The concept of an animal feigning death, known broadly as thanatosis, is a widespread survival mechanism observed across various species. This strategy is an act of deception where an animal deliberately adopts a state of stillness to deter a threat. Certain fish species exhibit behaviors that closely resemble “playing dead,” despite their different physiology.
Defining Playing Dead in Fish
The behavior of playing dead in fish is categorized under two related terms: thanatosis and tonic immobility (TI). Thanatosis, derived from the Greek word for death, refers to the intentional behavior of feigning death. TI is a more specific, reflexive state of temporary unresponsiveness and motor inhibition, typically triggered by physical restraint or extreme threat.
In fish, TI is characterized by an almost complete suspension of movement, making the animal appear lifeless. This state is distinct from being sick or stressed, as the fish remains largely unresponsive to external stimuli. It is an innate, involuntary reflex that occurs late in a predator-prey interaction, usually after the animal has been captured or is under immediate threat.
This state can last from seconds up to several minutes. While thanatosis involves deliberate deception and TI is a reflex, both result in the fish adopting a death-like posture. The fish’s brain often remains active and vigilant, ready for an opportunity to escape when the perceived danger subsides.
The Survival Strategy
The primary function of feigning death is predator evasion, providing an evolutionary advantage in a life-or-death scenario. This behavior is a last-resort defense mechanism, employed when the fish can no longer flee. The strategy works because many predators ignore prey that appears dead, often due to an instinct to avoid diseased or decaying flesh.
The temporary cessation of movement interrupts the predator’s attack sequence. Once the fish becomes motionless, the predator may lose interest or momentarily relax its concentration, believing the struggle is over. This brief lapse in attention is the window of opportunity the fish needs to suddenly “revive” and escape while the threat is distracted.
The decision to enter tonic immobility is a trade-off. It carries the risk of being ignored by a predator that prefers live prey, but offers a slight chance of survival when all other options are exhausted. This behavior is considered a secondary defense, occurring only after strategies like fleeing have failed.
Specific Examples and Observed Behavior
Many bony fish species, including goldfish, trout, and Oscars, can be induced into tonic immobility when physically restrained or inverted onto their backs. This action causes the fish to go limp, often resulting in a reduced rate of gill movement and a temporary loss of the righting reflex. Anglers sometimes use this state to quiet a fish while removing a hook.
In larger species, sharks, such as lemon sharks, enter tonic immobility when inverted and gently restrained. During this process, the shark’s dorsal fins straighten and its breathing becomes more relaxed, a state which can persist for around 15 minutes. This induced state is exploited by orcas, which have been observed ramming sharks to flip them and maintain the immobile state, preventing water flow over the gills.
A unique example of thanatosis is seen in certain African cichlids, such as Nimbochromis livingstonii, which use the behavior for hunting. This predatory fish lies on its side on the lakebed, displaying mottled coloration that mimics a decaying corpse. Unsuspecting smaller scavenger fish approach the seemingly dead body, only for the cichlid to spring back to life and ambush its prey. Other predatory cichlids, like Parachromis friedrichsthalii, exhibit a similar form of aggressive mimicry to lure prey.