11.1 Eusociality and insect societies

Eusociality represents the pinnacle of social organization in the animal kingdom. It's characterized by cooperative brood care, overlapping generations, and a reproductive division of labor. This advanced form of sociality has evolved independently in various groups, including insects, crustaceans, and mammals.

Insect societies, particularly ants, bees, wasps, and termites, showcase the most complex eusocial systems. These societies exhibit intricate communication methods, specialized castes, and sophisticated colony organization. Understanding eusociality provides insights into the evolution of cooperation and social behavior in nature.

Defining eusociality

• Eusociality is a highly advanced form of sociality characterized by cooperative brood care, overlapping generations, and reproductive division of labor
• Eusocial animals live in complex societies with intricate social structures and communication systems
• Eusociality has evolved independently in various taxa, including insects, crustaceans, and mammals

Key criteria of eusociality

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• Three main criteria define eusociality: overlapping generations, cooperative brood care, and reproductive division of labor
• These criteria distinguish eusocial societies from other forms of social organization, such as communal living or cooperative breeding
• All three criteria must be met for a species to be considered truly eusocial

Overlapping generations

• In eusocial societies, multiple generations of individuals coexist within the same colony or social group
• Offspring remain with their parents and contribute to colony tasks, such as brood care, foraging, and nest maintenance
• Overlapping generations allow for the transfer of knowledge and skills from older to younger individuals (social learning)

Cooperative brood care

• Eusocial animals engage in cooperative care of the colony's offspring, regardless of their direct genetic relatedness
• Workers, which are often non-reproductive individuals, feed, protect, and nurture the developing brood
• Cooperative brood care ensures the survival and well-being of the colony's future generations

Reproductive division of labor

• Eusocial societies exhibit a clear division of reproductive roles, with one or a few individuals (queens) specializing in reproduction while the majority (workers) are sterile or have reduced reproductive potential
• The reproductive division of labor allows for efficient allocation of resources and energy within the colony
• Queens focus on egg production, while workers perform tasks essential for colony maintenance and growth

Evolution of eusociality

• The evolution of eusociality has been a topic of intense research and debate among evolutionary biologists
• Several theories and hypotheses have been proposed to explain the origins and maintenance of eusocial systems
• Understanding the evolutionary drivers of eusociality provides insights into the complex social behaviors observed in eusocial animals

Inclusive fitness theory

• Inclusive fitness theory, proposed by W. D. Hamilton, suggests that individuals can increase their genetic representation in future generations by helping close relatives (kin selection)
• Eusociality can evolve when the benefits of helping relatives outweigh the costs of reduced personal reproduction
• Inclusive fitness theory helps explain the high levels of cooperation and altruism observed in eusocial societies

Haplodiploidy hypothesis

• The haplodiploidy hypothesis proposes that the unique sex determination system in hymenopteran insects (ants, bees, and wasps) predisposes them to the evolution of eusociality
• In haplodiploid systems, females develop from fertilized eggs and are diploid, while males develop from unfertilized eggs and are haploid
• This system results in higher relatedness between sisters (r=0.75) than between mothers and daughters (r=0.5), potentially favoring the evolution of worker behavior

Ecological factors

• Ecological factors, such as resource distribution, predation pressure, and habitat stability, can influence the evolution of eusociality
• Eusociality is more likely to evolve in stable, resource-rich environments where the benefits of group living and cooperative foraging are high
• Harsh or unpredictable environments may favor solitary or less social lifestyles

Phylogenetic constraints

• The distribution of eusociality across taxa suggests that certain lineages may be predisposed to evolving eusocial behavior due to shared ancestral traits or evolutionary history
• Eusociality has evolved multiple times independently in insects, particularly in the orders Hymenoptera (ants, bees, and wasps) and Isoptera (termites)
• Phylogenetic analyses can help identify the evolutionary transitions and key innovations that have led to the emergence of eusociality in different lineages

Insect societies

• Insects, particularly those in the orders Hymenoptera and Isoptera, have evolved some of the most complex and diverse eusocial societies
• Insect societies exhibit a wide range of social structures, communication systems, and division of labor
• Studying insect societies provides valuable insights into the ecology, evolution, and organization of eusocial systems

Ants

• Ants (family Formicidae) are one of the most successful and diverse groups of eusocial insects, with over 12,000 described species
• Ant colonies can range in size from a few dozen to millions of individuals, with highly specialized castes and complex division of labor
• Ants have evolved various adaptations for communication (pheromones), foraging (trail networks), and defense (soldier castes)

Bees

• Bees, particularly honey bees (genus Apis) and stingless bees (tribe Meliponini), exhibit advanced eusocial behavior
• Honey bee colonies consist of a single reproductive queen, thousands of sterile workers, and male drones
• Bees are known for their elaborate communication systems, such as the waggle dance used to convey information about food sources

Wasps

• Some wasp species, such as paper wasps (family Vespidae) and hover wasps (family Stenogastrinae), have evolved eusocial behavior
• Wasp societies often have smaller colony sizes compared to ants and bees, with a few dozen to a few hundred individuals
• Wasps exhibit a range of nesting behaviors, from simple paper nests to complex mud structures

Termites

• Termites (order Isoptera) are eusocial insects that have evolved independently from Hymenoptera
• Termite colonies can consist of hundreds to millions of individuals, with specialized castes such as workers, soldiers, and reproductives
• Termites play important ecological roles as decomposers and have evolved unique adaptations for wood digestion and nest construction

Colony organization

• Eusocial insect colonies exhibit highly organized social structures and division of labor
• Colony organization is essential for the efficient functioning and survival of the colony as a whole
• Various factors, such as caste systems, age polyethism, and task specialization, contribute to the complex organization of insect societies

Caste systems

• Eusocial insects often have distinct castes, such as queens, workers, and soldiers, each with specific morphological and behavioral adaptations
• Castes are determined by a combination of genetic factors, environmental cues, and developmental processes (e.g., nutrition, hormones)
• The presence of castes allows for the division of labor and the optimization of colony performance

Queen vs worker roles

• Queens are the primary reproductive individuals in eusocial insect colonies, responsible for laying eggs and ensuring the colony's continued growth
• Workers are typically sterile or have reduced reproductive potential and perform various tasks related to colony maintenance, such as brood care, foraging, and nest defense
• The distinct roles of queens and workers are essential for the reproductive division of labor and the overall success of the colony

Age polyethism

• Age polyethism refers to the age-related changes in task performance among workers in eusocial insect colonies
• As workers age, they tend to transition from tasks within the nest (e.g., brood care) to tasks outside the nest (e.g., foraging)
• Age polyethism allows for the efficient allocation of labor and the minimization of risks to the colony (e.g., older workers perform more dangerous tasks)

Task specialization

• Within the worker caste, individuals may specialize in specific tasks, such as foraging, nest maintenance, or defense
• Task specialization can be influenced by factors such as individual morphology, genetics, and experience
• Specialization allows for increased efficiency and performance in specific tasks, contributing to the overall productivity of the colony

Communication in insect societies

• Effective communication is crucial for the coordination and organization of eusocial insect colonies
• Insect societies have evolved sophisticated communication systems that allow individuals to convey information about food sources, nest sites, and potential threats
• Chemical, vibrational, and visual communication are the primary modes of information transfer in eusocial insects

Chemical communication

• Chemical communication, particularly through pheromones, is the most widespread and important mode of communication in eusocial insects
• Pheromones are chemical signals that elicit specific behavioral or physiological responses in other individuals
• Eusocial insects use pheromones for various purposes, such as trail marking, alarm signaling, and reproductive control

Pheromones

• Pheromones are secreted by specialized glands and can be detected by other individuals through olfactory receptors
• Different types of pheromones include trail pheromones (for recruitment to food sources), alarm pheromones (for warning of threats), and queen pheromones (for regulating reproduction)
• The complex pheromone communication systems in eusocial insects allow for the coordination of colony activities and the maintenance of social cohesion

Vibrational communication

• Some eusocial insects, such as termites and certain ant species, use vibrational signals for communication
• Vibrational signals are produced by body movements (e.g., drumming, stridulation) and are transmitted through the substrate (e.g., soil, wood)
• Vibrational communication can convey information about food sources, nest sites, and alarm signals

Dances in honey bees

• Honey bees have evolved a unique communication system based on dance language
• The waggle dance, performed by forager bees, conveys information about the distance, direction, and quality of food sources to other colony members
• The round dance, a simpler version of the waggle dance, is used for food sources close to the hive
• The honey bee dance language is one of the most sophisticated forms of non-human communication and has been extensively studied by ethologists

Foraging strategies

• Foraging is a critical activity for eusocial insect colonies, as it provides the resources necessary for colony growth and reproduction
• Eusocial insects have evolved various foraging strategies to optimize resource acquisition and allocation
• Foraging strategies can involve individual or group foraging, recruitment mechanisms, and resource allocation decisions

Individual vs group foraging

• Eusocial insects can forage individually or in groups, depending on factors such as resource distribution, colony size, and species-specific traits
• Individual foraging is more common in species with small colony sizes or when resources are widely dispersed (e.g., some ant species)
• Group foraging, also known as mass recruitment, is more prevalent in species with large colony sizes and when resources are clustered (e.g., army ants, honey bees)

Recruitment mechanisms

• Recruitment is the process by which successful foragers communicate the location and quality of food sources to other colony members
• Eusocial insects use various recruitment mechanisms, such as trail pheromones, tandem running, and waggle dances (in honey bees)
• Effective recruitment allows colonies to exploit profitable food sources efficiently and adapt to changing resource availability

Resource allocation

• Once resources are brought back to the colony, they must be allocated among colony members according to their needs and roles
• Resource allocation can be influenced by factors such as caste, age, and reproductive status
• Efficient resource allocation ensures that the colony's energy and nutrient requirements are met, promoting colony growth and survival

Optimization models

• Foraging strategies in eusocial insects can be analyzed using optimization models from behavioral ecology
• Central place foraging theory, which considers the costs and benefits of foraging at different distances from a central location (e.g., the nest), has been applied to eusocial insect foraging
• Other optimization models, such as the marginal value theorem and the ideal free distribution, have been used to understand resource exploitation and competition in eusocial insects

Nest construction and defense

• Nest construction and defense are essential for the survival and reproduction of eusocial insect colonies
• Eusocial insects have evolved diverse nesting strategies and defensive behaviors to protect their colonies from predators, parasites, and environmental stressors
• Nest architecture, building materials, and specialized defensive castes all contribute to the success of eusocial insect colonies

Nest architecture

• Eusocial insects construct nests with complex architectures that serve various functions, such as shelter, microclimate regulation, and brood rearing
• Nest architecture can vary widely among species, from simple underground chambers to elaborate mounds or arboreal structures
• The organization and spatial arrangement of nest components (e.g., brood chambers, storage areas) reflect the specific needs and life histories of each species

Building materials and techniques

• Eusocial insects use a variety of building materials, such as soil, wood, resin, and secreted substances (e.g., wax, silk), to construct their nests
• Building techniques can involve excavation (e.g., subterranean ant nests), assembly of collected materials (e.g., termite mounds), or secretion of structural components (e.g., wax combs in honey bee hives)
• The choice of building materials and techniques is influenced by factors such as environmental conditions, resource availability, and species-specific adaptations

Defensive behaviors

• Eusocial insect colonies have evolved various defensive behaviors to protect against predators, parasites, and competitors
• Defensive behaviors can include physical aggression (e.g., biting, stinging), chemical defense (e.g., formic acid spraying in ants), and alarm communication (e.g., alarm pheromones)
• Collective defense, where multiple individuals cooperate to defend the colony, is a hallmark of eusocial insects and enhances the overall survivability of the colony

Soldier castes

• Many eusocial insect species have evolved specialized soldier castes with morphological and behavioral adaptations for colony defense
• Soldier castes can have enlarged mandibles, reinforced exoskeletons, or chemical weaponry (e.g., termite soldiers with frontal glands)
• The presence of soldier castes allows for a division of labor in colony defense, with soldiers focusing on protection while workers perform other tasks

Mating systems and reproduction

• Mating systems and reproductive strategies in eusocial insects are diverse and have significant implications for colony structure and genetic relatedness
• Eusocial insects exhibit various mating systems, including monogyny, polygyny, and polyandry, which influence the genetic composition of colonies
• Reproductive control mechanisms, such as queen pheromones, help maintain the reproductive division of labor within colonies

Monogyny vs polygyny

• Monogyny refers to a colony structure where a single queen monopolizes reproduction, mating with one or more males
• Polygyny refers to the presence of multiple reproductive queens within a single colony, each contributing to egg production
• The occurrence of monogyny or polygyny can depend on factors such as colony size, resource availability, and ecological conditions

Mating flights

• Many eusocial insects, particularly ants and termites, engage in mating flights, where reproductives (alates) leave their natal nests to mate and establish new colonies
• During mating flights, males and females from different colonies congregate in swarms, mate, and then disperse to found new colonies
• Mating flights ensure outcrossing and the dispersal of reproductive individuals, promoting genetic diversity and colonization of new habitats

Sperm storage

• Female reproductives in eusocial insects often have specialized organs called spermathecae, which store sperm from their mates
• Sperm storage allows queens to fertilize eggs throughout their lifetimes, even in the absence of additional mating events
• The long-term storage of viable sperm is essential for maintaining colony reproductive output and genetic diversity

Queen pheromones and control

• In many eusocial insect species, queens produce pheromones that regulate the reproductive development and behavior of other colony members
• Queen pheromones can suppress ovary development in workers, preventing them from laying their own eggs and maintaining the colony's reproductive hierarchy
• The presence of queen pheromones also influences worker behavior, such as foraging, brood care, and nest maintenance, ensuring the cohesion and functionality of the colony

Social parasitism

• Social parasitism is a phenomenon where certain eusocial insect species exploit the social structure and resources of other species
• Social parasites can invade host colonies, manipulate their behavior, and rely on the host workers for brood care and foraging
• The evolution of social parasitism has led to fascinating adaptations and host-parasite coevolutionary dynamics

Inquilines

• Inquilines are social parasites that coexist with the host species within the same colony, often relying on the host workers for brood care and resource provisioning
• Inquiline species can be closely related to their hosts (e.g., within the same genus) and may have evolved from non-parasitic ancestors
• Examples of inquilines include some species of ants (e.g., Solenopsis daguerrei) and bees (e.g., Psithyrus spp.)

Slave-making ants

• Slave-making ants, also known as dulotic ants, raid the nests of other ant species to capture their brood, which are then reared as workers (slaves) in the slave-maker colony
• Slave-making ants rely on their enslaved workers for foraging, brood care, and nest maintenance, while the slave-makers specialize in raiding and reproduction
• Examples of slave-making ants include species in the genera Polyergus and Harpagoxenus

Brood parasitism

• Brood parasitism occurs when a social parasite lays its eggs in the nest of a host species, relying on the host workers to rear the parasitic offspring
• Brood parasites can exploit the brood care behavior of the host species, often at the expense of the host's own reproduction