11.2 Dominance hierarchies

Dominance hierarchies shape social structures in animal groups, determining status and resource access. These hierarchies can be linear or non-linear, stable or unstable, influenced by factors like aggression, physical attributes, age, and resource holding potential.

Establishing and maintaining dominance involves agonistic interactions, ritualized displays, and reinforcement. High-ranking individuals often enjoy benefits like priority resource access and reproductive advantages, but may face costs like energy expenditure and injury risks.

Types of dominance hierarchies

• Dominance hierarchies are social structures that determine the relative status and access to resources among individuals in a group
• The type of dominance hierarchy can vary depending on the species, group size, and ecological factors

Linear vs non-linear hierarchies

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• Linear hierarchies have a clear ranking order where each individual dominates all those below it and is dominated by all those above it (baboons, chickens)
• Non-linear hierarchies have more complex relationships where the relative dominance between individuals may not be transitive or consistent across all contexts (spotted hyenas, cichlid fish)
• Linear hierarchies tend to be more stable and predictable, while non-linear hierarchies can be more dynamic and context-dependent

Stable vs unstable hierarchies

• Stable hierarchies maintain a consistent ranking order over time with few changes in dominance relationships (wolves, gorillas)
• Unstable hierarchies are characterized by frequent challenges and changes in dominance status among individuals (chimpanzees, horses)
• Factors such as resource availability, group composition, and individual attributes can influence the stability of dominance hierarchies

Factors influencing dominance

• Multiple factors can contribute to an individual's dominance status within a hierarchy
• The relative importance of these factors can vary across species and contexts

Role of aggression

• Aggressive behaviors such as threats, displays, and physical attacks are often used to establish and maintain dominance (red deer, elephant seals)
• Individuals with higher levels of aggression or the ability to win aggressive encounters are more likely to achieve higher dominance status
• Aggression can also be used to challenge and displace higher-ranking individuals

Physical attributes and size

• Larger body size, strength, and weaponry (horns, antlers, canines) can provide an advantage in dominance contests (bighorn sheep, elephants)
• Physical attributes may be more important in species where dominance is primarily determined through direct competition and fighting

Age and experience

• Older individuals often have higher dominance status due to their greater experience, knowledge, and social connections (meerkats, African elephants)
• Age-related dominance can be influenced by factors such as physical condition, reproductive status, and the presence of allies or kin

Resource holding potential

• Resource holding potential (RHP) refers to an individual's ability to acquire and defend resources such as food, mates, and territory
• Individuals with higher RHP, determined by factors such as size, strength, and motivation, are more likely to achieve higher dominance status (red-winged blackbirds, hermit crabs)
• RHP can be context-dependent and may vary across different types of resources

Establishment of dominance hierarchies

• Dominance hierarchies are established through a series of social interactions and contests among individuals
• The outcomes of these interactions determine the relative dominance relationships within the group

Agonistic interactions and contests

• Agonistic interactions involve aggressive or threatening behaviors used to establish dominance (staring, chasing, biting)
• Contests can range from ritualized displays to physical fights and are often used to settle dominance disputes (mountain goats, cichlid fish)
• The winner of an agonistic interaction or contest typically achieves a higher dominance status relative to the loser

Ritualized displays and postures

• Many species use ritualized displays and postures to signal dominance or submission without engaging in direct physical contact (wolves, chimpanzees)
• Dominant displays may include enlarged body postures, vocalizations, and exposing weapons or ornaments
• Submissive displays often involve crouching, avoiding eye contact, and exposing vulnerable body parts

Winner and loser effects

• The outcome of a dominance contest can have lasting effects on an individual's future success in establishing dominance
• Winners of previous contests are more likely to win future contests, while losers are more likely to lose (cockroaches, crickets)
• These winner and loser effects can contribute to the formation and maintenance of stable dominance hierarchies

Maintenance of dominance hierarchies

• Once established, dominance hierarchies must be actively maintained through ongoing social interactions and reinforcement
• Various factors can influence the stability and persistence of dominance relationships over time

Reinforcement through interactions

• Dominant individuals reinforce their status through repeated agonistic interactions and displays towards subordinates (baboons, chickens)
• Subordinates may also signal their acceptance of the dominance relationship through submissive behaviors and avoidance
• Regular reinforcement helps to maintain the stability of the dominance hierarchy

Challenges to dominance status

• Subordinate individuals may challenge the dominance status of higher-ranking individuals in an attempt to improve their own position (chimpanzees, horses)
• Challenges can be triggered by factors such as changes in physical condition, resource availability, or social support
• Successful challenges can lead to a reorganization of the dominance hierarchy

Factors affecting stability

• The stability of a dominance hierarchy can be influenced by various factors such as group size, composition, and ecological conditions
• Larger groups and higher levels of competition for resources may lead to more frequent challenges and instability (spotted hyenas, red deer)
• The presence of kin or allies can help to stabilize dominance relationships by providing social support and reducing the likelihood of challenges

Benefits of high dominance status

• Individuals with high dominance status often enjoy a range of fitness advantages compared to subordinates
• These benefits can have important consequences for survival and reproductive success

Priority access to resources

• Dominant individuals typically have priority access to limited resources such as food, water, and shelter (African elephants, mountain gorillas)
• This priority access can be particularly important during times of scarcity or competition
• Improved access to resources can lead to better physical condition and increased chances of survival

Reproductive advantages

• High dominance status often confers reproductive advantages, particularly in males (red deer, elephant seals)
• Dominant males may have exclusive or preferential access to mating opportunities, leading to higher reproductive success
• Dominant females may also benefit from improved access to resources and social support, which can enhance offspring survival

Reduced stress and aggression

• Dominant individuals often experience lower levels of stress and aggression compared to subordinates (baboons, meerkats)
• This may be due to their greater control over resources and social interactions, as well as the submissive behaviors of subordinates
• Reduced stress can have positive effects on health, longevity, and reproductive success

Costs of high dominance status

• While high dominance status confers many benefits, it can also be associated with significant costs and trade-offs
• These costs can limit the advantages of dominance and influence the evolution of dominance hierarchies

Energy expenditure in defense

• Maintaining high dominance status often requires significant energy expenditure in the form of aggressive displays, contests, and vigilance (red deer, elephant seals)
• This increased energy demand can lead to reduced body condition and survival, particularly during times of resource scarcity
• The costs of dominance defense may be particularly high in unstable hierarchies or when faced with frequent challenges

Increased risk of injury

• Engaging in aggressive interactions and contests to establish and maintain dominance can increase the risk of injury (bighorn sheep, chimpanzees)
• Injuries can range from minor wounds to severe or fatal injuries, depending on the species and the intensity of the contest
• The risk of injury may be higher for individuals who are frequently challenged or who engage in escalated contests

Stress associated with challenges

• High-ranking individuals may experience increased stress associated with the constant threat of challenges from subordinates (baboons, horses)
• This chronic stress can have negative effects on health, immune function, and reproductive success
• Stress levels may be particularly high in unstable hierarchies or when dominance status is frequently contested

Dominance hierarchies in social groups

• Dominance hierarchies play an important role in the social organization and functioning of many animal groups
• The structure and dynamics of dominance hierarchies can have significant implications for individual fitness and group-level processes

Role in group cohesion and stability

• Dominance hierarchies can promote group cohesion and stability by reducing the frequency and intensity of aggressive interactions (wolves, gorillas)
• Clear dominance relationships provide a framework for social interactions and help to minimize conflicts over resources and mating opportunities
• Stable dominance hierarchies can facilitate cooperation and coordination among group members

Influence on reproductive success

• Dominance status can have a strong influence on reproductive success, particularly in species with high reproductive skew (meerkats, lions)
• In some species, dominant individuals may monopolize breeding opportunities, leading to high reproductive success for a small number of individuals
• Subordinate individuals may have limited or no reproductive opportunities, or may engage in alternative reproductive strategies

Inheritance of dominance status

• In some species, dominance status can be inherited or influenced by the dominance rank of an individual's parents (spotted hyenas, Japanese macaques)
• Inherited dominance status can provide a significant advantage to offspring, particularly in terms of access to resources and mating opportunities
• The degree of dominance inheritance can vary across species and may be influenced by factors such as maternal rank, group size, and ecological conditions

Interspecific dominance hierarchies

• Dominance hierarchies can also occur between different species that interact within the same ecological community
• Interspecific dominance can have important implications for resource use, habitat selection, and community structure

Dominance between species

• Interspecific dominance occurs when one species consistently dominates or displaces another species in competitive interactions (African lions and spotted hyenas)
• Dominance between species can be influenced by factors such as body size, group size, and ecological specialization
• Interspecific dominance can lead to the spatial or temporal partitioning of resources and habitat use

Factors determining interspecific dominance

• The relative dominance between species can be determined by various factors, including physical attributes, behavioral strategies, and ecological context
• Larger body size or the presence of specialized weaponry may confer a dominance advantage in direct competitive interactions (African elephants and black rhinoceroses)
• Behavioral strategies such as aggression, boldness, or social coordination can also influence interspecific dominance relationships

Ecological implications

• Interspecific dominance hierarchies can have significant ecological implications, influencing the distribution, abundance, and behavior of species within a community
• Dominant species may exclude or limit the access of subordinate species to preferred resources or habitats (grey wolves and coyotes)
• The presence of a dominant species can also have cascading effects on other species and ecosystem processes through direct and indirect interactions

Measuring dominance hierarchies

• Quantifying and analyzing dominance hierarchies requires systematic behavioral observations and the use of appropriate statistical methods
• Various approaches can be used to measure and describe the structure and properties of dominance hierarchies

Behavioral observations and sampling

• Collecting data on dominance interactions requires careful observation and sampling of social behaviors (aggression, submission, displacements)
• Focal animal sampling, where a single individual is observed for a set period, can provide detailed data on dominance interactions
• All-occurrence sampling, where all observed dominance interactions are recorded, can provide a more comprehensive picture of the hierarchy

Dominance matrices and linearity

• Dominance interactions can be summarized in a dominance matrix, where individuals are ranked based on the outcomes of their interactions
• Linearity refers to the degree to which dominance relationships are transitive and can be arranged in a linear hierarchy
• Linearity can be assessed using measures such as Landau's linearity index or de Vries' improved linearity index

Statistical analysis of hierarchies

• Various statistical methods can be used to analyze dominance hierarchies, including matrix-based approaches and network analysis
• Matrix-based methods, such as the I&SI method, use dominance interaction data to calculate dominance scores and rank individuals
• Network analysis can be used to visualize and quantify the structure of dominance hierarchies, including measures of centrality and clustering

Evolution of dominance hierarchies

• Dominance hierarchies have evolved in many animal species as a means of regulating social interactions and access to resources
• The evolution of dominance hierarchies can be influenced by various selective pressures and evolutionary processes

Adaptive significance

• Dominance hierarchies can provide adaptive benefits to individuals and groups by reducing the costs of aggression and competition
• For individuals, high dominance status can confer fitness advantages through improved access to resources and mating opportunities
• At the group level, dominance hierarchies can promote stability, cohesion, and cooperation, enhancing overall group success

Phylogenetic patterns

• The presence and structure of dominance hierarchies can vary across taxonomic groups and may show phylogenetic patterns
• Closely related species may exhibit similar dominance hierarchies due to shared evolutionary history and ecological factors
• Comparative studies can provide insights into the evolutionary origins and diversification of dominance hierarchies across species

Role of sexual selection

• Sexual selection, particularly intrasexual competition, can play a significant role in shaping the evolution of dominance hierarchies
• In many species, dominance status is closely linked to reproductive success, with dominant individuals having greater access to mates
• The intensity of sexual selection can influence the degree of reproductive skew and the stability of dominance hierarchies within a population