Bigger, faster, but still outfoxed: how prey escape predators

Predators are typically larger, faster, and more powerful than the animals they hunt. Yet in nature, most attacks fail. A new study, published in the Proceedings of the National Academy of Sciences (PNAS), by researchers from the University of Amsterdam Institute for Biodiversity and Ecosystem Dynamics (IBED), asks: why do prey get away so often? The key, the researchers found, lies in something the original model overlooked: reaction times.

A game of tag

Cat chases mouse. Falcon pursues pigeon. Shark trails fish. Across land, air, and water, the same game plays out between predator and prey. Pursuit predation, where a predator actively chases a fleeing prey, is one of the most fundamental animal behaviors. 'It's something you see throughout the animal kingdom', says first author Lars Koopmans, a PhD candidate at IBED. 'From flies that weigh less than a gram to killer whales weighing thousands of kilograms.'

Benjamin Martin, Assistant Professor in IBED's Department of Theoretical and Computational Ecology and one of the paper's authors, likes it to a game of tag: 'Imagine playing tag with someone five times faster than you. How could you ever get away?'

But prey do escape, frequently. For decades, scientists have explained this using a simple idea: maneuverability. Because prey are smaller, they can often turn more sharply. A classic model, known as the turning gambit, proposes that a well-timed evasive turn allows prey to slip out of a predator's path, even if the predator is faster. The model even specifies exactly how much more maneuverable prey need to be for this to work. But in the half-century since this model was proposed, no one had tested whether its predictions hold across land, air, and water.

Head starts

The new study compiled data on animal traits such as body mass, speed, and turning ability, to test the model's predictions. The results revealed a mismatch between theory and reality. Across all environments, prey are generally not maneuverable enough to compensate for their speed disadvantage. Paradoxically, aquatic environments, where the model predicted predators should hold a huge advantage, turned out to have the lowest capture success in nature. Predators caught prey in only around one in ten attacks.

So, if not maneuverability, what explains how prey get away so often? The key, the researchers found, lies in something the original model overlooked: reaction times. No predator can respond instantaneously to a prey's evasive turn. Seeing, processing, and reacting all take time. While these delays are short – just a small fraction of a second – they can make a huge difference.

'It's this little head start, or benefit of starting to turn earlier, that gives prey enough space to evade', says Koopmans. 'In the water where animals are especially maneuverable, prey can even slip behind the predator before it realizes what has happened.'

This exceptional maneuverability has a simple physical explanation: water is roughly 1,000 times denser than air, giving aquatic animals something far more substantial to push against to generate a sharp turn.

Next steps

While the model offers a new explanation for how prey evade faster predators, more research is needed. For example, to avoid capture, the model predicts that aquatic prey must time their evasive turns within a narrow window of around 100 milliseconds, waiting until the predator is nearly within striking distance. But whether animals can actually achieve such precision is still unknown. To find out, researchers are now filming real predator-prey interactions between fish on coral reefs.

More broadly, the findings suggest that predator–prey interactions are shaped not only by physical performance, but also by the speed of perception and decision-making.

'The main focus has been that predator-prey interactions are just a biomechanical arms race where one goes faster, and the other one also tries to go faster to compensate', explains Koopmans. 'This changes the view – it's also a neural arms race.'

More information

• Article: The allometry of vertebrate pursuit predation.

Text: Institute for Biodiversity and Ecosystem Dynamics (IBED)
Image: Twan Stoffers (lead photo: Caribbean reef shark)