Mice that kill at the flip of a switch may reveal how hunting behavior evolved hundreds of millions of years ago.
The mice became aggressive predators when two sets of neurons in the amygdala were activated with laser light, a team reported Thursday in the journal Cell.
"The animals become very efficient in hunting," says Ivan de Araujo, an associate professor of psychiatry at Yale University and an associate fellow at The John B. Pierce Laboratory in New Haven. "They pursue the prey [a live cricket] faster and they are more capable of capturing and killing it."
Activating the neurons even caused the mice to attack inanimate objects, including sticks, bottle caps and an insectlike toy. "The animals intensively bite the toy and use their forepaws in an attempt to kill it," De Araujo says.
But the aggressive behavior is reserved for prey. Mice didn't attack each other, even when both sets of neurons were activated.
The results hint at how the brain changed hundreds of millions of years ago when the first animals with jaws began to appear. This new ability to pursue and kill prey "must have influenced the way the brain is wired up in a major way," De Araujo says.
Specifically, the brain needed to develop hunting circuits that would precisely coordinate the movements of a predator's jaw and neck. "This is a very complex and demanding task," De Araujo says.
The researchers expected to find these hunting circuits in mice because many mice kill and eat insects. And one species known as the killer mouse "basically feeds on live prey, including sometimes even other mice," De Araujo says.
Sure enough, the scientists found one set of neurons in the amygdala, a structure involved in emotion and motivation, that became active when a mouse was pursuing prey. They found a second set of neurons in the amygdala that became active when the animal was biting and killing.
Then the team used a technique called optogenetics to create mice in which both sets of neurons could be controlled using light from a laser. That gave the researchers "an on-off switch for either or both of the circuits," De Araujo says.
"When we stimulate [both sets of] neurons it is as if there is a prey in front of the animal," De Araujo says. "They assume the body posture and actions usually associated with real hunting."
Researchers have found evidence of similar hunting circuits in rats and other species, including humans, whose survival once depended on their ability to hunt and kill large animals.