For decades, it has been taken for granted that insectivorous bats navigate in darkness using their ultrasonic calls. The fact that they have retained eyes through evolution was explained by their need to orient over long distances.

But now, an international study shows that the common noctule (Nyctalus noctula) also actively uses vision during the hunt – and catches insects much faster when there is light.

A research team led by biologist Laura Stidsholt, associate professor at Department of Biology at Aarhus University, equipped wild noctule bats with miniature sensors and showed that the animals integrate sight and hearing while hunting. The result: they capture insects significantly faster in light than in darkness. The study has just been published in the prestigious journal PNAS.

Tiny backpacks on flying hunters

The researchers fitted electronic “backpacks” on 21 common noctules. The backpacks recorded light, sound and movement down to the smallest wingbeat while the bats hunted insects over Berlin.

In dark conditions, the bats relied on their classic ultrasonic calls – high-pitched sounds that create echoes and reveal the prey’s position. But as soon as there was light, their tactics changed: they called only half as often, but more loudly, and at the same time accelerated. In darkness, they flew at an average of 5.2 metres per second. In light, they raced ahead at nearly 8 metres per second.

In other words: when bats can both hear and see, they receive more reliable information from their surroundings – and can catch their prey more quickly.

“It’s just like us: if we want to cross a road, we solve the task better by using both sight and hearing. If we cannot hear, we may double-check and make absolutely sure before crossing – but if we can also hear, we can better estimate whether we can make it across or not,” explains Laura Stidsholt.

Vision plays a bigger role than assumed

That nocturnal bats are able to see is not in itself news. As early as the 1960s, scientists discovered that they use their eyes to navigate over longer distances. But researchers assumed that insectivorous species left the actual hunting almost entirely to their ears – because they hunt in darkness, where eyes were thought to provide little useful information.

“However, many bats actually take off before the sun has completely set, and in the hours after sunset they can see quite well. They can also make use of streetlamps and other artificial light sources. This means that they often have two sets of information to hunt with,” says Laura Stidsholt.

An eye-opener

For Laura Stidsholt and her colleagues, the discovery is quite literally an eye-opener.

“This is the first time it has been documented that bats use multiple senses – both vision and echolocation – for hunting. In fact, it is also the first time it has been proven that a predator depends on multisensory information while hunting. Of course, we assume that a shark, for example, most likely both smells and sees its prey at the same time, but it is very difficult to prove this in a wild predator,” says Laura Stidsholt.

She emphasizes that the research team has so far only studied one bat species.

“But it is a very common species in Europe, and it is logical to assume that other insect-eating bat species also use their eyes,” she says.