One for all, all for one, and most for me? Good question, according to which criteria so many pack animals make their decisions - or who can tell them where to go. Cockroaches apparently rely on information from neighbors.
A cockroach rarely comes alone - where one of these unloved six-legged creatures runs across the ground, the numerous relatives are usually not far. But the group seems to be organized in a purely anarchistic way: people live together and side by side, without any claim to leadership from individuals. Foraging for food, finding hiding places – everyone takes care of that themselves. Unlike social insects, there is no right to subordinate everything to the well-being of the community.
A gathering of egocentric individualists then? Not necessarily - the animals really appreciate not simply going solitary paths. Clinging offers advantages: where others gather, there is probably something to eat, and the danger of ending up in a hungry robber's stomach is minimized with every companion that could serve as an alternative. On the other hand, conspecifics mean competition for food, and crowded sanctuaries have limited appeal to cockroaches.
In this way, a cockroach loses its wanderlust more and more, the more conspecifics have already gathered around it in the hiding place. For those outside the door, on the other hand, the fuller it is, the less interested they are in joining them. So the question remains: How do they base their decision to stay or to lace up their hiking boots after all?
Helps just check, decided Jean-Marc Amé and José Halloy from the Free University of Brussels and their colleagues. They sent young cockroaches, who were still completely inexperienced in all things in life, into the ring of a Petri dish, in which, depending on the experiment, they placed two or more equivalent plastic boxes as places of refuge. They also juggled with the number of animals: sometimes the shelters offered plenty of space for everyone, sometimes it just worked out, sometimes not everyone fit in even when they were very close together. Finally, after 24 hours, the researchers counted how many animals had gathered where for the siesta and developed general models from the data.
If there isn't enough space for everyone, it follows quite logically: All available sleeping places are filled, if you arrive late you have to stay outside. If there is more space than roaches, all alternative sanctuaries are filled equally - not one stuffed and the others sparsely filled, but in balanced proportions. If, on the other hand, everyone fits into one bed chamber, the second – and every other one – actually remains empty. Here chance alone decides which retreat is chosen.
Does everything sound very regular? It is: The researchers were able to express the possible variants with a simple model without any problems. When they also calculated the benefits for the animals, the picture was confirmed.
So without having explored the entire area, without having received detailed information from fellow species or precise instructions from a leader, the young cockroaches decide on the optimal solution that will bring them the greatest possible benefit solely on the basis of the companions they encounter. The only condition is that the animals remain flexible, i.e. can change their decision at any time.
Such behavior, which depends on the density of encountered colleagues, is already known from many animals and has been well studied in ants in particular. Interestingly, then, while in cockroaches it maximizes the welfare of an individual, the same mechanisms lead to the best welfare of all. Perhaps, the researchers speculate, their simple model applies well beyond the Scabworld.