Structure and Location of Pheromones
The general size of phermone molecules can be limited to about 5 to 20 carbons and a molecular weight between 80 and 300. This is because below 5 carbons and a molecular weight of 80, very few kinds of molecules can be manufactured and stored by glandular tissue. Above 5 carbons and a molecular weight of 80, the molecular diversity increases rapidly and so does the olfactory efficiency. Once you get above 20 carbons and a molecular weight of 300, the diversity becomes so great and the molecules are so big that they no longer are advantageous. They are also more expensive to make and transport and are less volatile. In general, most sex pheromones are larger than other pheromones. In insects, they have a molecular weight between 200 and 300 and most alarm substances are between 100 and 200. (Sociobiology: The Abridged Edition, 1980, 114)
Click here for a table of ant trail pheromones (Table taken from: Pheromone Communication in Social Insects, 1998, 17)
Pheromones, like visual communication, use single, individual messages as well as composite chemical messages. For example, in many insects and mammals, each endocrine gland produces a phermone with a different meaning. However, many times pheromones from different glands are secreted at that same time. Female wasps release attractant phermones from their head and sexual excitant pheromones from their abdomen at the same time. This can increase the effectiveness of the message or create an entirely new message. It is also possible for different substances with different meanings to be generated in the same gland. (Sociobiology: The Abridged Edition, 1980, 96)
Click here for a picture of ant gland location (Picture taken from: Pheromone Communication in Social Insects, 1998, 4)
Click here for a picture of wasp gland location (Picture taken from: Pheromone Communication in Social Insects, 1998, 4)
Click here for a picture of bee gland location (Picture taken from: Pheromone Communication in Social Insects, 1998, 4)
Click here for a picture of termite gland location (Picture taken from: Pheromone Communication in Social Insects, 1998, 4)
As mentioned in the previous section, organisms use special modes of enrichment during chemical communication. The ratio of the amount of pheromone emitted (Q) and the amount of phermone that is at the threshold concentration of the receiving animal (K) has been developed to help study phermones. Q is measured in molecules per unit of time and K is measure in molecules per unit volume. The time between pheromone discharge and the fade-out time of the pheromone can be determined using this ratio. The rate of information transfer can also be calculated. It can be increased by lowering the emission rate (Q) or raising the threshold concentration (K) or both. By doing this, the phermone has a shorter fade-out time and is more accurate in time and space. (Sociobiology: The Abridged Edition, 1980, 95)
However, in order to increase the active space, Q must be increased or K must be decreased or both. Decreasing K is much more efficient because it can be altered over many orders of magnitude by chemoreceptors. Increasing Q requires the organism to enormously increase pheromone production and capacity of gland reservoirs. A lower Q/K is a characteristic of alarm and trail systems. (Sociobiology: The Abridged Edition, 1980, 95)
Click here for a table of termite trail pheromone thresholds (Tables taken from: Pheromone Communication in Social Insects, 1998, 197-9)
Another way to shorten the duration of a pheromone signal is by enzymatic deactivation. For example, in honeybees, more than 95% of the trans-9-keto-2-decenoic acid fed to worker bees is converted to 9-ketodecanoic acid, 9-hydroxydecanoic acid, and 9-hydroxy-2-decenoic acid within 72 hours. All of these are inactive substances. (Sociobiology: The Abridged Edition, 1980, 95)
One important system in many mammals is chemoreception. Chemoreception is made up of all the systems that are used to sense chemicals in the environment. In humans, it is made up of the olfactory system, the vomeronasal organ (VNO) or Jacobson's organ, and the trigeminal nerve. The trigeminal nerve is a network that spreads over the front of the skull, tongue, and teeth. It is a hazardous chemical warning system that alerts us to the presence of ammonium hydroxide, making tears when we cut onions, or reporting the progress the dentist is making while drilling our teeth. ("Processing of odorous signals in humans", Brain Research Bulletin, 2001, 54:307-12)
Many studies have been done to show that the detection of pheromones by mammals and reptiles is located in the vomeronasal organ, which is located behind the nostrils. However, anatomists and other researchers don't think humans have a functional VNO, but they also aren't sure we need one to detect pheromones. Some recent studies have shown that there might be telltale pits just inside the nostrils which might detect pheromones, but they also might do nothing at all. This remains a very controversial topic in the research world. ("Identification of non-functional human VNO receptor genes provides evidence for vestigiality of the human VNO", Chemical Senses, 2001, 9:1167-74)
Click Here for a Picture of a Hamster Vomeronasal Organ (Picture taken from: http://neuro.fsu.edu/research/vomer.htm)
Click Here for a Picture of a Snake Vomeronasal Organ (Picture taken from: http://neuro.fsu.edu/research/vomer.htm)
Click Here for a Picture of a Human Vomeronasal Organ (Picture taken from: http://neuro.fsu.edu/research/vomer.htm)
The exact chemical nature of human pheromones is still unknown, but there have been more than a thousand sex-attractant pheromones identified for insect species. One mammal who's chemical identity of a pheromone that has been isolated is the Asian elephant. It is known as (Z)-7-dodecenyl acetate and is located in the female urine. It tells the males that the female is ready to mate. This pheromone actually has the same chemical makeup as a major sex pheromone used by moths. The identification of the exact chemical identity of human pheromones is getting closer. ("Pheromones: what's in a name?", Bioscience, 1998, 48:505-11)
Copulins are one specific type of human pheromones found in vaginal secretions. They were first isolated in Rhesus monkeys and are composed of C2-C5 aliphatic acids. Astrid Jutte of the Ludwig Boltzmann Institute for Urban Ethology in Vienna continued the study done by McClintock mentioned above and showed that copulin blend changes during the menstrual cycle and can send a specific signal to males during ovulation. Now that these pheromones have been isolated, they are being used to make synthetic pheromones. ("Battle of odors: significance of pheromones for human reproduction", Gynakol Geburtshilfliche Rundsch, 1997, 37:150-3 (German Article))
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