Animal behavior
Agonism is a broad term which encompasses many behaviours that result from, or are triggered by biological conflict between competing organisms. It is defined as "survivalist animal behaviour that includes aggression, defense, and avoidance". Approximately 23 shark species are capable of producing such displays when threatened by intraspecific or interspecific competitors, as an evolutionary strategy to avoid unnecessary combat. The behavioural, postural, social and kinetic elements which comprise this complex, ritualized display can be easily distinguished from normal, or non-display behaviour, considered typical of that species' life history. The display itself confers pertinent information to the foe regarding the displayer's physical fitness, body size, inborn biological weaponry, confidence and determination to fight. This behaviour is advantageous because it is much less biologically taxing for an individual to display its intention to fight than the injuries it would sustain during conflict. Agonistic displays are essential to the social dynamics of many biological taxa, extending far beyond sharks.
Characteristics.
Definition.
Agonistic displays are ritualized sequences of actions, produced by animals belonging to almost all biological taxa, in response to conflict with other organisms. If challenged or threatened, animals may employ a suite of adaptive behaviours, which are used to reinforce the chances of their own survival. Behaviours which arise from agonistic conflict include: 
Each of these listed strategies constitute some manifestation of agonistic behaviour, and have been observed in numerous shark species, among many higher taxa in Kingdom Animalia. Displays of this nature are influenced and reinforced by natural selection, as an optimal strategy for avoiding physical conflict, and the costs of such interactions. In nature, agonistic behaviour is both species and situation-specific, and is often expressed as a graded response to imposed threats.
Triggers.
The most effective triggers of agonistic behaviour in sharks include:
The intensity and duration of the resulting display graded in proportion to the shark's perceived degree of endangerment during the conflict.
Physical characteristics.
Apart from species which incorporate unique elements into their display, a typical threat display will include the following four postural elements: elevated snout, pectoral fin depression, arching of the back, and lateral flexing of the body to display total size, and two locomotory elements: jerky side-to-side movements and rolling or spiral looping in the water column. There are always exceptions to this rule. Certain species will display unique visual cues, outside of the displays discussed here, as individual species face their own biological stresses that drive the selection and reinforcement of new adaptive behaviours.
Outside the typical display, certain species have been reported displaying the following behaviours as part of a species-specific display:
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Regardless of the precise sequence, agonistic displays function as accurate, predictive warnings to competitors which communicate a willingness to engage in more intense behaviour, if further provoked. Displays of this nature will always either result in a fleeing behaviour, or rapid, slashing attack from the displaying individual.
Characteristics of normal behaviour.
To recognize and identify true agonistic behaviour, it is first important to understand what constitutes "normal" behaviour in sharks, specific to the lifestyle of the species of interest. For sharks, normal behaviour is considerably different than the exaggerated appearance of the agonistic display. A shark which is in distress will behave in a manner that is easily recognizable, due to the sheer oddity of the sequence of movements and torsional elements characteristic of agonistic displays - juxtaposed to normal behaviour, which appears visibly more relaxed and natural.
The following table summarizes the characteristic physical elements of sharks displaying agonistic behaviour and non-displaying sharks:
Evolutionary relevance.
As an apex predator, sharks are no stranger to biological conflict. To successfully hunt and kill prey, sharks regularly encounter competing conspecific and heterospecific individuals, each willing to fight them to earn dominion over the meal, which in itself is a rare, valuable resource. As part of any biological conflict, natural instinct dictates that the shark fights to outcompete their competitors, or to subdue prey. In the past, combat was the sole means to determine which competing organism attained dominion over valuable resources, including access to prey, territory and reproductive partners. During combat, however, animals are highly susceptible to injury which significantly reduces overall and reproductive fitness. Sustained injuries impede each individual's ability to perform necessary biological functions, including foraging, courtship and mating behaviour. Thus, given the maladaptive nature of physical combat, animals have since developed refined methods to communicate their intent and willingness to fight, as a strategy to evade suffering the costs of physical combat.
Through the process of ritualization, the initial sequences of the animal's fighting behaviour, referred to as intention movements, are received by other animals as sign stimuli that the signaller is ready to fight. Through evolutionary time, sharks specifically have developed a suite of adaptive agonistic behaviours to communicate their intentions during such conflict. Sharks display recognizable sequences of behaviour, which mimic the initial phases of their fight sequence, to signal their degree of agitation, along with their intent to fight. The stereotyped postures and movements displayed by the shark are subsequently interpreted by competing organisms as honest, predictive signals of their level of commitment.
Producing the agonistic display incurs very slight costs for the displaying organism, but are performed nevertheless, as they mitigate the potential risks of sustaining a life-threatening injuries in combat. Instead of fleeing or attacking the competitor, a displaying organism sacrifices its competitive edge to perform the display, to warn and hopefully deter the challenger. If the displaying organism is larger or more robust than its competitor, the display would save it from having to fight to prove its dominance. The smaller opponent would withdraw after recognizing its own inadequacies during this short display period, and retreat from the dominant opponent. In this sense, displays are evolutionarily beneficial as they cost very little in the grand-scheme, but maintain the potential benefit of not sustaining life-threatening injuries during combat, which would negatively impact the organism's fitness.
The following is a summary of the costs and benefits attributed to shark threat displays in comparison to mounting a physical attack as a primary response to natural conflict:
Case studies.
Gray reef shark.
Available literature suggests that the standardized threat display has been observed and appropriately documented in many shark species. Additionally, some express dissimilar behavioural patterns, in a species-specific context. Of the described incidences where agonistic displays have been observed, the gray reef shark is the most commonly reported, as they hold a reputation for having a very exaggerated, recognizable display, which incorporates all of the most commonly reported physical and dynamic elements. Thus, gray reef sharks are often selected as model organisms to describe incidences of agonistic behaviour in sharks, as theirs is the most visually distinctive, providing a near perfect representation of the behaviour. 
Sharksucker-induced agonistic pseudodisplays.
By definition, pseudodisplays of aggression are "apparent" physical displays that closely resemble agonistic behaviour patterns elicited in response to conflict, but are not themselves produced as a result of conflict. While the visual cues performed during the pseudodisplay appear visually similar to the genuine agonistic display, sharks only exhibit the true elements of this display in response to biological conflict. The overlapping kinetic and postural elements of the pseudodisplay and the typical agonistic display include twitching, shaking, body rolling, flexure or depression of pectoral and caudal fins, jaw gaping, and accelerated, jerky movements. Biologists and behavioural scientists often mistake one or more of these elements as part of their stereotyped threat display, when only a proportion of the observed instances truly are.
A significant number of displays are mistaken for the actions produced during attempts to lessen the irritation caused by sharksuckers ("Echeneis naucrates)" along the length of their body. Sharks are known to have sensitive skin, due to the high concentration of electroreceptors distributed across their face and body. For sharks, irritation is a regular occurrence when they spend as much time as they do being cleaned by sharsuckers. In recognition that the majority of behavioural elements exhibited during a genuine agonistic encounter overlap with those performed as part of the pseudodisplay, it is easy to understand how and why accurately differentiation between the two is a challenge. 
Sharks are extremely limited by their physiology, in the sense that the distribution of key appendages or structures limit overall flexibility and maneuverability, based solely on their apparent location on the body. As a result, sharks rely on a select few features to accomplish all necessary biological functions. Due to these limitations, the available appendages inherit multifunctional capabilities, and the shark can perform tasks this include signalling, swimming and removal of irritants, all with the same appendages and sequences of movement. Despite the inherent redundancy of their body plan, sharks are able to effectively communicate their intentions to nearby organisms, such as when they produce their agonistic display. However, due to the nature of this overlap in function, agonistic behaviour is often mistaken/misidentified as pseudodisplays, resulting from sharksucker-induced irritation. The very slight distinction between true agonistic behaviour and the irritation-borne pseudodisplay has historically, and will continue to confound scientists dedicated to this area of research, since the two are nearly indistinguishable from each other, at first glance.
References.
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