Many animals come together to find safety in numbers. In teeming herds, flocks, shoals and swarms, it’s harder for a predator to track and isolate any single individual. This is the standard view of animal groups, as espoused in countless nature documentaries. It rests on one big assumption—that the animals are in charge of their own behaviour. And that is not always true.

French scientists Nicolas Rode and Eva Lievens have found that two species of shrimp form swarms because they are being controlled by parasites in their bodies. Some of these need to get into birds to complete their life cycle. Others need to pass from one shrimp to another. They all achieve this by controlling their hosts and making