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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 someodd behaviors,it can also help us better understand our own,sometimes counterintuitive,responses to violence.

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.

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.

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.

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.

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.

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.

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.

Species are sometimes introduced into new habitats through natural factors,like storms,ocean currents,or climate shifts.The majority of invasive species,though, are introduced by humans.Often this happens unintentionally,as when the zebra mussel was accidentally brought to Lake Erie by cargo ships.But as people migrate around the world,we have also deliberately brought our plants and animals along,rarely considering the consequences.But now that we're learning more about the effects of invasive species on ecosystems,many governments closely monitor the transport of plants and animals,and ban the imports of certain organisms.But could the species withthe most drastic environmental impactbe a group of primates who emergedfrom Africa to cover most of the world?Are we an invasive species?

But even minor changes in one factor can upset this balance,and the sudden introduction of non-native organisms can be a pretty major change.A species that is evolved in a separate habitat will be susceptible to different limiting factors,different predators,different energy sources,and different climates.If the new habitat's limiting factors fail to restrict the species growth,it will continue to multiply,out-competing native organisms for resources and disrupting the entire ecosystem.

So why does this keep happening?Most of the world's ecosystems are the result of millennia of coevolution by organisms,adapting to their environment and each other until a stable balance is reached.Healthy ecosystems maintain this balance via limiting factors,environmental conditions that restrict the size or range of a species.These include things like natural geography and climate,food availability,and the presence or absence of predators.For example, plant growth depends on levels of sunlight and soil nutrients.The amount of edible plants affects the population of herbivores,which in turn impacts the carnivores that feed on them.And a healthy predator population keeps the herbivores from becoming too numerous and devouring all the plants.

And in Australia, European rabbits eat so many plants that they wipe out the food supply for themselves and other herbivores.They're a pretty recent addition,intentionally introduced to the continent because one man enjoyed hunting them.Like the Burmese pythons,various factors in their native habitat keep their numbers in control.But in Australia, the lack of predators and a climate perfect for year-long reproduction allows their populations to skyrocket.

Meanwhile in Florida's Everglades,Burmese pythons,thought to have been released by pet owners,are the cause of decreasing populations of organisms.They're successfully outcompeting top predators,such as the alligator and panther,causing a significant reduction in their food sources.They're not a problem in their native Asia because diseases, parasites, and predators help to control their population size.

The kudzu vine, for example,had grown quality in its native east Asia,eaten by various insects and dying off during the cold winters.But its fortunes changed when it was imported into the southeastern United States for porch decoration and cattle feed.Its planting was even subsidized by the government to fight soil erosion.With sunny fields, a mild climate,and no natural predators in its new home,the vine grew uncontrollably until it became known as the plant that ate the South.

Massive vines that blanket the southern United States,climbing as high as 100 feet as they uproot trees and swallow buildings.A ravenous snake that is capable of devouring an alligator.Rabbit populations that eat themselves into starvation.These aren't horror movie concepts.They're real stories,but how could such situations exist in nature?All three are examples of invasive species,organisms harmful not because of what they are,but where they happen to be.