Pheromones
Zoologist Tristram Wyatt – about mulberry silkworm, rabbit feeding and environmentally friendly pest control
Pheromones are invisible chemical signals between members of the same species. This is probably the most common way of communication in the animal kingdom.
The discovery of pheromones
We meet mentions of the existence of pheromones from a very long time ago. The ancient Greeks were familiar with the invisible signals that dogs emit during estrus and that males feel. They knew that if the female's secretion was placed on a towel, the male would follow the towel. It's not a sound that the females made―it's a smell.
The problem is that the number of pheromones is very small, and most people are not able to smell such signals between animals. It was only in the 1950s, after twenty years of work by a group of chemists led by Adolf Butenandt, who received the Nobel Prize for his work on human steroid molecules, that a work with the chemical definition of the first pheromone was published. It was bombicol, the female sexual pheromone of the silkworm.
Bombicol molecule // wikimedia.org
How pheromones function
Pheromones are chemical signals that pass between the animal emitting them and the recipient. They are usually carried by water or by air. When they reach the recipient animal, they are almost always determined by the sense of smell (in the case of mammals, by the nose, in insects and crustaceans, for example, in lobster, through the antennae). Special nerve cells in the nose or on the antennae, which have a special receptor that binds to the pheromone molecule, are responsible for detecting pheromones.
When a pheromone molecule hits a receptor, it irritates the nerve, which sends a message to the brain, and it detects this pheromone. What happens next depends on the signal and the specific animal. If it is a male moth that has detected a female sex pheromone, it takes off and after a series of responses to this pheromone finds a female.
But pheromones are used in different situations. One of the pheromones is contained in the milk of a nursing rabbit. Milk pheromone helps rabbits to find the nipple. This is a very specific reaction, it allows rabbits to find the nipple very quickly, because they have only three to four minutes a day when they can get milk from their mother.
The pheromone signal acts the same as any other, except that the molecule has to overcome the space between the signaling and receiving individuals. This allows you to use them in the dark, but implies that you are more or less close. Such a signal differs from an auditory or visual signal, which can potentially be used at a much greater distance.
Types of pheromones
Pheromones can act in any situation and at any stage of life, depending on the type of animal. We have already mentioned sex pheromones in moths, dogs and milk pheromones in rabbits. In social insects, including bees, wasps, ants and termites, there are a huge number of different pheromones that provide for an extensive variety of actions, for example, the alarm pheromone. A honey bee hive is a very valuable resource, it attracts bears (and people) who want to steal it. Bees have a collective defense system, so when a bee bites a bear, the sting remains in the bear's skin and continues to emit an alarm pheromone. It works as a beacon that directs the rest of the bees to attack the bear. In ants, the anxiety pheromone is important when colonies are fighting: it attracts more soldiers into battle. Of course, ants also have trace pheromones, with the help of which scouts organize large groups of workers to collect food.
There are also types of pheromones that do not directly affect behavior, but change the psychology of recipient animals. For example, mice have pheromones that accelerate or slow down sexual development, which delay puberty. There are pheromones, unlike most others, that bypass the olfactory system. For example, honey bees feed their larvae with special food – royal jelly. It contains pheromones, which include the protein royalactin. If you feed the larvae exclusively with royal jelly, they will become queens. If you also give them pollen, they will turn into workers. That is, pheromones in royal milk change the expression of genes, they have the ability to switch the ways of development of larvae.
The use of pheromones
80 years before the first pheromones were discovered, entomologist Joseph Lintner kept several female silkworms on the window of his office in New York. Through the open window, their pheromones attracted large males who fluttered over the sidewalk, gathering crowds of passers-by below. Lintner was sure that the strong attraction must be caused by some chemical that is secreted by female moths. If this attractive chemical could be synthesized and started to be produced in large quantities, perhaps there would be a way to control pest moths. There are no problems from adult moths, but caterpillars do harm: they spoil apples and other agricultural crops. If it becomes possible to interrupt mating, prevent adults from finding each other and laying eggs, then it will be possible to stop the caterpillars.
Mulberry silkworm // wikimedia.org
In 1960, to test this idea, experiments were conducted with orchards, corn and cotton plantations. Ten years later, it became clear that this was a very successful idea. She proposed a solution to the problem that pesticides stop working because pest moths become resistant to them. Pheromones turned out to be safe for the environment and have become the most effective way of disinfection in the ratio "price ― quality". They do not kill moths, but prevent adult individuals from meeting, so they rid us of pest larvae.
Mating inhibition and pheromone pest control are used on at least ten million hectares worldwide: on cotton in the USA, on tomatoes in Mexico, on eggplants in Pakistan and on peaches in South Africa. Unlike pesticides, synthetic pheromones do not kill predatory insects and other natural enemies, which allows you to control the population of harmful insects. At the same time, there is a problem: each type of pest has its own pheromone, which needs to be studied and optimized for effective pest control.
The Myth of human pheromones
Human pheromones have been discussed for a long time. This is a very tempting idea, and, of course, it has deep roots in folk traditional knowledge. There are stories of Central European villagers carrying a handkerchief under their arm and waving it at village dances in front of women they like. Those, of course, fall in love with them. Real stories are rather disappointing. If you search the Web, a huge number of commercial sites will try to sell you, as they claim, "real pheromones". And if you look at the scientific literature, there you will also find a lot of studies that are designed to show the effectiveness of what they usually call "presumably human pheromones."
A few years ago I was interested in this question and looked for sources of this idea in the scientific literature. What I found really surprised me. It turned out that most of the published studies were based on one paper from 1991. This was a study of two molecules – androstadienone and estratetraenol, which were supposed to be human pheromones, male and female, respectively. Nevertheless, the article, which is included in the conference materials, does not provide evidence. In the section of the article devoted to the method, it is written that the molecules are provided by the Erox Corporation. And I wondered: what did the corporation "Erox" do? All I found was a series of patents that described the synthesis of molecules in detail, but there was no evidence in them that they were human pheromones. On what grounds did they even decide that they were pheromones?
To understand the principle of pheromone discovery, it is necessary to return to the work on the silkworm of 1959. In his pioneering work, Butenandt set the gold standard for how to discover and demonstrate real pheromones. First, you should carefully separate the molecules and check if your extract has the same effect as the natural secretion. Then you identify the molecules, synthesize them and show that synthetic molecules produce the same effect that you investigated at the very beginning. When it came to people, it turned out that these key steps had never been done by anyone. These two molecules are practically taken from the air and are regarded as pheromones. They just said that they were pheromones, but there was no evidence of proper testing, no evidence that their use causes a specific behavior or psychological reaction. There was no evidence at the very core.
But there was a lot of literature that took on faith that these molecules are pheromones. In my research in 2015, I showed that there are many reasons to think that this is not the case. The molecule, passed off as a female pheromone, was found only in the urine of a woman who was in the third trimester of pregnancy. It is unlikely that she plays the role of a sexual pheromone that attracts a spouse. The claimed male "pheromones" were found in the man's armpit, but there are hundreds of other molecules present. It just doesn't make sense to choose these two molecules. I'm afraid people are just too interested in sex pheromones.
A real human pheromone
The first human pheromone that was precisely identified turned out to be not sexual at all, despite the unprecedented interest in them in the past. Instead, the detected pheromone is involved in communication between mother and child. In general, to determine a pheromone, you need to conduct a simple but reliable experiment that you can repeat at any stage of determining the molecule. In the case of a person, there is a behavior that potentially depends on the smell, the bioassay and the direct interaction between mother and child. This work was done in France by Professor Binoist Schaal. His team found a secretion that is produced around the nipple of a nursing mother, which attracts anyone ― not just her own ― baby to breastfeed. The researchers improved a simple bioassay by recording the reaction of infants to secretion, which allowed them to track and determine the pheromone in the molecules. Maybe it's a human analog of the milk pheromone that was found in rabbits.
In the light of an entertaining story that takes place in the world of pheromones in general, it turned out that by studying them, we are surprisingly engaged in both animal behavior and neuroscience at the same time. In the case of a moth, we have a certain pheromone emanating from a female individual, and a highly specialized male structure ― antennae that catch a pheromone signal. Specific pheromone receptors are known, which are located on many thousands of sensitive neurons in the olfactory organs and go to special "antennal" parts of the male brain.
In many parts of the world, researchers are looking at how the signal enters the sensitive neurons of the olfactory organs and how it is processed at different levels of the brain, eventually causing the wings to fly. Each stage can be investigated both at the level of psychology and at the genetic level. Thus, we can look at the genetics and evolution of enzymes that create a pheromone signal in females. Scientists are also investigating the evolution of receptors in the antennae and the nervous system in the brain of moths. There are similar studies of mammals and other vertebrates, but they are at a much earlier stage due to difficulties with the study of the nervous system in mammals. Among the pheromones, mouse darsin and ESP1 were the most studied.
Open questions of chemosensory
A whole field of research on chemosensory has begun, albeit belatedly, to be equated with the research of other senses. This is indicated by the dates of the Nobel Prize. Prizes were awarded for the discovery of the mechanisms of hearing and vision in 1971 and 1967. And Richard Axel and Linda Buck won the award for sense of smell only in 2004. They discovered a group of 1,000 olfactory receptors in mice in 1992. Each receptor is tuned to specific odor molecules. One odor molecule can stimulate more than one type of receptor. The brain can distinguish which molecule was "inhaled" by a combination of receptors (and their neurons). This is facilitated by the convergence of neurons that transmit signals to specific receptors that converge in the tangle (glomerulus) of the olfactory bulb. For this reason, there are as many tangles as there are types of receptors.
We learned about the mechanism of smell perception quite recently, but we are moving extremely fast. However, much remains unexplored. We are still trying to detect pheromones in the simplest species. The last example would be a domestic dog. Although we know that a male can detect a pheromone released during estrus, and from a sufficiently long distance, but we do not know exactly which molecules cause this reaction. So we still have a lot to discover.
This is a translation of an article from our English-language publication Serious Science. You can read the original version of the text by following the link.
About the author:
Tristram D. Wyatt – Senior Research Associate Emeritus Fellow, Kellogg College, University of Oxford.
Portal "Eternal youth" http://vechnayamolodost.ru
25.05.2017