Eusociality and Division of Labor

Social Organization

Varying levels of social organization are observed within the animal kingdom, ranging from simple to highly complex. These systems can greatly enhance the survival and reproductive success of an individual or a population. Of these, eusociality is the highest level of social organization, involving divisions of labor based on social castes. However, this system is rarely observed in nature, as it requires individuals to help others in the population at great survival or reproductive costs to themselves¹. When this behavior pattern, termed altruism, was originally observed in species like the honeybee, it caused early evolutionary biologists to reconsider their antiquated definition of evolutionary fitness. Original theories considered fitness only on an individual level. However, in light of eusocial systems, W. D. Hamilton first proposed a theory of inclusive fitness. This postulates that the measure of an individual’s fitness can include the reproductive fitness of family members, who also carry a proportion of that individual’s genes. In this way, it can make evolutionary sense for individuals to make personal sacrifices for the benefit of related individuals.

Relatedness

Hamilton’s theory is driven by the concept of relatedness, which is defined as the proportion of shared genes between two individuals. For example, 50% of an individual’s genes are shared with direct offspring, while only 25% are present in the following generation. Accordingly, investment in a closely-related individual confers greater fitness than investment in those less closely-related. The following equation describes this relationship:

r (relatedness) × B (benefit to the recipient) > C (cost to the altruist)

In this equation, “benefit to the recipient” (B) is equivalent to the increase in number of offspring produced by that individual while “cost to the altruist” (C) is equivalent to the decrease in the number of offspring produced by the altruist. In situations where the product of “r” times “B” is not greater than “C”, altruism is not favored by natural selection. However, as relatedness increases, so does the value of “r” times “B.” Thus, altruistic behavior becomes more likely when the altruist and recipient are more closely related.

Eusociality

Not every species that exhibits altruistic behavior is considered eusocial. Ultimately, eusociality is characterized by three main traits. First, the division of labor (including reproduction) into social castes leads some individuals to forego reproduction to enhance the reproductive success of family members. Other characteristics include overlapping generations, in which individuals of multiple generations live and work together, as well as cooperative brood care, in which non-parent individuals assist in the care of offspring.

One well-known example of eusociality is displayed by the honeybee. A typical beehive contains different groups of bees performing varied tasks. Only one reproductive female, the queen bee, repopulates the hive while other females act as workers. Reproductive males, called drones, are fairly uncommon. Honeybees belong to the order Hymenoptera along with ants and wasps^2-3. This order contains the most eusocial species, but is not the only order with eusociality. Other groups that contain eusocial species include termites, marine snapping shrimp, and naked mole rats. Some have even proposed that humans may be eusocial. Though overlapping generations and cooperative brood care are generally present, humans appear to lack a separation between reproductive and non-reproductive groups. Though it is not a perfect fit, aspects of eusociality and altruism are frequently observed in humans and play a large role in our own social structure.

Though non-eusocial communities structures are much more commonly observed, eusociality confers evolutionary advantages to the species in which it is found. Two main hypotheses have been proposed to explain the evolution of eusociality: the ecological hypothesis and haplo-diploidy hypothesis.

The first, the ecological hypothesis, considers the many ecological drivers that may favor eusocial structures. These include the ability of eusocial communities to utilize common nest sites, facilitate group protection from predators, and reduce competition among individuals. The second theory, the haplo-diploidy hypothesis, takes into account the complex genetic structure of many eusocial groups, including bees and ants. In these groups, males commonly exhibit haploidy. This means that they carry only one copy of each chromosome, and thus contain half the genetic information of females. As a result, fathers pass 100% of their genes to their daughters while mothers, who are diploid, pass on 50%. Accordingly, when calculating the relatedness between sisters in a haplo-diploid species, 75% of genes are shared. This is significantly more than the 50% observed in diploid species. Increased relatedness among sisters may explain why workers in a colony are generally exclusively female and are able to work together for the good of the colony. This pattern is seen in honeybees which include haploid males and diploid females⁴. However, not all eusocial species exhibit haplo-diploidy. Both the ecological and the haplo-diploidy hypotheses likely play a role in the evolution of eusociality.

Further Reading:

1. Sun, Q., et al. (2018). 'Managing the risks and rewards of death in eusocial insects.' Philos Trans R Soc Lond B Biol Sci 373(1754).
2. Krasnec, M. O. and M. D. Breed (2012). 'Eusocial evolution and the recognition systems in social insects.' Adv Exp Med Biol 739: 78-92.
3. Meunier, J. (2015). 'Social immunity and the evolution of group living in insects.' Philos Trans R Soc Lond B Biol Sci 370(1669).
4. Holland, J. G., et al. (2013). 'Queen control of a key life-history event in a eusocial insect.' Biol Lett 9(3): 20130056.

Charles Darwin had a bee problem. Bees built hives working for a queen, but the workers, who gathered food and cared for the young, had no young of their own. This behavior is referred to as altruism, the behavior of an animal that benefits another at its own expense. If evolution via natural selection was driven by the ability of the fittest to reproduce, why did bees exist? Such simple instincts as bees making a beehive could be sufficient to overthrow my whole theory, Darwin wrote. At the time, he had no good explanation for such self-sacrificing behavior, though he later went on to suggest that perhaps the trait could exist in a population if it helped the family.

Darwin was missing key knowledge – genetics - which meant that he didn't know how traits were inherited or passed down. But the monk, Gregor Mendel, did. His founding work on genetics and inheritance was published in 1866 opening up potential clues. Other scientists then suggested that relatedness, how many genes two individuals share, was the key to the evolution of altruism. For example, siblings share 50% of their genes while first cousins share 12.5%. There had to be a point where, from a genetic standpoint, it was better to help your relative even if it hurt you than it was to help yourself.

In 1964, evolutionary biologist and economist WD Hamilton created an equation to describe mathematically when this point was. The equation is defined as r x B > C. “R” is the genetic relatedness between the actor and recipient. “B” is the fitness benefit to the recipient…and “C” is cost to the actor. If r x B > C, it is beneficial to act altruistically.

WD Hamilton's equation helps to explain how bee society can evolve and exist. Today, we call bee society and societies like it eusocial, a phrase coined by Suzanne Batra in 1966 as she studied a type of bee that practiced cooperative brood rearing. Since then, scientists have documented eusociality across numerous animal societies. Many like bees, ants, and termites are insects…but some are mammals like naked mole rats.

Eusociality is considered the highest level of social organization that animals practice. There are three things that most eusocial societies have in common. They practice communal brood rearing, which means, like in bees, many animals will work together to rear another's young. They have overlapping generations, which means siblings - like this older naked mole rat - can take care of younger siblings…and there is a division of labor, usually between reproductive castes, as shown by these leaf-cutter ants.

There are two hypotheses for the evolution of eusociality. The first is the haplo-diploidy hypothesis. Bees and ants don't have two sex chromosomes like humans do. Male bees only have one set of chromosomes. They get all of their genes from the queen that laid them. Females, on the other hand, are diploid and get one set from their queen, who is also diploid, and one set from a male, typically from a different hive. Queens store sperm from mating, meaning that most workers will be created using sperm from only a few potential male partners. This means that on average, sisters share more genes with each other than they do with their mothers or hypothetical daughters, so it's evolutionary beneficial to work together to raise their siblings. However, not all eusocial species have this type of sex determination. The ecological hypothesis suggests that an animal's habitat could make eusocial living beneficial. For example, termites need to work together to gather food from dead wood and living together can help them fight neighboring colonies or find protection from predators.

In this lab, you and your classmates will perform a series of exercises to simulate both eusocial and solitary societies. Does one strategy work better than the other?