Eusociality: what it is and how it occurs in social animals

The human being is characterized by an anthropocentric vision of life, that is, it considers our species as the center of all things and the absolute end of creation. Being the Homo sapiens the measure and axis of the totality of existence, it is common to think that there are no more complex social organizations than ours, where countries, laws, superior figures and interpersonal relationships dominate our day to day and identity.

If that is your conception, you are wrong: living beings specialize in nature according to the environmental pressures and sometimes sacrificing individual identity is necessary for the species to persist. As much as it is hard for us to understand, in the animal world the life of the individual is not important, as long as its genetic line is perpetuated over time.

Thus, there are much more intricate levels of socialization than those presented by humans, where the reproduction and functionality of some specimens is sacrificed for the common good. We are talking about eusociality, and in the following lines we tell you everything you need to know about it.

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What is eusociality?

Eusociality is defined as the highest level of social organization found in certain animals, especially in invertebrates of the order Hymenoptera, which includes insects such as ants and bees. The basis of this hierarchy is the division of labor: this allows specialized items to be created both at a morphological and ethological level with a specific purpose, encompassed in a term called "caste".

The types of social life in animals can be categorized in various terms. which we are not going to cover due to the complexity they report, but they all have 3 pillars in common:

• Adults jointly care for young and live in groups.
• Two or more generations always live in a nest, that is, they overlap.
• The individuals in the colony are divided into a “royal” reproductive caste and an infertile and hard-working caste, the “worker”.

The non-reproductive castes are in charge of caring for the larvae and maintaining the nest, while that the only function of reproductive individuals is usually that: to give rise to offspring during their entire cycle vital.

According to other authors, there would be a fourth idea that characterizes eusociality: the point of no return. Simply, this concept tries to convey that the specimens that are part of this system Biologically they are "fixed" in a specific breed, generally before the reproductive age mature. So that, a worker cannot become a queen and therefore needs to live in that social system so that her genetic information is perpetuated in some way, even if it is taking care of the offspring that another “superior” specimen has left in her care.

The evolution of eusociality in animals

For a long time, the theory of kin selection or kin selection has explained many of the eusociality mechanisms present in living beings. This strategy is based on the fact that certain animals favor the reproductive success of a relative with their behaviors, even when this may cost them their own success and survival. In the natural world, this might sound counter-intuitive, right?

To understand this reality, we have to understand the term inclusive fitness.. Unlike classical biological fitness, inclusive fitness recognizes that genetic information need not only be passed between generations through direct descent, but another option is that it be transmitted through blood relatives other than their own children.

That is to say, the biological aptitude of the animal is not based solely on its reproduction, since, after all, its relatives share a high proportion of genes with it. On these bases are based phenomena as complex as altruism in the animal kingdom.

Thus, the presence of these genes that "encode" biological altruism (or that an animal rejects have offspring to take care of others) will increase in frequency when the following condition is met equation:

R X B > C

Where R is the genetic relationship between the recipient and the donor, B is the additional reproductive benefit received by the recipient from the "altruistic" act, and C is the reproductive cost suffered by the donor.

This simple equation, in itself, could eventually explain the behavior of worker bees and ants., who give their lives to save the queen's. If a soldier ant protects with its life two or three sisters who may become queens, it is maintaining its genetic lineage much more than if it lasted itself over time. After all, the relationship rate is very high between the two breeds and the soldier ant could not reproduce either.

This general rule tells us that cooperativeness (and therefore eusociality, its extreme expression) is favored by selection natural when the degree of relatedness between individuals (R) is greater than the cost/benefit ratio (C/B). For example, eusociality could theoretically be fostered if the benefit of keeping a brother or sister alive doubles the biological cost of the altruist, that is, a value of R=½.

• You may be interested in: "Is there altruism in non-human animals?"

Examples of Eusociality in the Animal Kingdom

As we have said, the order of insects Hymenoptera is the maximum exponent of eusociality, since it includes in its interior to bees, wasps and ants, where there is clearly a reproductive caste (the queens) and a worker (the workers). This very clear division of labor has been observed, for example, in the species Polistes versicolor, a clearly eusocial type of wasp.

In this colony, the matriarchs are in charge of laying eggs and building the honeycomb cells where the larvae will be raised. while the workers deal with daily tasks, such as feeding the offspring and searching for food in the abroad. In this specific species, it has been observed that the dominant queens perform only 18.6% of the tasks in the comb, while the workers occupy more than 80% of the total. Undoubtedly, in these cases work is a matter of caste.

If we leave the world of invertebrates, eusociality becomes much less common, and it is striking to know that only two species of mammals are known to perform it, both from the family Bathyergidae, historically known as naked mole rats. In this case, the majority of individuals in a colony care for the offspring produced by a single queen, which gives rise to the offspring. As you may have imagined, the kinship rates in this system are very high between individuals, otherwise it would not be sustained at a biological level.

The fall of classical eusocial theory

Although we have convinced you plain and simple that kin selection theory explains eusociality without issue, the current reality is very different. Several biologists argue today that the mathematical model presented is flawed, since it does not represent evolutionary dynamics or take genetic distribution mechanisms into account.

In addition, the basis of fitness or inclusive fitness fails in a very important premise: according to it, the biological fitness of an individual depends on additive components that are caused by individual actions. This is not the case at all in a general panorama, since all competition relationships (both intra- and inter-specific) and many other factors would have to be factored into this equation. It is a set of theories that have sinned as reductionists and, therefore, are openly challenged today.

Summary

Thus, the phenomenon of eusociality has been orphaned as far as explanation is concerned today. There are new theories that try to explain this type of fascinating hierarchies, for example taking into account account factors as key as the cohesion and persistence of groups of the same species at the evolutionary. So that, the alleles of the genes that code for eusociality will be transmitted in a more centralized way the less dispersed the population has been throughout its history, promoting eusocial systems.

We are dealing with completely conjectural explanations because, once the inclusive aptitude and the selection of kinship for the eusociality, there is still a lot to investigate and raise before finding another explanation that convinces the scientific community general.

Bibliographic references:

• Grafen, A. (1984). Natural selection, kin selection and group selection. Behavioral ecology: An evolutionary approach, 2, 62-84.
• Johnson, R. M., Harpur, B. A., Dogantzis, K. A., Zayed, A., & Berenbaum, M. R. (2018). Genomic footprint of evolution of eusociality in bees: floral food use and CYPome “blooms”. Insectes Sociaux, 65(3), 445-454.
• Jones, d. (2018). Kin selection and ethnic group selection. Evolution and Human Behavior, 39(1), 9-18.
• Murphy, G. P., Swanton, C. J., VanAcker, R. C., & Dudley, S. TO. (2017). Kin recognition, multilevel selection and altruism in crop sustainability. journal of ecology.
• Nowak, M. A., Tarnita, C. E., & Wilson, E. EITHER. (2010). The evolution of eusociality. Nature, 466(7310), 1057-1062.
• Thorne, B. L. (1997). Evolution of eusociality in termites. Annual Review of Ecology and Systematics, 28(1), 27-54.