A chemical substance (see semiochemical) emitted by an organism into the environment as a specific signal to another organism, usually of the same species. Pheromones play an important role in the social behaviour of certain animals, especially insects and mammals, and are also released by the hyphae of fungi to establish compatibility between mating types. They are used to attract mates, to mark trails, define territories, and to promote social cohesion and coordination in colonies (see queen substance). Pheromones are usually volatile organic acids or alcohols and can be effective at minute concentrations. The volatility depends on the nature and size of the molecule, and determines the role played by the pheromone. For example, a highly volatile pheromone will disperse rapidly and widely and serve effectively as a transient alarm signal, whereas a less volatile, more persistent pheromone is useful for marking territories or trails.