Is an ant colony one animal?

Biologists call an ant colony a superorganism, because it can be viewed as one animal. A superorganism is a group of synergetically interacting organisms of the same species, most often a social unit of eusocial animals with highly specialised division of labour whose individuals cannot survive alone for long. Ants are the best-known example. Think of the workers as the cells and tissues of a body: each caste has a role that contributes to the whole colony and cannot survive outside it.

But there is no brain at the top. Despite the title of queen, she does not delegate tasks to the workers; the ants choose their tasks based on individual preference. Much of the coordination comes from signals between ants. A forager that finds food lays a pheromone trail on the way back to the nest, and the trail gets stronger as more ants tread it, signalling that food is still there. This kind of feedback is why ants are typically seen travelling in single file.

Jobs also follow age. A new worker spends her first days caring for the queen and the young, then moves on to digging and other nest work, and later to defending the nest and foraging. It has been suggested that foraging and defence go to older ants because those jobs are so deadly that they are an acceptable risk only for ants likely to die soon anyway.

So is the colony one animal? The superorganism idea says it can be viewed that way, and biologists widely regard the origin of superorganisms as a major evolutionary transition, like the origin of multicellular organisms. But the concept also raises the question of what is to be considered an individual.