Pheromones In Animals

Pheromones

Pheromones are invisible chemical signals between members of the same species. This is probably the most common mode of communication in the animal kingdom.

The discovery of pheromones

We meet references to the existence of pheromones from very ancient times. 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 secretions were placed on a towel, the male would follow the towel. It's not the sound the females made, it's the smell.

The problem is that the number of pheromones is very small, and most people are not able to smell such signals between animals. 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 molecules - steroids, was a paper published with the chemical definition of the first pheromone. It was bombykol, the female sex pheromone of the silkworm.

How Pheromones Work

Pheromones are chemical signals that travel between the emitting animal and the recipient. They are usually carried by water or air. When they reach the recipient animal, they are almost always detected by smell (in the case of mammals, through the nose; in insects and crustaceans, such as the lobster, through the antennae). Special nerve cells in the nose or antennae, which have a special receptor that binds to the pheromone molecule, are responsible for detecting pheromones.

When a pheromone molecule hits the 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 particular animal. If it is a male moth that has detected a female sex pheromone, it flies up and after a series of responses to this pheromone finds a female.

But pheromones are used in different situations. One of the pheromones is found in the milk of a nursing rabbit. Milk pheromone helps rabbits find the nipple. This is a very specific reaction, it allows the 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 works just like any other, except that the molecule has to traverse 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 is different from the 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 involve a wide variety of actions, such as 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 stings a bear, the sting stays in the bear's skin and continues to emit an alarm pheromone. It works like a beacon that directs the rest of the bees to attack the bear. In ants, the alarm pheromone is important when colonies are fighting: it attracts more soldiers to fight.

The use of pheromones

Eighty years before the first pheromones were discovered, entomologist Joseph Lintner kept several female silkworms in his New York office window. Through the open window, their pheromones attracted large males that fluttered over the pavement, gathering crowds of passers-by below. Lintner was sure that the strong attraction must be caused by some kind of chemical that the female moths secrete. If this addictive chemical could be synthesized and mass-produced, 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.

In 1960, experiments were carried out with orchards, corn, and cotton plantations to test this idea. Ten years later, it became clear that this was a very successful idea. She offered a solution to the problem that pesticides stop working because pest moths become resistant to them. Pheromones proved to be safe for the environment and became the most effective method of disinfestation in terms of price-quality ratio. They do not kill moths but prevent the meeting of adults, 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 US, on tomatoes in Mexico, on eggplant 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 the women they like. They, of course, fall in love with them. The real stories are rather disappointing. If you search the web, a huge number of commercial sites will try to sell you what they claim to be "real pheromones". And if you look at the scientific literature, you'll also find a lot of research that is designed to show the effectiveness of what they commonly call "supposedly human pheromones."

A few years ago, I was interested in this issue 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 relied on one work in 1991. It was a study of two molecules - androstadienone and estratetraenol, which were supposed to be human pheromones, male and female, respectively. However, the article, which is included in the conference proceedings, does not provide evidence. In the section of the article devoted to the method, it is written that the molecules were provided by the Erox Corporation. And I asked myself: what did Erox Corporation do? All I found was a series of patents that detailed the synthesis of molecules, but there was no evidence that they were human pheromones. On what basis did they even decide that these are pheromones?

To understand the principle behind the discovery of pheromones, one must go back to the 1959 work on the silkworm. In his pioneering work, Butenandt set the gold standard for how real pheromones should be discovered and displayed. First, you must carefully separate the molecules and see if your extract has the same effect as natural secretion. Then you define the molecules, synthesize them, and show that synthetic molecules produce the same effect that you explored in the beginning. When it came to the people, it turned out that these key steps had never been taken by anyone. These two molecules are practically taken from the air and are regarded as pheromones. They just said it was pheromones, but there was no evidence of proper testing, no evidence that their use elicits a specific behavior or psychological response. There was no proof at the 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 masquerading as a female pheromone was only found in the urine of a woman who was in her third trimester of pregnancy. It is unlikely that she plays the role of a sex pheromone that attracts a spouse. Claimed male "pheromones" were found under the armpit of a man, but there are hundreds of other molecules present. It just doesn't make sense to choose those two molecules. I'm afraid people are just too interested in sex pheromones.

Real human pheromone

The first human pheromone to be accurately identified turned out not to be sexual at all, despite the unprecedented interest in them in the past. Instead, the detected pheromone is involved in the communication between mother and child. In general, to determine the pheromone, you need to conduct a simple but reliable experiment that you can repeat at any stage of the molecule definition. In the case of humans, there is a behavior that is potentially dependent on smell, bioassay, and direct interaction between mother and child. This work was done in France by Professor Benoist Schaal. His team found a secretion that is produced around a nursing mother's nipple that attracts any - not just her own - baby to breastfeed. Researchers have improved a simple biotest by recording the reaction of infants to secretion, which allowed them to track and detect pheromone in molecules. Maybe it's the human equivalent of the milk pheromone found in rabbits.

In the light of the fascinating history that is happening in the world of pheromones in general, it turned out that by studying them, we surprisingly do both animal behavior and neuroscience at the same time. In the case of the moth, we have a specific pheromone coming from the female, and a highly specialized male structure, the antennae, that picks up the pheromone signal. Specific pheromone receptors are known, which are located on many thousands of sensory neurons in the organs of smell and go to special "antennae" parts of the brain of males.

In many parts of the world, researchers are looking at how the signal gets to the sensory neurons of the sense of smell and how it is processed at different levels of the brain, ultimately causing the wings to fly. Each step can be explored both at the level of psychology and at the genetic level. Thus, we can look at the genetics and evolution of the enzymes that create the 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 in mammals and other vertebrates, but they are at a much earlier stage because of the difficulty in studying the nervous system in mammals. Among pheromones, mouse darsin and ESP1 have been the most studied.

Open questions in chemosensorics

A whole field of chemosensory research has begun, albeit belatedly, to be equated with research on other senses. This is evidenced 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 received the award for smell only in 2004. They discovered a group of 1000 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 is "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 coils as there are types of receptors.

We have only recently learned about the mechanism of smell perception, but we are advancing extremely quickly. 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 dog can detect the pheromone released during estrus, and from a sufficiently large distance, we do not know exactly which specific molecules cause this reaction. So we still have a lot to discover.

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