Chapter 11 Summary

Two individuals competing for the same nonshareable resource item represent the maximum degree of sender-receiver conflict, with both preferring that the other party back down. But both benefit if a physical fight can be avoided. Two competitors for the same resource are rarely equal in fighting ability, motivation, or experience, each of which can affect the likelihood of winning a fight. Animals make use of prior information, cues, and signals to assess their likelihood of winning a fight against a particular opponent. If one party estimates that it is likely to lose, it makes the decision to quit. If communicating cannot resolve the likely winner, the contest will escalate to costly physical fighting. The signals and tactics employed during contests are collectively referred to as agonistic behaviors, which include both aggressive and submissive acts.
Most evolutionary game models of conflict resolution assume that individuals possess some knowledge of their own fighting ability. Engaging in contests and remembering the outcome is the best way to acquire this information. Winning increases one’s estimate, while losing decreases it. This winner-loser effect has a physiological basis that affects subsequent aggressive behavior. A cognitive sense of self is not required, only simple rules of thumb that adjust effort invested in a contest based on the outcome of previous contests. Formation of linear dominance hierarchies in group-living species is an emergent property of such winner-loser effects.
Animals fight to gain access to limited resources such as food, mates, shelters, and territories. The value of the resource item, the contestants’ need for the resource, and the availability of alternative resource items determine how motivated a contestant is to fight for the contested item. Resource value can be measured as the difference between the fitness gain if the contest is won and the fitness loss if it is lost. Resources such as territories, shelters, and sometimes mates may be defended for some length of time, leading to a level of ownership. Owners are likely to value the resource more than intruders because of their investment in defending the resource, and they may have better knowledge of its specific value. A high resource valuation is assumed to lead to an increased motivation to fight.
Species employ different fighting strategies—the signals, tactics, and combat methods used during extended conflicts. Fighting strategy models differ in their assumptions about the way contestants gather information during the interaction, which types of signals and tactics are used, and how they make the decision to end it. The sequential assessment model assumes that animals have little information about their rival’s relative fighting ability at the beginning of a contest, but acquire this information in a mutual assessment process by performing repeated mutual displays. Increasingly costly signals and tactics are used in stages if cheaper signals do not lead to the resolution of the likely loser. When one contestant is relatively certain it will lose, it makes the decision to quit.
Two other fighting models are based on the notion that a contestant makes the decision to quit based only on self-assessment of its own fighting ability, rather than by mutual assessment. The energetic war of attrition model assumes that contestants engage in a matched and costly chasing or grappling contest, A contestant’s decision about whether to continue or quit an extended agonistic interaction is based on its assessment of its current ability to continue in this activity. The first individual to quit loses the contest. The cumulative assessment model assumes that that contestants inflict significant stress and injury upon each other during the conflict, and a contestant’s decision about whether to continue or quit a fight is based on the sum of the costs it has sustained.
The three models differ in many of their predictions. Empirical tests of the alternative models have provided support for each one in different species. However, the energetic war of attrition model applies only to a few species that fight with extended chases. The cumulative assessment model is likely to characterize the escalated stage of fighting in some species. Most species undergo at least some initial signaling and mutual assessment, as described by the sequential assessment model, before escalating to a physical fight.
Other types of asymmetric information may be used to help settle conflicts. Resource value asymmetry—in which the contestants place different values on the resource—is expressed in the assessment phase of the contest and involves signals that indicate motivation to fight as well as fighting ability. For conflicts involving owned resources, an owner/intruder role asymmetry may be used to settle the contest. The bourgeois strategy is a solution in which contestants assess which one of them is the owner, and the invader then retreats. Although owners usually win against invaders, recent models and empirical results indicate that this simple rule is not stable, and that owners usually win because they have the better proven fighting ability and value the owned resource more.
Most species have a rich repertoire of agonistic signals, including signals that indicate body size, aggressive motivation, offensive and defensive threat, dominance status, victory, submission, appeasement, and retreat. Fighting ability signals include index signals and amplifiers of body size, and handicap signals indicating stamina. Stamina is usually dependent on condition, and informative signals include performance of energetically costly displays, as well as color and acoustic signals affected by condition. Weapons serve a tactical function during fights by enabling contestants to jab, wrestle, block, or flip the opponent. Extremely enlarged weapons occur in a few taxa where males defend burrows or compete intensely for females. Fighting is highly ritualized, and the size of weapon primarily serves as an assessment signal of body size and condition.
Because they are used only in conditions where there is a strong conflict of interest, signaling costs are needed to guarantee some level of signal reliability. Honesty-guaranteeing costs include receiver retaliation, proximity risk, physical/physiological constraints, and energetic costs, and thus may be classified as conventional, proximity threat, index, and handicap signals, respectively.
Aggressive motivation signals are important for species in which resource value and need vary significantly between contestants. The signals indicate a sender’s willingness to fight for a resource item, and can be subdivided into three categories. Challenge signals are targeted toward a specific rival from a distance. Conventional visual challenge signals include directed looking postures, and acoustic signals include song matching and frequency matching. General aggressive motivation signals indicate level of arousal (desire for the resource and willingness to fight for it) and often take the form of energetically costly repeated displays. Offensive threats are given close to the opponent, where there is a significant proximity risk of injury, and are strongly correlated with subsequent attack. Examples include intention-to-attack postures and low-amplitude acoustic signals.
Dominance signals reflect an animal’s prior fighting experience. Visual and olfactory badges of status are dominance signals that appear to be conventional in form but require a physiological link to aggressive hormones and neurotransmitters, in addition to an immediate retaliation cost, to remain honest. Territory ownership signals are site-based dominance signals that declare the presence of the owner, define the defended area, and repel intruders. Ownership signals usually encode individual identity as well as the owner’s motivation and condition. Boundaries between neighbors are often dynamic and negotiated with acoustic (e.g., countersinging) or olfactory (e.g., overmarking) interactions. Victory displays are postcontest signals given by the winner, and may serve to browbeat the loser, demonstrate confidence, or advertise the victory to third-party receivers. De-escalation signals are antithetical in form to aggressive threat signals and are designed either to move the quitting individual quickly away from the dominant one, or to indicate the intention not to attack.