RACHEL MARTIN, HOST:
If you're scared of mice, this next story is not going to help you get over that fear. We're going to hear about how lab mice can be turned into predatory killers with just the flip of a switch. NPR's Jon Hamilton reports on an experiment that let researchers take control of brain circuits that tell animals to hunt.
JON HAMILTON, BYLINE: Mice are often portrayed as meek creatures nibbling on crumbs, but Ivan de Araujo of Yale University says they have a dark side.
IVAN DE ARAUJO: There is, for example, one species of mouse that is known as the killer mouse that basically feeds on live prey including sometimes even other mice.
HAMILTON: And many mice kill and eat insects when they get the chance. De Araujo and a team of scientists found evidence that this hunting behavior was controlled by neurons in a part of the brain called the amygdala. De Araujo says the team used a technique called optogenetics to control the brain cells. The cells were triggered by light from a laser.
DE ARAUJO: When we stimulate these neurons, it is as if we were telling these neurons that there is a prey in front of the animal.
HAMILTON: The scientists put mice in a cage with a live cricket. Even without stimulation, the mice would eventually hunt and kill the insect. But De Araujo says when the laser went on, the rodents got serious.
DE ARAUJO: The animals become very efficient in hunting, so they pursue the prey faster, and they are more capable of capturing and killing the insects.
HAMILTON: Even more dramatic, though, is what happened when the scientists put an insect-like toy in the cage. When the laser was off, the mice would move away from the toy.
DE ARAUJO: But when the circuit is activated, then the animals intensively bite it and use their forepaws in an attempt to kill it.
HAMILTON: This behavior was just about hunting, the mice never attacked each other. De Araujo says the research helps explain how the brain evolved to hunt. The team located the hunting circuit in two sets of neurons - one that controlled muscles involved in pursuing prey, the other controlled jaw muscles. He says these circuits probably began evolving hundreds of millions of years ago, when the first animals with jaws started to appear.
DE ARAUJO: It must have influenced the way the brain is wired up in a major way.
HAMILTON: And now, De Araujo says, these hunting circuits exist in a wide range of animals including humans. The research appears in the journal Cell. Jon Hamilton, NPR News.
(SOUNDBITE OF KID LOCO'S "FLYIN' ON 747")