What are pheromones? A pheromone is a substance emitted by one organism that elicits a behavioral or physiological response in another organism of the same species and (for most pheromones) of the opposite sex. (Ben-Ari, 1998) This response, in order for the substance to be classified as a pheromone should be innate and should not require specific learning. Pheromones are carried in the air and received via the vomeronasal organ (VNO from now on) in the nose. The VNO connects to the accessory olfactory bulb behind the nose, which leads to the amygdala (for emotion) and the thalamus (for distribution to the rest of the brain). This is completely different from the regular olfactory system, which goes through the main olfactory bulb. Pheromones are believed to have similar properties as odorants, which are the chemicals detected as odors.
The first pheromone found was a sex attractant in silkworm moths. This led to the belief that pheromones are only used on the opposite sex, which we now know to be false.
There are two types of pheromones: releaser and primer pheromones. Releaser pheromones are odors that are fast-acting and behavior-changing. They mostly function as alarm pheromones, although some organisms use them for sex as well. An example of a releaser pheromone is the scent released by aphids to tell other aphids to leave a particular leaf. Primer pheromones function much differently: they induce delayed, behavioral or psychological responses. An example of a primer pheromone is the one that causes the Lee-Boot effect: When female rats are grouped together without the presence of a male rat, they release pheromones that lengthen or stop their estrous cycles. Martha McClintock, a researcher who published an article in Nature in 1971 about pheromones, found that this effect also happens in human females. (Ben-Ari, 1998)
Controversy about Pheromones
There is still a lot of controversy about pheromones, mostly centered around whether they exist for humans. Recent findings suggest that there is a human VNO, although many researchers, including McClintock believe that the human VNO is vestigial. McClintock suggests that not all pheromonal signals are transmitted via the VNO, though, and not all signals transmitted via the VNO count as pheromones: garter snakes use earthworms' chemical signals to track them. McClintock defines pheromones as intraspecies, not interspecies.
George Preti and Charles Wysocki of the Monell Chemical Senses Center say that humans have two small pits on either side of the septum that fit the descriptions of openings to the VNO. Wysocki also says that there is no anatomical evidence to suggest that this VNO connects to the brain. Indeed, many animals have evolved out of VNOs: dolphins, whales, and birds all lack one. We are not sure if Old World primates, such as chimpanzees and gorillas, have functional vomeronasal systems, although these findings could prove essential to the human pheromone field of research. Both Wysocki and McClintock say that human pheromones probably don't go through the VNO, but through the main olfactory system or terminal nerve. (Ben-Ari, 1998) Wysocki is concerned the motivation to find the human VNO comes from commercial interests, as the pheromone industry stands greatly to gain from the results. He also says that most of the funding on pheromone research comes from the human pheromone industry, so the results may be heavily biased.
David Berliner, CEO of Pherin Pharmaceuticals, believes that humans have a functional VNO. He is trying to make synthetic compounds, which he calls "vomeropherins." He cites studies that show when certain vomeropherins are puffed onto the VNO, they generate responses that resemble responses of receptor neurons, as well as physiological responses, such as reduced heart rates. Pherin is currently developing products to relieve social phobia, hot flashes, and PMS. (Ben-Ari, 1998)
Evolutionary Reasons for Pheromones
What evolutionary benefit do pheromones provide? Here are a few examples of pheromonal mechanisms in animals that increase their evolutionary advantages:
- Rats
Female rats who coordinate ovulation times, and thus pregnancies, have healthier and heavier pups because rats pool their litters and take turns nursing each other's pups. This fine-tuning of ovulation provides an advantage to the male exuding the pheromones because it means that his offspring will be sturdier. It follows, therefore, that his genetic material has a better chance to be passed on.
- Hamsters
Female hamsters use pheromones from vaginal secretions to bring on mating behavior in male hamsters, which means that a male who has never mated before can still perform successfully on the first time. This is clearly an evolutionary advantage for the whole species because it increases the likelihood that hamsters will successfully mate, thus increasing the population and gene pool of all of the hamsters.
- Goldfish
Female goldfish release pheromones into the water that stimulate hormonal changes that boost sperm count and quality in male goldfish.
- Aphids
As mentioned earlier, aphids produce an alarm pheromone that causes other aphids to leave the plant. However, this pheromone also causes pea aphids to spawn offspring with wings that fly away from the host plant when they have reached maturity. This provides an essential evolutionary benefit to aphids since they can effect migration from a dangerous area with their pheromones.
- Humans
The synchrony that occurs in rats also occurs in females, although McClintock says that this synchrony is an "epiphenomenon--a fossilized remnant of some other, evolutionarily advantageous effect of human pheromones that can regulate the timing of ovulation or perhaps affect other factors related to reproduction." She emphasizes that this is one small piece of the phenomenon. (Ben-Ari, 1998)
Chemistry of Pheromones
Pheromones have only been seen so far in insects, vertebrates, and goldfish. For insects, says Wendell Roelofs, of Cornell University, pheromones of a single chemical consist of complex compounds that are not found elsewhere in nature, whereas pheromones of a mixture of chemicals have some very common compounds where the particular combination is what triggers the response. More than a thousand different sex-attractant pheromones have been identified for insects, one of which-- (Z)-7-dodecenyl acetate--is the same pheromone used in elephants. (Ben-Ari, 1998)
The best characterized pheromone for mammals is the one that controls reproductive behavior in pigs: androstenone. This pheromone causes sows to assumes the mating position, known as lordosis.
