We often see attacks from predators in nature documentaries, such as lionesses hunting gazelles or sharks attacking huge schools of fish. In both cases, the threatened animals coordinate their movements, remain united as a collective and they flee in the same direction, but how do they do it?

Until now, most researchers assumed that social animals need to follow visual cues to guide your collective movement and avoid predators, but the design of these signals was unknown.

Now, an investigation published in the magazine Proceedings of the Royal Society B and led by Spanish researchers from the Doñana Biological Station of the Higher Center for Scientific Research (EBD-CSIC) explains that most flocks of birds, groups of mammals and schools of fish avoid their predators with stripes or stripes on their body that indicate to the rest of the group in which direction to flee.

To do the study, the researchers looked at about 800 species divided into four groups of animals in which there were mammals -mainly ruminants such as gazelles, zebras or giraffes-, waders (marsh), geese like ducks and geese -of the same phylogenetic group-, and reef fish coral.

“Each type of animal has a different signaling pattern: stripes, stripes … and using the comparative method we analyze what they have in common,” the researcher and former director of the EBD-CSIC, Juan José Negro, explained in statements to Efe.

The analyzes found that the lateral body bands are more frequent in social species and they are less present in solitary species or less vulnerable to predation, such as geese or swans, which, being large, do not have predators such as small ducks that are the target of hawks.

As indicator arrows

The researchers concluded that these body patterns are a communication mechanism which serves individuals to form compact groups and take the same direction without causing chain collisions and also to confuse predators with moving bands.

“We believe that the stripes are like arrows that indicate the direction of movement And this is so that when one of them has to pull the others – because it has detected a predator or because it has decided to go in one direction – the others follow, “he adds.

And it is that, in social groups, survival is based on group unity: “When a gazelle separates itself from the rest or isolates itself, it is dead. That is why the visual signals are not oriented in any way, but show where the group should move, where it has to go to flee.”

Exceptions

However, although the research raises this hypothesis with a comparative (statistical) method, there are also exceptions because not all social species have stripes, nor do solitary species lack them.

“We know that there are very gregarious species that do not have stripes like the common starling, for example, which is grouped in thousands and even in groups of a million individuals, and they do not have stripes. Some researchers have proposed that these birds – which form flocks very compact- they use the beams of light between individuals as signals. “

Other striped animals such as skunks waves wasps, for example, they have those signals for the opposite: “They pretend to be very visible to predators, which is known as aposematism for be associated with danger and avoid attacks “, the biologist points out.

Black on white

The study also determined that the stripes are usually achromatic, located on the scale from white to black, and without colors, because this allows “this mechanism to be used even by species that do not see in color. “

Using dark stripes on a light background allows species with monochromatic or dichromatic vision to take advantage of this “simple and universal mechanism” which makes it possible for all species to use it even in variable light conditions, such as sunrise or sunset, when the colors are barely distinguishable, the research concludes.