Animals make behavioral choices every day. But how do they prioritize one need over another—especially when those choices are both necessary for survival and reproduction? Researchers at the University of Birmingham used Drosophila melanogaster to study this behavioral conflict. They found that sex- and food-deprived male flies prioritize feeding over courtship initiation. They also showed that this decision can be fine-tuned by changes to the quality of the food or the animal’s internal state and pinpoint the neuronal impulses triggered in flies’ brains when faced with the critical choices of feeding or mating.

The study, published in Current Biology, is titled, “A neuronal mechanism controlling the choice between feeding and sexual behaviors in Drosophila.

“We are often exposed to conflicting situations where we must prioritize one goal over others. For an animal in nature this could mean having to choose between feeding, mating, or fighting for resources,” noted Carolina Rezaval, PhD, group leader at the University of Birmingham. “How does the animal know what to do? The fruit fly Drosophila is a great experimental system to understand how crucial behavioral decisions are made in the brain. We can identify neural elements that direct behaviors with great resolution and decipher the underlying mechanisms.”

In the study, male flies were kept away from both food and females, and then offered a choice of both. The team discovered that mating was consistently overridden by hunger in flies that were starved, with the behavioral tipping point occurring after about 15 hours of starvation. Once fed, the researchers found the male flies turned their attention to courtship—sometimes within just a few seconds.

The team then used genetic tools to label neurons in the brain with fluorescent markers. They further switched on or off a small number of neurons and tested the effects on behavior. With these tools, they asked how the fly brain responds when there are conflicting options available, and how it chooses amongst them. Using two-photon calcium imaging, the researchers monitored the neurons in the brain of live flies. This enabled them to pinpoint activated neurons in the flies’ brains as they made decisions about what to prioritize.

More specifically, the team identified the tyramine signaling pathway as an essential mediator of this decision. They wrote that, “Tyramine biosynthesis is regulated by the fly’s nutritional state and acts as a satiety signal, favoring courtship over feeding. Tyramine inhibits a subset of feeding-promoting tyramine receptor (TyrR)-expressing neurons and activates P1 neurons, a known command center for courtship. Conversely, the perception of a nutritious food source activates TyrR neurons and inhibits P1 neurons. Therefore, TyrR and P1 neurons are oppositely modulated by starvation, via tyramine levels, and food availability.”

“The neurons that tell the fly to go and eat, or to go and mate, are essentially competing with each other,” explained Rezaval. “If the need to eat is most urgent, the feeding neurons will take over, if the threat of starvation is less, then the urge to reproduce will win.”

The researchers also found that the behavioral choice was not absolutely fixed and is affected by context. For example, although feeding was prioritized when the fly was low on energy, this decision could also be affected by the quality of the food, with flies rejecting bad food and choosing to mate, even when hungry.

Saloni Rose, a PhD student and an author on the study added: “We have so much more still to learn from the fruit fly, for example, what happens when other threats are introduced—how would the fly decide whether to feed or escape from a predator, or what would happen if a female fruit fly were confronted with similar choices? All these insights help us to build up a picture of complex decision-making in the brain.”

Graphical abstract for Cheriyamkunnel et al.