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…
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?
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?
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Q1: Why did Darwin struggle to explain altruistic behavior in bees?
Darwin observed that worker bees sacrificed their own reproduction to care for the queen's offspring, contradicting his theory that natural selection favored individual reproduction. He lacked knowledge of genetics and inheritance, so he couldn't explain how self-sacrificing traits could evolve. Later scientists discovered that genetic relatedness between individuals was key to understanding altruism.
Q2: What does Hamilton's equation reveal about altruistic behavior?
Hamilton's equation (r × B > C) mathematically determines when altruism benefits an individual genetically. The equation compares genetic relatedness (r), fitness benefit to the recipient (B), and cost to the altruist (C). When the product of relatedness and benefit exceeds the cost, altruistic behavior becomes evolutionarily favorable, especially among close relatives.
Q3: What are the three defining characteristics of eusocial societies?
Eusocial societies exhibit communal brood rearing, where multiple individuals care for offspring together. They have overlapping generations, allowing siblings to care for younger relatives. Finally, they display division of labor based on reproductive castes, with some individuals forgoing reproduction to support colony members, as seen in bees, ants, and termites.
Q4: How does haplo-diploidy increase genetic relatedness among sisters?
In haplo-diploid species like bees and ants, males carry one chromosome set while females carry two. Males inherit 100% of genes from their mother, while females inherit 50% from each parent. This means sisters share 75% of genes with each other, compared to 50% in diploid species, making cooperation more genetically beneficial.
Q5: What does the ecological hypothesis suggest about eusocial evolution?
The ecological hypothesis proposes that environmental factors favor eusocial living. For example, termites benefit from cooperating to gather food from dead wood and defending against neighboring colonies. Group living provides protection from predators and reduces competition, making eusocial organization advantageous regardless of genetic relatedness alone.
Q6: How does inclusive fitness theory redefine evolutionary success?
Inclusive fitness theory extends the measure of evolutionary success beyond individual reproduction to include the reproductive success of genetic relatives. Since relatives share genes, helping them reproduce can increase an individual's overall genetic contribution to future generations, explaining why altruistic behavior toward kin can evolve through natural selection.
Q7: Which animal groups besides insects exhibit eusocial behavior?
While most eusocial species are insects like bees, ants, and termites, naked mole rats represent eusocial mammals. Marine snapping shrimp also display eusocial characteristics. Understanding population growth exponential logistic growth patterns helps explain how eusocial colonies expand and sustain themselves over time.