Imagine you’re being attacked
by a ferocious predator. With no chance of escape, you do what any courageous,
self-respecting possum would do: curl into an immobile
state called catatonia, stick out your tongue, drool, and ooze some foul-smelling
liquid from your anal glands. Disgusted, your assailant
loosens its grip, decides you’re not the dinner
it was looking for, and departs. After 10 minutes,
you resurrect and merrily saunter on. From lemurs to lizards, ants to amphibians, sharks to chickens, hundreds of animals
"play dead" as a survival tactic. Nicknamed "playing possum"
after its star performer, feigning death is also called thanatosis. That’s from Thanatos,
the ancient Greek deity of death. But most scientists call it
tonic immobility, or TI. How and why TI occurs depends
on the species and situation. Spewing stench and adopting odd postures
are common and often play important roles. Other animals sacrifice their neighbors: quail chicks that freeze
while their kin run amok have a better chance of survival
when pursued by a cat. Speaking of cats, feline mothers can pinch
the napes of their kittens’ necks and induce another kind of
immobility called clipnosis. This keeps their kittens quiet
and easy to transport. Most of the physiological mechanisms
underlying these theatrics originate in
the parasympathetic nervous system, better known for controlling cycles
of resting and digesting. In possums, the parasympathetic nervous system causes
their heart rates to drop by nearly half, respiration by a third, and body temperatures by more than half a
degree Celsius for up to an hour. The neurotransmitter dopamine
also plays a part. Flour beetles with low dopamine levels
play dead more frequently than those with high levels, and anything blocking dopamine receptor
sites can lengthen catatonia. But maintaining a death ruse isn’t easy. The performers are constantly
gauging their surroundings for cues on when it’s safe to rise. Chickens, for instance, can sense
when a predator’s eyes are upon them. Researchers know this because when they
used a stuffed hawk in an experiment, their chicken subjects came out of their
catatonia quicker when the hawk’s eyes were averted. Other animals use TI for purposes
other than defense. When the sleeper cichlid feels peckish, it sinks to the lake floor
and lies motionless, its splotchy coloration making it
seem like a rotting carcass. If a small scavenger investigates,
this undead trickster strikes. Some animals even feign death
as a sexual ploy. Male nursery spiders offer gifts of
silk-wrapped insects in hopes of wooing females. But those females are known
to eat love-seeking males. By playing dead while the female
eagerly devours her snack, these males can cautiously revive and improve their chances
of successfully mating. So TI can work to an animal’s advantage,
unless someone else knows its secret. California orcas can flip over
young great white sharks, inducing TI for so long
the immobilized sharks, who must move to respire,
essentially suffocate. Humans can also flip sharks into TI. By stroking a shark’s
electrically-sensitive snout and gently turning it over, researchers can induce TI
that lasts up to 15 minutes. That’s enough time to insert tags, remove hooks, and even perform surgeries. There are risks however: TI can hamper respiration and induce
hyperglycemia, a sign of stress. So this technique should only
be used when necessary. Humans can also experience TI when they
freeze with fear during violent assaults. Recognizing this ancient, involuntary
form of self-defense has significant implications
when trying to understand why some victims don’t flee
or fight in the face of danger. So, studying TI in non-human animals not only helps us better understand some
odd behaviors, it can also help us better
understand our own, sometimes counterintuitive, responses to violence.
