Scientists studied why similar species of mice either run or freeze when faced with the same predator.
Introduction
You might have heard of the fight or flight response. Do you run away? Do you freeze? Do you fight? This is your instinctive response to a threat. An organism needs to respond correctly or they might die. Natural selection is the process that shapes how these responses evolve. Organisms that respond correctly to threats will survive and reproduce.
Responding to a threat is complex. First, an organism needs to sense a threat. Then information about the threat travels through neurons to the brain. The brain’s escape center receives the information. It processes the information and tells the rest of the body how to respond. But we don’t know much about what happens in the escape center during a threat.
Mice are good model organisms to help us study these threat responses. Mice are frequently hunted by predators. They also live in different environments. Plus, they don’t all react the same way to a threat. Some mice escape and run away the moment they detect a predator. Other mice freeze. They stay perfectly still until the threat level increases – and then they run away.
We wanted to know what was happening in the brains of these mice. How is the brain responsible for differences in behavior? And how does this relate to survival and natural selection? Mouse physiology is very similar to humans. So by studying how mice brains respond to threats, we can learn how ours respond too!
Methods
We worked with two closely related mice species.
Deer mice (Peromyscus maniculatus): they often live in dense vegetation and forests.
Oldfield mice (Peromyscus polionotus): they often live in open areas, including on beaches.
We compared their behavior and brain activity in response to a threat. We put the mice in an arena one at a time. Then we exposed them to a visual representation of a predator. This mimicked a bird swooping toward the mouse and then looming above them. In some experiments we made the shadow of the fake predator lighter or darker. The darker the shadow, the greater the threat.
To examine which parts of the brain are related to behavior, we did three things:
We looked at the amount and location of a specific protein in the brain. The brain produces this protein when neurons are active. To do this we took tissue samples from the brains of each mouse after the behavioral experiments.
We recorded neuron activity in the brain while mice were shown the fake predator. We used a special in vivo system with a separate design that restricted the mouse’s movement.
We turned on the escape center of the brain by modifying the neurons there to activate in response to light (Fig.1). We then turned off the escape center by modifying the neurons in a different way so they wouldn’t respond at all. We thought that by turning it on or off we could make our mice species either run away or freeze. It could also confirm that this area of the brain caused the behavior in our mice.
Results
Both mouse species froze before running away when the threat was low (light shadow). When the threat was high (dark shadow), more mice of both species escaped. We saw that deer mice had a lower threat threshold than oldfield mice. For example, 24 out of 25 deer mice ran away at a medium threat level. In contrast, only 6 out of 28 oldfield mice did.
We also saw different protein levels in the escape center of the brain (Fig. 2). Deer mice had more protein in this area than oldfield mice. We also observed that the greater the neural activity in this area, the faster the deer mice ran.
When we turned on the escape center, deer mice ran away immediately. But oldfield mice slowed down and froze. When we turned off the escape center, deer mice froze just like oldfield mice.
Discussion
These two types of mice react very differently to the same threat. Deer mice run away because of their low threat threshold. Their escape center switches on easily. We found that the escape center in oldfield mice doesn’t work the same way. They take longer to run when they see the same threat. Even when we turned on the escape center, they still froze! This may mean other parts of the brain are also involved in their response to threats.
Differences in escape behavior are often related to how organisms detect threats. For example, how well an organism can see or smell predators. These senses are controlled by peripheral neurons outside of the brain. Our research shows a rare example of behavior directly connected to neurons deep in the brain.
Natural selection may be responsible for these differences in escape behavior. Running away might help deer mice survive because they live in places with many hiding spots. In contrast, oldfield mice live in open areas. So, freezing may help them not be seen and survive.
Conclusion
We can learn a lot about human behavior from these mice. People have a similar complex network of regions working together to process threats and decide what to do. They kick in when we face challenges, like a big test or a tough conversation. Understanding this can help us notice our reactions. We can then choose how to respond instead of reacting automatically. When you feel stressed or scared, first take a deep breath. And remind yourself that your brain is trying to protect you!
