When the food ran out, evolution answered—not with strength, but with instinct.

Content warning

• Hunger
• Starvation

The short story

No one knew why the humans had disappeared. One day, the city roared and reeked of food – trash bins overflowing, sidewalks littered with crumbs, alleyways dripping with grease. The next, it was silent. The machines stopped, the lights blinked out. The food… vanished. The rats had lived with humans for so long, they barely remembered a time before them. Their maps were restaurant backdoors, their clocks set by the hum of refrigerators. They didn’t hunt. They didn’t need to forage. They waited – and they feasted.

But now? Now they starved.

In the belly of a rusted-out vending machine, a young female rat named Vira curled her tail tightly around her ribs. She hadn’t eaten in three days. Around her, the others grew weaker. Slower. Timid males paced around anxiously, sniffing the air and arguing over rumors of stale crackers in a distant food cart. None dared to venture far. The city had become a foreign land without its human heartbeat.

“We should go underground”, one male muttered.

“Hide and wait. The humans might return”, another added.

But Vira… she felt something else. A pressure. A pulse. It started as a chill in her bones, then spread like fire in her veins. Her stomach twisted – not in pain, but in focus. Her ears picked up sounds too small to name. Her nose caught scents no one else seemed to notice. The world slowed.

Vira didn’t feel fear. She felt ready.

Without a word, she darted into the open. The others gasped, but she was already gone – a blur in the half-light, skimming gutters, leaping drains, climbing walls like a shadow with claws. She didn’t know where she was going – but her hunger did. She remembered the stories of her ancestors that her grandma used to tell her. When starving, something in them… awakened. Especially in the females of the ancient tribe. And though no rat believed in the ancestors’ stories anymore, as they had been living in affluence, something had awoken in Vira.

Hours later, she returned. Her mouth was stained with berry pulp, her fur dusted in flour from a forgotten bakery sealed behind a warped door she had squeezed through. The males stared, wide-eyed.

“Where did you go?”, one asked.

“How did you know?”, whispered another.

She didn’t answer. Not because she was rude – but because she didn’t know. Not in words. Only in hunger. But her stomach was full now and what had awakened was gone. She didn’t dare to take another step onto the streets. Over the weeks, when the hunger came back, she taught others. Most males stayed behind, they feared the open streets, the echoing emptiness. For some reason what awakened in Vira was not sufficient to make them go out in the open. But the females? They followed. And in them, too, the fire began to stir. In hunger, they saw clearer. Smelled farther. Feared less. A new kind of rat rose from the silence of humankind’s fall – one that trusted their gut instinct. And Vira, the first of them, became legend. Not a queen, not a ruler. Just a name passed from nest to nest, whispered when the food runs dry and the wind smells wrong.

“Trust the hunger, be like Vira.”

This story was originally written in English and without any help of AI.

The paper

Börchers, S., Krieger, J. P., Maric, I., Carl, J., Abraham, M., Longo, F., Asker, M., Richard, J. E., & Skibicka, K. P. (2022). From an empty stomach to anxiolysis: Molecular and behavioral assessment of sex differences in the ghrelin axis of rats. Frontiers in Endocrinology, 13, 901669. https://doi.org/10.3389/fendo.2022.90166

Connection between story and paper

Scientists have long known that a hormone called ghrelin rises when we’re hungry. Ghrelin cannot only affect our appetite but also our emotions. The study behind this story looked at whether ghrelin might also help reduce anxiety - and whether it works differently in male and female rats. To test this, the behavior of hungry rats was tested in anxiety-related tasks, like exploring new spaces or open areas. One group of rats also received a synthetic form of ghrelin or a drug that blocks the effects of ghrelin. Researchers also compared how the ghrelin-related molecules in their brains, blood, stomachs, and livers differ. We found out that female rats were more sensitive to the calming (anxiolytic) effects of ghrelin than males. They also became more anxious when ghrelin was blocked. This might be because they naturally respond with higher ghrelin levels to hunger, and because brain areas that are involved in hunger and anxiety appear to be more sensitive to ghrelin in females. This is important to know because blocking our hunger hormone ghrelin is of big interest for drug development for weight management. Females might be more affected by side effects such as anxiety.

The story is illustrating the evolutionary explanation of why a hunger hormone makes rats less anxious: to survive. By using Vira as the hero in the story it also portrays the sex difference that was found in the study.

The author

Stina is a PhD Candidate in Neuroscience at the University of Gothenburg in Sweden. Her research revolves around the neurobiology of appetite regulation. She is passionate about researching sex differences and has also been an active science communicator since 2016.