What is a secreted or excreted chemical factor that triggers a social response in members of the same species?

A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to impact the behavior of the receiving individuals. There are alarm pheromones, food trail pheromones, sex pheromones, and many others that affect behavior or physiology. Pheromones are used from basic unicellular prokaryotes to complex multicellular eukaryotes. Their use among insects has been particularly well documented. In addition, some vertebrates, plants and ciliates communicate by using pheromones.

Olfactory processing of chemical signals like pheromones has evolved in all animal phyla and thus is the oldest phylogenetic receptive system shared by all organisms including bacteria. It has been suggested that it serves survival by generating appropriate behavioral responses to the signals of threat, sex and dominance status among members of the same species.

Furthermore, it has been suggested that in the evolution of unicellular prokaryotes to multicellular eukaryotes, primordial pheromone signaling between individuals may have evolved to paracrine and endocrine signaling within individual organisms.

Some authors assume that approach-avoidance reactions in animals, elicited by chemical cues, form the phylogenetic basis for the experience of emotions in humans.