Eusociality: what is it and how does it occur in social animals?

Let's see how eusociality, a very complex type of social behavior in animals, occurs.

Human beings are characterized by an anthropocentric view of life, that is, they consider our species as the center of all things and the absolute end of creation. As the Homo sapiens the measure and axis of the totality of existence, it is common to think that there are no social organizations more complex than ours, where countries, laws, superior figures and interpersonal relationships dominate our daily life and identity.

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

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

What is eusociality?

Eusociality is defined as the highest level of social organization that occurs in certain animals, especially in invertebrates.especially in invertebrates of the order Hymenopterawhich includes insects such as ants and bees. The basis of this hierarchy is the division of labor: this allows specialized specimens to be created both morphologically and ethologically for a specific purpose, encompassed in a term called "caste".

The types of social life in animals can be categorized into several terms which we will not cover here due to their complexity, but all of them have 3 pillars in common:

• Adults jointly care for the young and live in groups.
• Two or more generations always live in a nest, i.e. they overlap.
• The individuals of the colony are divided into a "real" breeding caste and an infertile, hard-working "worker" caste.

The non-breeding castes are responsible for the care of the larvae and the maintenance of the nest, while the only function of the breeding individuals is usually that of giving rise to offspring throughout their life cycle.

According to other authors, there is a fourth idea that characterizes eusociality: the point of no return. Simply put, this concept tries to convey that the specimens that are part of this biological system are "fixed" in a particular caste, generally before the mature reproductive age. Thus, a worker cannot become a queen and, therefore, needs to live in that social system in order for her genetic information to perpetuate in order for her genetic information to be perpetuated in some form, even if it is by caring for the offspring that another "superior" specimen has left in her care.

The evolution of eusociality in animals

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

To understand this reality, we need 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 that another option is for it to be transmitted through Blood relatives other than one's own offspring.

In other words, the biological fitness of the animal is not based solely on its reproduction, since after all, its relatives share a high proportion of genes with it. This is the basis for such complex phenomena as altruism in the animal kingdom.

Thus, the presence of these genes that "encode" biological altruism (or that an animal refuses to have offspring in order to care for the offspring of others) will increase in frequency when the following equation is satisfied:

R X B > C

Where R is the genetic kinship between the recipient and the donor, B is the additional reproductive benefit received by the recipient of the "altruistic" act, and C is the reproductive cost incurred by the donor.

This simple equation, in itself, could come to explain the behavior of worker bees and ants, who give their lives to save the life of the donor.who give their lives to save the queen's life. If a soldier ant protects with her life two or three sisters who may become queens, she is maintaining her genetic lineage far more than if she were to survive on her own over time. After all, the kinship rate is very high between the two castes and the soldier ant would not be able to reproduce either.

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

• You may be interested in "Does altruism exist in nonhuman animals?"

Examples of eusociality in the animal kingdom.

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

In this colony, the matriarchs are responsible for laying eggs and building the honeycomb cells where the larvae will be reared, while the workers take care of daily tasks, such as feeding the offspring and foraging outside. In this particular 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 eusociality, both of them from the family Bathyergidaeboth 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 offspring. As you can imagine, the kinship rates in this system are very high between individuals, otherwise it would not be sustained at a biological level.

The fall of the classical eusocial theory

Although we have convinced you in a simple and straightforward way that the theory of kin selection explains eusociality without problems, the current reality is very different. Several biologists argue today that the mathematical model presented is flawed because it does not represent evolutionary dynamics and does not take into account the mechanisms of genetic distribution.

In addition, the basis of 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 by no means the case in a general overview, since all competitive relationships (both intra- and inter-specific) and many other factors would have to be factored into this equation. This is a set of theories that have sinned of reductionism and are therefore openly challenged today.

Summary

Thus, the phenomenon of eusociality has been orphaned in terms of explanation at present. There are new theories that try to explain this type of fascinating hierarchies, for example, taking into account such key factors as the cohesion and persistence of groups of the same species at the evolutionary level. Thus, the alleles of the genes coding for eusociality will be transmitted more centrally the less dispersed the population is throughout its historyand eusocial systems will be encouraged.

Estamos ante explicaciones completamente conjeturales pues, una vez desestimada la aptitud inclusiva y la selección de parentesco para la eusocialidad, aún queda muchísimo por investigar y plantear antes de encontrar otra explicación que convenza a la comunidad científica general.

Referencias bibliográficas:

• Grafen, A. (1984). Natural selection, kin selection and group selection. Behavioural 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., Van Acker, R. C., & Dudley, S. A. (2017). Kin recognition, multilevel selection and altruism in crop sustainability. journal of ecology.
• Nowak, M. A., Tarnita, C. E., & Wilson, E. O. (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.