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: • fight or flight response • threat display to warn competitors and signal honest intentions • defence behaviour • simulated
paralysis • avoidance behaviour • withdrawal • settling behaviour. 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: • hunger • crowding by human divers (independent or group dives) and submersible machinery • sustained targeted pursuit • invasion of the shark's idiosphere, without appropriate warning or consent • natural competition for resources with other organisms • obstructions on the
benthic floor which block escape routes or reduce scope of visibility • crowding by boats. 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: • stiff posture • tail flexure • tail depression • head shaking • jaw gaping •
ritualistic jaw snapping • open jaw tooth raking • gill pouch billowing • torso thrusting • clasper flexion • tail slapping • flank displaying • body shivering • reduced swimming efficiency • charging • ramming with snout • rapid withdrawal from threat 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 ==