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 affect 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 by many organisms, 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. The ecological functions and evolution of pheromones are a major topic of research in the field of chemical ecology.
A fanning honeybee exposes Nasonov's gland (white – at tip of abdomen) releasing pheromone to entice swarm into an empty hive
Aggregation of bug nymphs
Aggregation of the water springtail Podura aquatica
Male Danaus chrysippus showing the pheromone pouch and brush-like organ in Kerala, India
In animal communication, an alarm signal is an antipredator adaptation in the form of signals emitted by social animals in response to danger. Many primates and birds have elaborate alarm calls for warning conspecifics of approaching predators. For example, the alarm call of the blackbird is a familiar sound in many gardens. Other animals, like fish and insects, may use non-auditory signals, such as chemical messages. Visual signs such as the white tail flashes of many deer have been suggested as alarm signals; they are less likely to be received by conspecifics, so have tended to be treated as a signal to the predator instead.
Alarm calls have been studied in many species, such as Belding's ground squirrels.
Vervet monkey in Dar es Salaam, Tanzania
Diana monkeys produce alarm calls that differ based on caller, threat type, and habitat.
Chimpanzees produce alarm calls in response to threats.