It has all the trappings of a classic horror plot: a group of normally timid individuals are transformed by scientists into instinctive killers, programmed to pursue and sink their jaws into almost anything that crosses their path.

However, this hair-raising scenario was recently played out in a study of laboratory mice, designed to uncover the brain circuits behind the predatory instinct.

The research revealed that one set of neurons triggers the pursuit of prey, while another prompts the animal to clench its jaws and neck muscles to bite and kill. The study relied on the technique optogenetics, in which neurons can be artificially activated using light, effectively allowing scientists to switch the killer instinct on and off at will.

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When the laser was off in the experiment, the animals behaved normally, but at the flick of a switch they assumed qualities of “walkers” from The Walking Dead.

Ivan de Araujo, a psychiatry researcher at the Yale University School of Medicine and lead author said: “We’d turn the laser on and they’d jump on an object, hold it with their paws and intensively bite it as if they were trying to capture and kill it.”

In the study, the mice were seen to pursue almost anything in their path, including insects, robot insects and even inanimate objects such as bottle caps and wooden sticks.

However, De Araujo said the mice stopped short of displaying aggression towards fellow mice or the researchers, and seemed only to target objects that were smaller than themselves. “It had to be something that could be grabbed and contained, something they want to capture and subdue” he said. “It’s not that they got out of control and tried to kill everything. It had to be something that looks like food to them.”

In the study, published in the journal Cell, the scientists used a technique called optogenetics to pinpoint and eventually take control of the neuronal circuits involved in predation. The mice were genetically engineered so that specific groups of neurons were light-sensitive, meaning that these could be switched on and off by shining a laser into the mouse brain.

The scientists identified two separate clusters of neurons in the central amygdala, a brain area normally linked to emotion and motivation. These were shown to be communicating with other neurons in two motor areas – in one case, a region linked to the ability to run and change speed and, in the other, a region known to control jaw and neck movements.

In real life sensory cues, such as a small animal scurrying across the predator’s field of view, would trigger activity in the amygdala setting off this “chase and kill” neuronal chain of command, the scientists said.

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In the experiment, they were able to bypass the usual sensory requirements and could trigger the predation circuits artificially using lasers.

They found that the two clusters operated independently: if they only activated the “hunting” neurons, the animals would chase the prey, but the biting force of the jaw was decreased by 50%. “They fail to deliver the killing bite,” said De Araujo.



When they only activated the “biting” neurons, mice in empty cages would display “fictive feeding” behaviours, raising their paws as though they were chomping on something.

Hunger had a powerful influence on predatory behaviour – hungry mice were much more aggressive in their pursuit. “The system is not just generalised aggression,” said De Araujo. “It seems to be related to the animal’s interest in obtaining food.”

The same circuits are thought to be closely conserved in the human brain, although De Araujo said that the circuitry is more likely to be associated with our drive to find food, rather than with anger or the instinct to attack.

“My take on this is that predatory behaviour is more related to food intake itself,” he said. “I would be a little hesitant to associate this with aggression.”

Professor Candy Rowe, a zoologist at the University of Newcastle, said that the research provided a valuable insight into predation, although said it was unsurprising that mice have effective predation circuitry in the brain. “Mice are often represented as prey - take Tom and Jerry as an example - but in fact they are predators themselves, particularly of invertebrates. At this time of year, that might include worms or hibernating butterflies,” she said.

Rowe added that in future, scientists hope to gain a better understanding of what sensory information triggers pursuit and capture behaviours, and how prey might evolve strategies to evade this.

“There may be a tussle between predators and prey, with prey being selected to change their appearance so that they don’t trigger these predatory responses, and with predators being selected to have neural circuits that help them track changes in prey appearance and select the right objects in their environment to chase and capture,” she said.