Evading predators is more complex than 'run away!'

Animals’ escape response, the decision about where and how to escape from a looming threat, including predators, is more nuanced than previously thought, researchers report.

“Gauging an appropriate response to stimuli is a fundamental job of the brain in all animals, including humans…”

In a study of larval zebrafish, the researchers are the first to find that the animal’s innate escape response incorporates the speed of the approaching predator—the urgency of the threat—and not just the proximity of the predator in its calculation of how best to flee.

Prior to the new research, the escape behavior was thought to be driven by a proximity threshold where anything that gets within a certain distance triggers an escape. The researchers, however, found that at slower approach rates by a predator, the larval zebrafish’s fastest escape circuit is not deployed; instead, a different circuit produces a more delayed and variable escape behavior.

By attributing prey’s neural escape response to the predator’s velocity as well as proximity of approach, the research team has uncovered new information that can help scientists understand the neural mechanics that fuel the most elemental self-preservation instincts.

“A potential problem with basing the prey’s escape decision solely on the predator’s proximity is that it does not distinguish between predators approaching rapidly and those approaching slowly,” says Malcolm A. MacIver, professor of biomedical engineering and of mechanical engineering in Northwestern University’s McCormick School of Engineering.

“Our work contributes to understanding a fundamental tradeoff within neural systems: whether to rapidly initiate a canned, inflexible behavior that is more predictable or to delay a response and compute a more variable behavior that will be harder to predict.”

To study the neural underpinnings of the escape response, MacIver, David L. McLean, and biomedical engineering doctoral candidate Kiran Bhattacharyya chose the larval zebrafish.

“The larval zebrafish is transparent, which allows us to image the activity of whole groups of neurons with volumetric imaging techniques and monitor animal behavior at the same time,” says Bhattacharyya, first author of the paper.

“We can watch the brain light up with activity as the animal behaves,” says McLean, an associate professor of neurobiology at Northwestern University. “Studying a model organism such as the zebrafish helps us understand how the brain generates a diversity of behaviors. Gauging an appropriate response to stimuli is a fundamental job of the brain in all animals, including humans, and it is something we want to understand.”

“It seems that the animal is assessing risk, and if the approaching predator’s velocity passes a certain level, then the prey gets out of Dodge as fast as it can,” MacIver says. “If a predator is coming more slowly, the prey has more options and more time to decide between the options.”

Why predators and prey should recover together

Using multi-neuron imaging while simultaneously recording high-speed video of the escape behavior, the researchers have shown that the rate of approach of a threat sets the probability that a special high-speed escape mechanism is deployed (fired by special neurons called Mauthner cells). As the predator’s approach rate increases, so too does the probability of deploying this special escape mechanism.

The advantage of the special escape mechanism is that responses occur as fast as possible, but a disadvantage is that the movement is highly predictable, which allows certain predators to “hack” the circuit and trick prey into launching themselves straight into the predator’s mouth. At lower approach rates, the special escape circuit is not deployed (Mauthner cells do not fire), and a more variable, although delayed, escape behavior ensues.

“Our findings suggest these simple fish are calibrating their response to the perceived risk of the threat,” McLean says.

“Our own brain evolved from fish to weigh numerous variables before we act. Now that we know what fish are paying attention to, we can begin to explore the neural computations that govern this fundamental process,” he says.

While the special high-speed escape circuit that fish and amphibians deploy for urgent threats disappears in fully terrestrial animals like reptiles, birds, and mammals, the alternative high-variability circuit is preserved, MacIver says. As animals emerged from the water and started inhabiting land, their visual range increased dramatically, allowing the expression of more variable behaviors using this alternate circuitry.