Unveiling the mysteries of pheromones

Although we humans are more interested in everything audiovisual, most of the communication that occurs in living beings is carried out through chemical signals. Life is chemistry, more than anything else.

Communication between each cell inside organisms is fundamentally chemical, as is the communication between bacteria, plants to grow and multiply, or insects.

The laws that govern these chemical channels of communication remain largely a mystery.

Linguistic laws in chemical communication?

In a recent post on Biology Letterswe have explored in chemical communication the so-called linguistic laws, that is, the statistical regularities that are fulfilled in all human languages, from English to Swahili, passing through Catalan and Spanish.

The fundamental laws of communication, with a physical basis, have been verified both in the languages ​​spoken by humans around the world, and in other species. The acoustic communication of geladas or lemurs, for example, or even gestural communication between chimpanzees follow the same basic laws of all language.

The investigation of linguistic laws has spread in recent years in biology in various studies, ranging from genomics or proteomics to ecosystems. But are they also fulfilled in chemical messages?

For our research, we tested one of the most well-known linguistic laws: Zipf’s law of brevity.

Zipf’s law of brevity, or simply law of brevity, is the statistical tendency for the most frequent words, the most used, to be shorter. It is basically a principle of economy, energy saving, proven in human languages, both orally and in writing. Thus, we seek whether the law of brevity is fulfilled in the substances used in chemical communication.

Short words in the chemical language

The words of chemical language are called infochemicals or semiochemicals. After analyzing the Pherobase database, we generally verified the law of brevity: the most frequent infochemicals in ecosystems, those that are used by a greater number of species, tend to be shorter and therefore lighter carbon chains.

Potentially, these words short ones are easier to decipher. However, there is a notable exception when infochemicals are grouped according to their communicative function: pheromones.

Pheromones are chemical substances used in communication between members of the same species, which can be contrasted with allelochemicals, or substances used in communication between different species, with diverse functions in ecosystems. Some examples are shown in the figure following this paragraph.

Thus, some types of allelochemicals are the following:

• The allomones, substances that emit, for example, carnivorous plants to attract their prey and therefore benefit the emitter.

• Kairomones, which attract species that take advantage of the emitter, as happens with the terpenoids that some conifers emit and that cause pests such as the pine beetle

• Synomones are substances that are emitted in symbiosis, beneficial for both species, as in the case of clownfish and sea anemones.

• Attractants are molecules that we humans also synthesize, beneficial for the emitter and/or the receiver, depending on the case, and with diverse functions whose ecological impact is mostly to be determined.

An encrypted language?

The same infochemical can have different functions. That is, it can be used as a pheromone by one species, and at the same time, for example, be a kairomone for others (something common for predators of the same species). Well, we have found that pheromones, statistically, do not obey Zipf’s law of brevity.

This fact could have various explanations. One of them is the need to increase the complexity of the substance and its length when communicating with our own species to avoid detection by others. For example, this makes it more difficult for a predator to intercept that signal and, therefore, devour the sender.

That pheromones are not statistically brief could indirectly support the handicap hypothesis, classically enunciated by Amotz Zahavi, according to which organisms could tend to waste energy on chemical signals as a strategy of reproductive ostentation –as some birds do when decorating their nests– .

It makes perfect sense to strive for the emission of more complex substances when survival or reproduction is at stake. Meanwhile, in communication with other species, the principle of economy would prevail, that is, being able to send large quantities of infochemicals at a low cost.

This explanation for the exceptionality of pheromones is pending future work that corroborates the results of this pioneering study. Specific analyzes of the chemical channels of specific ecosystems are necessary since, it must be recognized, the Pherobase data mixes diverse ecosystems and allows only a general perspective.

This global vision could be useful in exobiology: the presence of infochemicals in the atmospheres of distant planets would be indirect evidence of the existence of life on those worlds. But first, what if we investigate the chemical communication of our unknown Earth? We still ignore the secrets of the chemical communication of our fellow travelers. Paraphrasing Rudyard Kipling, “Only if we start listening will the jungle speak to us”.