EXPLORE | BASIC INSTINCTS
Acting Dead Can Be Animals’ Advantage
Assuming a lifeless-looking,
immobile state in an effort
to discourage or ambush a
predator is known as tonic
immobility, death feigning,
or thanatosis. It’s been seen
in a wide variety of verte-
brates and invertebrates.PHOTO: RASSIM KHELIFAVIRGINIA OPOSSUM
When threatened, an opos-
sum will faint and look quite
dead—from its prone posture
to its drooling, tongue- lolling
grimace—which led to the
phrase “playing possum”
to refer to feigning death.HOGNOSE SNAKE
To make their fake death
throes as convincing as
possible, hognose snakes
of the North American genus
Heterodon have been known
to secrete a foul-smelling
fluid and even spew blood.THE MATING RITUAL of the moorland
hawker dragonfly—common around
the ponds and wetlands of Europe,
Asia, and North America—begins with
what biologist Rassim Khelifa calls “an
acrobatic aerial copulation.” While in
flight, the female Aeshna juncea con-
torts so that her genitals, which are
near the end of her body, connect with
the male’s genitals, which are near his
thorax. Thus joined in a lopsided-heart
shape (right), they land and complete
the sex act, whereupon the female will
head off to lay her eggs.
Before she can do that, other males
may show up seeking sex. Evolution
predisposes her to resist: She has lim-
ited eggs, her reproductive tract can be
damaged by repeated copulations, she’s
already been inseminated by the mate
she chose, and the dragonfly penis is
structured so that it removes any sperm
present before making a new deposit. So
to avoid further sex, she may fall down
dead—or more precisely, she may fake
death, dropping from the air and lying
motionless in the ground cover.
At the University of Zurich, Khelifa
conducted a study of the females’ death
feigning and found that usually “their
strategy works.” Most males buzzed
the crash site briefly, he says, then
flew off to look for other conquests.
Once the males left, the lifeless- looking
females stirred and went on their way.
—PATRICIA EDMONDSFAKING TO AVOID
SEX? SHE’S DEAD
SERIOUS ABOUT IT
PREDATORY CICHLID
At least two species of this
fish may lure prey by sinking
to the bottom of a river or
lake and lying still as a corpse.
If small fish approach to scav-
enge, the cichlid lunges and
makes a meal of them. —PE