Types of pheromones
Alarm pheromones, as the name suggests send SOS signals to other members of the same species. Such compounds are released when a specific organism is being attacked by a predator, leading to trigger of either a fright or flight response. Surprisingly, plants have also been known to emit these compounds, when being preyed upon. This leads to production of tannins by adjacent plants, compounds that make the plant less edible for the preying organism.
Perhaps the most studied of the pheromones are sex pheromones that greatly influence the sexual behavior of an animal. These sex pheromones perform several functions ranging from sending signals about availability of a partner for mating to detecting a potential mate. Certain male organisms also release pheromones that communicate information about their species and genetic constitution. The presence of sex pheromones in animals as diverse as moths, mice and monkeys have been well documented.
Aggregation pheromones is the collective term given to chemical substances produced by one or other sex of a particular species, that attract individuals of both sexes. An unclassified set of pheromones (bees and other mammals) also exists, about which active research is being conducted at the moment.
Of special interest are the human pheromones, which are claimed to enhance the libido (sex drive) of an individual. Several companies even market human pheromones, believed to possess aphrodisiac properties. The number of studies available on human pheromones is however very limited.