Many insects and animals, including humans, enter into a state of "fake death" immobility when threatened, but this seemingly passive frozen-with-fear state may be a selfish behavior that can lead to the killing of one's friends and relatives, according to a new study.
For humans, this can happen when an attacker enters a building and starts randomly targeting victims. People who play dead often survive, while their fleeing colleagues usually aren't so lucky.
The paper, published in the latest Proceedings of the Royal Society B, is the first to demonstrate the adaptive significance of playing possum, and how it's a selfish behavior.
"Death-feigning prey increase their probability of survival at the expense of more mobile neighbors," lead author Takahisa Miyatake told Discovery News.
Miyatake, a professor in the Laboratory of Evolutionary Ecology at Okayama University, and his team focused on a predator-prey system that, like the cartoon Road Runner and Wile E. Coyote, involves constant chasing. In this case, a hungry jumping spider frequently chases, bites and eats red flour beetles. The two species cohabit on rice bran or corn flour in cereal storehouses.
The researchers conducted experiments to see how well death feigning beetles survived when alone, or with other individuals either of their own, or similar, species. The scientists also checked to see if chemicals emitted by beetles playing dead somehow made them unattractive or unpalatable to the hungry spiders, which were starved for a week before the experiments began.
The chemicals didn't seem to make a difference, but having other moving individuals around did. During one experiment, around 60 percent of the beetles were eaten by the spider when they were alone and playing dead versus just 9.6 percent when additional mobile insects were nearby.
"Spiders appear to rely on prey locomotion in order to catch prey, and in addition may rely on tactile movement signals to initiate the kill behavior," the researchers explained, adding that "selfish prey" playing dead then wind up sacrificing their "neighbors in the group or community."
They suspect the findings could also apply to caterpillars, moths and other beetles that live in food storehouses. Sheep, group-living snakes and even humans may also fit the scenario.
"We can imagine such a (death-feigning) person might survive more," Miyatake said, referring to war, or war-like conditions.
He added that not everyone immediately goes into the play dead mode, however, probably due to each individual having either a "shy or bold" personality, which is partly controlled by his or her genes.
Something similar happens among fire ants, according to Deby Cassill of USF Petersburg's Biology Department. She and her colleagues observed how different aged fire ants acted when they were under attack from neighboring colonies.
"Days-old workers responded to aggression by death feigning, weeks-old workers responded by fleeing, and months-old workers responded by fighting back," Cassill and her team determined.
The older ants might have died holding down the fort, but they were four times more likely to perish than the younger ants that simply went into a catatonic state. Age and assessment of personal strength might therefore come into play when an individual has no choice but to flee, feign death or face the enemy.