Which Animals Form A Eusocial Society

Eusociality, the highest level of social organization, is characterized by cooperative brood care, overlapping generations within a colony of adults, and a division of labor, which often involves a reproductive division of labor (i.e., not all individuals get to reproduce). While often associated with insects, particularly ants, bees, and termites, eusociality is found in a fascinating, though limited, range of animals. This article delves into the world of eusocial animals, exploring the species that have evolved this complex social structure and the intricacies of their societies.

The Hallmarks of Eusociality

Before diving into specific examples, it's crucial to understand the three primary characteristics that define eusociality:

• Cooperative Brood Care: Members of the group assist in raising the young, even if they are not their own offspring. This can involve feeding, protecting, and grooming the young.
• Overlapping Generations: Multiple generations of adults live together in the same colony. This allows for the transmission of knowledge and skills from older to younger generations.
• Division of Labor: Individuals within the colony have specialized roles, with some individuals focusing on reproduction (reproductives) and others on non-reproductive tasks such as foraging, defense, and nest maintenance (workers). This division is often accompanied by morphological differences between castes.

Eusocial Insects: The Pioneers

Insects represent the vast majority of eusocial species. Their small size, rapid reproduction rates, and haplodiploid genetic system (in some groups) have likely contributed to the evolution of eusociality.

Ants (Family Formicidae)

Ants are arguably the most well-known eusocial insects. With over 12,000 identified species, they exhibit a remarkable diversity of social structures and ecological roles.

• Colony Structure: Ant colonies typically consist of one or more queens (reproductives), sterile female workers, and, periodically, males (drones) for reproduction.
• Division of Labor: Workers are further divided into sub-castes based on size and task. Smaller workers may tend to the brood, while larger workers may defend the colony or forage for food. Some species even have specialized "soldier" castes with enlarged heads and mandibles for defense.
• Communication: Ants communicate through chemical signals called pheromones, which they use to coordinate activities such as foraging, defense, and nest building.
• Examples: Leafcutter ants (genus Atta) are famous for their complex division of labor in harvesting leaves to cultivate fungi, their primary food source. Army ants (genus Eciton) are nomadic predators that form massive raiding swarms.

Bees (Superfamily Apoidea)

While most bees are solitary, several species have evolved eusociality, most notably honeybees and stingless bees.

• Colony Structure: Honeybee colonies (genus Apis) consist of a single queen, thousands of female workers, and drones that appear seasonally for mating.
• Division of Labor: Worker bees perform a variety of tasks throughout their lives, including nursing larvae, building comb, foraging for nectar and pollen, and defending the hive. They exhibit age-based polyethism, with younger bees performing tasks within the hive and older bees foraging outside.
• Communication: Honeybees communicate through a variety of methods, including the waggle dance, a complex behavior used to convey information about the location and quality of food sources.
• Examples: Stingless bees (tribe Meliponini) are another group of eusocial bees found in tropical and subtropical regions. They produce honey and pollen but, as their name suggests, lack a stinger.

Termites (Order Isoptera)

Termites are often mistakenly called "white ants" due to their superficial resemblance to ants. However, they are more closely related to cockroaches. All termites are eusocial.

• Colony Structure: Termite colonies can range in size from a few dozen to millions of individuals. They typically consist of a king and queen (reproductives), soldiers, and workers.
• Division of Labor: Workers perform tasks such as foraging, nest building, and caring for the young. Soldiers defend the colony with specialized morphological adaptations, such as enlarged mandibles or the ability to secrete toxic chemicals.
• Caste Determination: Unlike ants and bees, termite caste determination is not solely based on sex. Both males and females can develop into workers or soldiers. Environmental factors, such as pheromones and nutrition, play a role in determining caste.
• Examples: Macrotermes bellicosus builds massive mounds that can reach several meters in height. These mounds house millions of termites and are a testament to their sophisticated social organization.

Other Eusocial Insects

While ants, bees, and termites are the most well-known eusocial insects, eusociality has also evolved in other groups, including:

• Wasps (Order Hymenoptera): Several wasp species, such as paper wasps (genus Polistes) and yellowjackets (genus Vespula), exhibit eusocial behavior. Their colonies are typically smaller than those of ants, bees, or termites, and the division of labor may be less rigid.
• Aphids (Order Hemiptera): Some aphid species, such as the gall-forming aphids, exhibit a rudimentary form of eusociality. Soldiers defend the gall from predators.
• Thrips (Order Thysanoptera): Some thrips species exhibit eusocial behavior, with soldiers defending the colony against invaders.

Beyond Insects: Eusocial Mammals

Eusociality is exceedingly rare in mammals, with only two known species exhibiting this complex social structure.

Naked Mole-Rats (Heterocephalus glaber)

Naked mole-rats are hairless, subterranean rodents native to East Africa. They live in colonies of up to 300 individuals and exhibit a remarkable degree of social organization.

• Colony Structure: Naked mole-rat colonies consist of a single breeding female (the queen) and a few breeding males. The remaining individuals are non-reproductive workers.
• Division of Labor: Workers perform a variety of tasks, including digging tunnels, foraging for food (roots and tubers), and caring for the young. There is a division of labor based on size, with smaller workers performing tasks within the nest and larger workers digging tunnels.
• Repression of Reproduction: The queen maintains her reproductive dominance through pheromones and behavioral suppression. Workers are physiologically capable of reproduction, but their reproductive capacity is suppressed by the presence of the queen.
• Inbreeding: Naked mole-rats exhibit a high degree of inbreeding, which may have contributed to the evolution of eusociality by increasing the relatedness among colony members.

Damaraland Mole-Rats (Fukomys damarensis)

Damaraland mole-rats are another species of subterranean rodent native to southern Africa. They are closely related to naked mole-rats and also exhibit eusocial behavior, though their social structure is less extreme.

• Colony Structure: Damaraland mole-rat colonies typically consist of a single breeding pair and several non-reproductive workers.
• Division of Labor: Workers perform tasks such as digging tunnels, foraging for food, and caring for the young.
• Repression of Reproduction: Similar to naked mole-rats, the breeding pair suppresses the reproduction of other colony members through pheromones and behavioral dominance.
• Outbreeding: Unlike naked mole-rats, Damaraland mole-rats exhibit a higher degree of outbreeding.

The Evolution of Eusociality

The evolution of eusociality is a complex and fascinating topic that has been the subject of much research and debate. Several hypotheses have been proposed to explain the evolution of this unusual social structure.

Haplodiploidy Hypothesis

This hypothesis, initially proposed to explain the evolution of eusociality in Hymenoptera (ants, bees, and wasps), suggests that the unusual genetic system of haplodiploidy predisposes these insects to eusociality. In haplodiploid species, females develop from fertilized eggs (diploid) and males develop from unfertilized eggs (haploid). This means that sisters are more closely related to each other (relatedness of 0.75) than they are to their own offspring (relatedness of 0.5). Therefore, it may be more beneficial for a female to help her mother raise more sisters than to produce her own offspring.

However, the haplodiploidy hypothesis has been criticized because eusociality has also evolved in diploid species, such as termites and mole-rats. Furthermore, not all haplodiploid species are eusocial.

Kin Selection Theory

Kin selection theory, developed by W.D. Hamilton, provides a more general explanation for the evolution of altruistic behavior, including eusociality. This theory suggests that individuals can increase their inclusive fitness (the sum of their own reproductive success and the reproductive success of their relatives, weighted by their relatedness) by helping relatives reproduce. In other words, individuals may sacrifice their own reproduction to help relatives reproduce if the benefits to their relatives outweigh the costs to themselves.

Hamilton's rule states that altruistic behavior is favored when rB > C, where:

• r is the relatedness between the actor and the recipient
• B is the benefit to the recipient
• C is the cost to the actor

Ecological Factors

Ecological factors, such as harsh environmental conditions, high predation pressure, and limited resources, may also play a role in the evolution of eusociality. In these environments, individuals may benefit from living in groups and cooperating to survive and reproduce. For example, the subterranean lifestyle of naked mole-rats and Damaraland mole-rats may have favored the evolution of eusociality by making it difficult for individuals to disperse and establish their own colonies.

Fortress Defense

This hypothesis suggests that eusociality evolves in species that live in defensible nests or territories, such as wood-boring insects or gall-forming aphids. The benefits of group living and cooperative defense outweigh the costs of reproductive suppression.

The Benefits of Eusociality

Eusociality offers several advantages to the species that have evolved this social structure.

• Increased Efficiency: The division of labor allows individuals to specialize in specific tasks, which can increase efficiency and productivity.
• Improved Defense: Cooperative defense can protect the colony from predators and competitors.
• Enhanced Resource Acquisition: Group foraging can increase the efficiency of resource acquisition.
• Extended Lifespan: Queens in eusocial species often have remarkably long lifespans compared to solitary individuals.
• Stable Environment: The colony provides a stable and buffered environment for the developing young.

The Costs of Eusociality

Eusociality also has several costs associated with it.

• Reproductive Suppression: The majority of individuals in the colony are non-reproductive, which can be a significant cost.
• Inbreeding Depression: Inbreeding, which is common in some eusocial species, can lead to reduced fitness due to the expression of deleterious recessive genes.
• Competition: Competition for resources within the colony can be intense.
• Disease Transmission: High population densities within the colony can increase the risk of disease transmission.

Conclusion

Eusociality is a fascinating and complex social structure that has evolved in a limited number of animal species. While most eusocial species are insects, eusociality has also evolved in two species of mammals, the naked mole-rat and the Damaraland mole-rat. The evolution of eusociality is likely driven by a combination of genetic and ecological factors, including kin selection, harsh environmental conditions, and the benefits of cooperative living. Eusociality offers several advantages, such as increased efficiency, improved defense, and enhanced resource acquisition, but it also has several costs, such as reproductive suppression and inbreeding depression. Studying eusocial animals provides valuable insights into the evolution of social behavior and the complex interactions between genes, environment, and social organization.