Chemicals produced and emitted by an individual which produce changes in the social or sexual behaviour when perceived by other individuals of the same species. The precise role of these odours – for it is by their smell that they are recognised – in humans is still not clear, but there is growing evidence of the part they play in the animal kingdom. Thus, if a strange male rat is put into a group of female rats, this may cause death of the fetus in any pregnant rats, and this is attributed to the pheromones emitted by the male rat.