There is not human specific threat display

by Dr. Erich Ritter
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A being threatened display


The general appearance of this display consists of lowered pectoral fins and a hunched back connected to a rather exaggerated swim motion. The assumption is that this behavior is meant as a threat against a person for getting to close for example. However, this behavior has a different meaning than believed. A behavior is caused by a need, urge, or any other stimulus that demands an action of any kind. Should the situations then repeat themselves, with the same causing factors involved, the behavior does not have to be relearned anymore but remains ready to be used (if it was not forgotten in the meantime). But this behavior, called “threat display,” is also shown in sharks that never had contact with humans hence it can’t reflect a previous encounter and thus has likely been misinterpreted. Several scenarios can create the lowering of pectoral fins and hunching (in an anatomical sense) by a shark. Lowering of the pectoral fins is used to increase maneuverability in tight spaces (the lowering of the fins increases the lateral surface) and to create short bursts of speed aka “pectoral burst.” If a diver corners a shark, the animal likely tries to get away. It will lower its pectoral fins when close to its inner circle (inner threshold) to increase maneuverability and be able to swim tighter patterns. From a shark’s point of view, this posturing occurs because the animal feels threatened or is stressed by the presence of the person. The lowering of the pectoral fins also creates a rather less harmonic swim motion. Sharks also perform a swim pattern similar to the number 8 when in close(r) range of humans, which has also been interpreted as a threat display but is merely the swimming back and forth to look for escape routes and then can look more or less like the figure 8. Here as well, the posturing of the shark is caused by the potential threat of the diver. Something similar can also be observed when a diver is close to a cleaning station. As already described, a person’s presence can be stressful for a shark, leading to a displacement behavior since it feels itself “trapped” between the urge to get closer to the cleaning station and the compulsion to stay away from the person because of the potential threat. Depending on the intensity of the situation, sharks then often swim up and down, making tight circles. Another scenario in which pectoral fins are often lowered and backs hunched is triggered by sharksuckers that attach themselves to sensory and hydrodynamic sensitive areas. A shark may then try to get rid of these bony fishes (or at least force the sharksucker to move to another less sensitive spot on its body) by bending or squeezing the respective skin part to loosen the suction pressure and than look very similar to the presumed “threat display.” Such body bending can sometimes be accompanied very intense shaking of the entire body. The commonly assumed “threat display” is indeed most likely either a stress related behavior when a shark feels threatened, represents a displacement behavior, or is caused by irritating sharksuckers or through other “itches.”


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