When an association of animals (or, by extension, people) is described as a "herd", that means that the group tends to act together (for example, all moving in the same direction at a given time), but that this does not occur as a result of planning or co-ordination. Rather, each individual is choosing behaviour that corresponds to that of the majority of other members, possibly through imitation or possibly because all are responding to the same external circumstances. A herd can be contrasted with a co-ordinated group where individuals have distinct roles. Many human groupings, such as an army detachments or sports teams, show such co-ordination and differentiation of roles, but so do some animal groupings such as those of eusocial insects, which are co-ordinated through pheromones and other forms of animal communication. Conversely, some human groupings may behave more like herds.