both the gene and the mechanism behind the circadian clock were conserved across species, resulting in a shared Nobel Prize for the fruit fly researchers in 2017, said Ja.

Rezaval said her lab's next plan is to study how the female fruit flies behave when facing the same conflict. It is "important to do research in fruit flies … because it really opens venues to understand very complex questions that are difficult to tackle in mammalian organisms," she said.
 

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Karen Kwon is a science journalist based in the Washington, D.C. area and was an intern with Inside Science during the summer of 2021. She is also a graduate student in the Science, Health & Environmental Reporting Program (SHERP) at New York University. Originally from Seoul, Korea, she was a 2020 AAAS Mass Media Fellow at Scientific American and has a Ph.D. in chemistry. Follow her on Twitter @ykarenkwon.